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I'm a doctoral student in evolutionary ecology; D & T is my personal 'blog, and my top topics are science, religion, and politics, with particular interest in the interface between science and religion.
Jeremy Yoder
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- August 9, 2011
- 09:06 AM
- 508 views
Flowers stay open for pollinators, not daylight
by Jeremy Yoder in Denim and Tweed
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A honeybee explores the depths of a dandelion, one of the species used in Fründ et al.'s experiments. Photo by je-sa.If you've ever stopped to admire morning glory flowers opening first thing in the morning, then noticed they've closed by evening, you're at least dimly aware of one of the longest-established ideas in plant biology: that flowers open and close on a reliable daily schedule. Different species are open at different times of day, of course, but each flowering plant has its preferred open period, and it sticks to that schedule during its flowering season.
This idea led Carolus Linneaus, the father of modern biological taxonomy, to propose an Horologium florae, or "floral clock" using plantings of species with known flowering times to mark the hours. You can find his table of proposed species in the online version of Linneaus' 1783 treatise Philosophia Botanica, if you're not averse to Latin. Studies of flowers' daily schedules go back to well before English was the language of international science, and continue to the present day [$a].
Yet no one seems to have spent much time considering how flowers' schedules might respond to the activity of their very reason for being: pollinators. Flowers don't open just to be open in a particular kind of sunlight—they're open to attract animals that can carry pollen to another plant, and maybe leave some, too. If a flower receives enough pollen to make seeds by noon, why would it stay open until two o'clock?
According to some new experimental results, the answer to that question is that they don't [$a].
Jochen Fründ, Carsten F. Dormann, and Teja Tscharntke set out to see whether a selection of European wildflowers adjusted their opening schedules in response to pollination, with two major experiments and a broader-scale observation project. The experiments address whether pollinator activity could change flowers' schedules; the observations help determine how important those changes might be in studies of plant-pollinator interaction.
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A floral clock in Geneva—not quite what Linneaus had in mind. Photo by aranmanoth.In the first experiment, the team planted wildflowers—Crespis capillaris, a close relative of common dandelions—in experimental plots spaced across a field. Plots were either caged or left open to insect visitors, and Fründ et al introduced bees into some of the caged plots. So some plots had a controlled set of pollinators, some had none at all, and some had whatever pollinators were already active in the field.
The team then watched the flowers' daily opening and closing in the experimental plots. (They had a lot of help—a long list of names in the paper's Acknowledgements section ends with "and many others.") Over the same period of time, flowers in the un-caged plots received more insect visitors than flowers in either other treatment, and had mostly closed by midafternoon; flowers in the caged plots with bees introduced received fewer visitors and closed hours later; and flowers in the plots with no pollinators at all stayed open till evening.
So flowers experiencing the same daylight pattern closed earlier if they received more pollinator visits. The team followed up this result by hand-pollinating flowers of C. capillaris and a handful of closely related species growing in the same field, including dandelions—and flowers of three out of four species closed more rapidly when hand pollinated. Dandelions didn't respond to hand pollination, a result the authors explain by noting that dandelions often self-pollinate, and so don't need to wait for animal pollinators.
Finally, the team compiled observations of plant-pollinator interactions from sites similar to their study field located across Germany, and divided them into observations taken before solar noon, when the focal flower species from the experiments above tend to be open, and after solar noon. Which pollinator species visited which flowering plants depended significantly on when the observations were made—to the extent that the apparent importance of C. capillaris and its relatives is entirely different before and after noon.
Of course, these results apply directly to only a handful of species representing a particular group of flowering plants—but it's a group with a lot of widespread and abundant members, and the result is straightforward and striking. Animal-pollinated plants may not behave much like clocks at all. Instead, they're more like the patrons of a singles bar: they show up at about the same time and hang around until they find someone to buy them a drink. That's a dynamic worth keeping in mind for studies of plant-pollinator interaction, since it suggests that the partners a pollinator chooses will depend, at least in part, on whether or not it's out after closing time. ◼
References
Ewusie, J., & Quaye, E. (1977). Diurnal periodicity in some common flowers. New Phytologist, 78 (2), 479-485 DOI: 10.1111/j.1469-8137.1977.tb04854.x
Fründ, J., Dormann, C., & Tscharntke, T. (2011). Linné’s floral clock is slow without pollinators - flower closure and plant-pollinator interaction webs. Ecology Letters DOI: 10.1111/j.1461-0248.2011.01654.x
von Hase, A., Cowling, R., & Ellis, A. (2005). Petal movement in cape wildflowers protects pollen from exposure to moisture Plant Eco... Read more »
Ewusie, J., & Quaye, E. (1977) Diurnal periodicity in some common flowers. New Phytologist, 78(2), 479-485. DOI: 10.1111/j.1469-8137.1977.tb04854.x
Fründ, J., Dormann, C., & Tscharntke, T. (2011) Linné’s floral clock is slow without pollinators - flower closure and plant-pollinator interaction webs. Ecology Letters. DOI: 10.1111/j.1461-0248.2011.01654.x
von Hase, A., Cowling, R., & Ellis, A. (2005) Petal movement in cape wildflowers protects pollen from exposure to moisture. Plant Ecology, 184(1), 75-87. DOI: 10.1007/s11258-005-9053-8
- July 26, 2011
- 09:53 AM
- 533 views
Of mice and men, making a living in rarefied air
by Jeremy Yoder in Denim and Tweed
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High-elevation populations of deer mice have evolved "stickier" hemoglobin to cope with the thin atmosphere. Photo via Animal Diversity Web.
It's easy to walk through the woods and fields of North America and never spot Peromyscus maniculatus, the deer mouse, but you've probably heard them scampering off through the leaf litter or under cover of tall grass. They're exceptionally widespread little rodents, found in forest undergrowth and fields from central Mexico all the way north to the Arctic treeline. In all this range, they look about the same: small and brown, with white underparts and big, sensitive ears.
That apparent sameness is deceptive, however.
A big, varied range presents lots of different environmental conditions to which a widespread species must adapt. And when that big, varied range includes the Rocky Mountains, one of those environmental conditions is as basic as the air itself. At high altitudes, atmospheric pressure is lower, which means lower partial pressure of oxygen, the gas that makes life as we know it work.
The fundamental problem at high altitude is to pull more oxygen from thinner air. Natural selection is good at solving problems, and it has multiple options for adapting a mammal to thinner air at high altitudes, to the extent that these traits are heritable. Selection could favor individuals who more readily respond to thin air by breathing faster and deeper, pulling in more air to make up for its lower oxygen content. Or selection could favor individuals who produce more red blood cells, so that a given volume of blood pumped through their lungs picks up more oxygen. Or, at the most basic level, selection could favor individuals whose individual red blood cells are better at picking up oxygen, via a new form of hemoglobin, the oxygen-binding molecule that packs every red blood cell.
This final option is the path selection took in deer mice. As the mammalogist Jay Storz has discovered, subspecies of Peromyscus maniculatus that live at higher altitudes have stickier hemoglobin, which soaks up more oxygen in thin air than the hemoglobin of deer mice from lower altitudes. This is a pattern repeated on a broader evolutionary scale across the mammals: llamas and vicuñas, close relatives that both evolved in the rarefied air of the Andes, have "stickier" hemoglobin than most other mammals [PDF].
The pattern is also repeated in another, even more widespread mammal: humans.
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Mount Everest, the pinnacle of the Himalayas. Photo by phobus.
The evolution of Homo sapiens in response to high-altitude low-oxygen conditions is one of the most thoroughly described examples of human evolution, thanks to years of work by Cynthia Beall and her colleagues. Previous research going back to the nineteenth century established that people native to the Andes produce more red blood cells and more hemoglobin [PDF] than people from lower altitudes—a naturally selected form of the "blood doping" practiced by élite athletic cheats that helps draw more oxygen from every breath.
Meanwhile, populations that have lived for generations on the Tibetan Plateau don't have high-capacity blood like native Andeans. Yet high-altitude Tibetans maintain higher concentrations of oxygen in their blood than lowland natives can manage at the same altitude, though. Instead of evolving new blood capacity, Tibetans make their circulatory system work harder, with a high-powered ventilatory response. This response is the sensitivity to low oxygen conditions that prompts you to breathe more rapidly—people native to low altitudes also increase their breathing rate in response to moving to high altitude conditions, but they don't keep it up for more than a few days. In high-altitude Tibetans, this physiological acclimation has become a permanent feature.
In other words, natural selection has come up with two separate solutions to the problem of low oxygen at high altitude, within the same species.
Beall and her collaborators traced the presence of this "high oxygen saturation" trait in Tibetan families to establish that it has a genetic basis, and even identified part of the specific fitness benefit associated with carrying the high oxygen saturation gene in the thin air of the Tibetan plateau. In villages located at 4,000 meters or higher, women with the high oxygen saturation trait are about as likely to become pregnant, and carry those pregnancies to term, as women without the trait—but their children were more likely to survive over the longer term.
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Children of Tibetan women carrying the "high oxygen saturation" allele have a higher survival rate. Photo by The AlleyTree.
This adaptation to the heights is one of the most well documented examples of natural selection in human populations, with more detail known even than in the case of adult lactose tolerance. It's a demonstration of natural selection's resourcefulness, acting on random mutations to come up with entirely different solutions in distantly related populations on opposite sides of the world—adapting living things of all kinds to life at the very ends of the Earth. ◼
This post has its origin in the 2011 National Academies Northstar Summer Institute for science education, where I learned about the full extent of Cynthia Beall's work.
References
Beall, C. (2004). Higher offspring survival among Tibetan women with high oxygen saturation genotypes residing at 4,000 m. Proc. Nat. Acad. Sciences USA, 101 (39), 14300-14304 DOI: 10.1073/pnas.0405949101
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Beall, C. (2004) Higher offspring survival among Tibetan women with high oxygen saturation genotypes residing at 4,000 m. Proc. Nat. Acad. Sciences USA, 101(39), 14300-14304. DOI: 10.1073/pnas.0405949101
Beall, C. (2006) Andean, Tibetan, and Ethiopian patterns of adaptation to high-altitude hypoxia. Integrative and Comparative Biology, 46(1), 18-24. DOI: 10.1093/icb/icj004
Beall, C. (2007) Two routes to functional adaptation: Tibetan and Andean high-altitude natives. Proc. Nat. Acad. Sciences USA, 104(S1), 8655-60. DOI: 10.1073/pnas.0701985104
Storz, J. (2007) Hemoglobin function and physiological adaptation to hypoxia in high-altitude mammals. Journal of Mammalogy, 88(1), 24-31. DOI: 10.1644/06-MAMM-S-199R1.1
Storz, J., Runck, A., Sabatino, S., Kelly, J., Ferrand, N., Moriyama, H., Weber, R., & Fago, A. (2009) Evolutionary and functional insights into the mechanism underlying high-altitude adaptation of deer mouse hemoglobin. Proc. Nat. Acad. Sciences USA, 106(34), 14450-5. DOI: 10.1073/pnas.0905224106
- July 19, 2011
- 11:29 AM
- 397 views
Post arising: Anole vs. anole vs. predators
by Jeremy Yoder in Denim and Tweed
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A brown anole, with dewlap extended. Photo by jerryoldnettel.Last June, I discussed a study with big ambitions: to experimentally compare the effects that competition and predators have on island populations of brown anoles, Anolis sagrei. Now the current issue of the journal that carried that study, Nature has a brief communication from the godfather of anole evolutionary ecology himself, Jonathan Losos. Losos and his coauthor Robert Pringle raise some serious questions [$a] about the results of that experiment.
The authors of the original study [$a], Ryan Calsbeek and Robert Cox, concluded that competition was more important than predation because natural selection acting on anoles was stronger on experimental islands with higher anole population density, while the presence or absence of predators on those islands made no difference in the strength of selection. Losos and Pringle object that anole population density is entangled with other factors that may make Calsbeek and Cox's results uninterpretable.
This experimental design is confounded in three fundamental ways. First, density is confounded with island area. All analyses treat lizard density as a surrogate for intraspecific competition. However, an inverse correlation with island area explains 95% of the variation in density, such that it is impossible to disentangle the two factors statistically. This is a crucial problem, because multiple factors related to both predation and competition are known to vary with island area. For example, as island area increases, so too do the number of bird species (which increases the number of potential predators) and mean vegetation height (which might increase lizards’ susceptibility to avian predation). Likewise, because larger islands have lower perimeter/area ratios, they receive relatively lower input of marine-resource subsidies and have lower arthropod densities; a study of A. sagrei in this system showed that lizard densities vary significantly with the amount of seaweed deposition, and that experimental seaweed deposition increased lizard densities by more than 60%. [In-text citations removed for clarity.]
That point alone is a pretty big problem with Calsbeek and Cox's result. Then Losos and Pringle re-analyze the data presented in the original study, and discover the very odd result that anoles in the experimental populations had higher rates of survivorship on the high-density islands—which is exactly the opposite of what you'd expect if competition for important resources were more intense in high-density populations. At the very least, this indicates that there could be more going on than Calsbeek and Cox originally supposed, in which case their data don't support their conclusions.
Losos and Pringle raise other objections, including the issue of small sample size I noted in my original post. You should read the whole thing [$a] for the details, as well as the response [$a] from Calsbeek and Cox.
References
Calsbeek, R., & Cox, R. (2010). Experimentally assessing the relative importance of predation and competition as agents of selection. Nature, 465 (7298), 613-616 DOI: 10.1038/nature09020
Calsbeek, R., & Cox, R. (2011). Calsbeek & Cox reply. Nature, 475 (7355) DOI: 10.1038/nature10141
Losos, J., & Pringle, R. (2011). Competition, predation and natural selection in island lizards. Nature, 475 (7355) DOI: 10.1038/nature10140
... Read more »
Calsbeek, R., & Cox, R. (2010) Experimentally assessing the relative importance of predation and competition as agents of selection. Nature, 465(7298), 613-616. DOI: 10.1038/nature09020
Calsbeek, R., & Cox, R. (2011) Calsbeek . Nature, 475(7355). DOI: 10.1038/nature10141
Losos, J., & Pringle, R. (2011) Competition, predation and natural selection in island lizards. Nature, 475(7355). DOI: 10.1038/nature10140
- July 12, 2011
- 09:05 AM
- 624 views
Choosing your partner is only as helpful as the partners you have to choose from
by Jeremy Yoder in Denim and Tweed
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Picking teammates. Original photo by humbert15.When you need partners for some sort of cooperative activity—say, teammates for a game of kickball—you'd probably like to have a choice among several candidates. That lets you weigh considerations about kicking strength and running speed—and who promised to give you his dessert at lunch period—to build a winning team. However, if the other team captain snaps up the good players first, the fact that you have a choice among the others might not make much difference.
Plants and animals looking for mutualists face a similar situation. Being able to choose among possible partners should allow the chooser to work with helpful partners and avoid unhelpful ones, but a new study suggests that in one widespread mutualism the process of choosing between partners can leave the chooser worse off than if it had no choice at all [$a].
Coauthors Erol Akçay and Ellen Simms focus on the effects of partner choice in the mutualism between plants and nitrogen-fixing bacteria—the interaction I'm studying in my current postdoc position, as it happens. All living things need nitrogen, but only some strains of bacteria are able to collect nitrogen from the atmosphere and "fix" it into a form that other organisms can use. Many plants, particularly members of the big and diverse bean family, have evolved to allow nitrogen-fixing bacteria to infect their roots—the plants form a nodule of root tissue around the infection and supply the tissue with sugar for the bacteria to feed on as they fix nitrogen. Eventually the nodule dries up and dies off, and the bacteria are freed into the soil, having multiplied many times over thanks to the food supply from the host plant.
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A plant's root nodules, some cut open to show the interior. Photo by pennstatelive.To see how this choice might work in practice, Akçay and Simms construct a mathematical model of a plant with two nodules. Each nodule produces some level of nitrogen, and recieves some level of sugar from the plant. The plant negotiates with the two nodules in what's called a "war of attrition" game: whichever partner wants a better deal cuts off the exchange of services, and holds out until the cost of losing the service it recieves is greater than the benefit it hopes to gain in the war of attrition.
Rather like ant-defended plants, plants that host nitrogen-fixing bacteria don't seem to screen potential mutualistic bacteria before allowing them to infect their roots. However, after root nodules are established, the success of the mutualism from the perspective of both partners depends on the genetics of each [PDF], and when host plants receive supplemental nitrogen, they put fewer resources into growing nodules [PDF]. Host plants have been observed with different strains of bacteria in different nodules, and some nodules could contain diligent nitrogen fixers while others are full of freeloaders. This may be the point at which the plant has a choice of partners—it can potentially direct sugar to helpful nodules, and cut off unhelpful ones.
Because the plant has two nodules to choose from, it can potentially outlast an uncooperative nodule by relying on the other one. This works if the plant can shunt more resources to the cooperative nodule and recieve more nitrogen from it in return. However, the success of this strategy depends on two traits of the bacteria inhabiting the nodules—how readily they ramp up nitrogen production in response to more sugar, and how stubborn they are in the war of attrition game.
If both nodules are stubborn but responsive to extra sugar, the plant can negotiate with one nodule by giving the other more sugar and receiving extra nitrogen. This lets the plant hold out longer in the war of attrition. On the other hand, nodules that are not responsive to extra sugar but also not very stubborn yield quickly in the war of attrition even though they don't help much in negotiations. In either of these two cases, the negotiations find an equilibrium in which the plant receives a benefit about intermediate between what it would recieve if both nodules were infected by the same strain of bacteria.
However, if the plant hosts a stubborn-responsive bacterial strain in one nodule and a yielding-unresponsive strain in the other, it finds itself in a trap: the yielding-unresponsive strain is no help in negotiation against the stubborn-responsive strain, and the help provided by the stubborn-responsive strain isn't an advantage in negotiating with the yielding-unresponsive strain. Over successive negotiations, the stubborn-responsive strain can ratchet up the sugar it extracts from the plant, and the plant ends up worse off than it would be if the two nodules were identical.
Just like humans haggling in a marketplace, the outcome of the interaction depends strongly on whether the other party plays along as expected.
Akçay and Simms find a way out of this trap by adding another wrinkle to the model. Much like the contract-theory models of mutualism I've discussed before, they modify the model to allow cooperative nodules to benefit from being cooperative. This makes a good deal of intuitive sense—if a nodule provides a better deal to the plant, the plant can potentially grow more leaves to produce more sugar, which would allow it to offer a better deal to the bacteria it hosts. Akçay and Simms call this "partner fidelity feedback," and they find that, if it is sufficiently strong, it can allow the plant to out-negotiate a stubborn strain of bacteria.
Although it has a good deal of intuitive appeal, the model presented by Akçay and Simms does a fair bit of speculating in the absence of data. This is also a problem for the contract-theory model, and really all models of this widespread and important interaction. We know a great deal about the chemical details of plants' interaction with nitrogen fixing bacteria. However, we don't have a good sense of whether and how plants can redirect resources among nodules to haggle with the bacteria they host, and we don't know whether and how bacteria could adjust their behavior to haggle with the plant. Akçay and Simms devote a big section of their online appendix [$a] to discussing just this point.
To figure out what's going on inside those nodules, we need to determine how different models of interaction between plants and their bacterial mutualists may shape patterns in things that are easier to observe—both in the compatibility between plant genotypes and bacterial strains in greenhouse tests, and in the broader population genetics of both partners.
References
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Akçay, E., & Simms, E. (2011) Negotiation, sanctions, and context dependency in the legume-rhizobium mutualism. The American Naturalist, 178(1), 1-14. DOI: 10.1086/659997
Heath, K. (2010) Intergenomic epistasis and coevolutionary constraint in plants and rhizobia. Evolution. DOI: 10.1111/j.1558-5646.2009.00913.x
Heath, K.D., Stock, A.J., & Stinchcombe, J.R. (2010) Mutualism variation in the nodulation response to nitrate. Journal of Evolutionary Biology, 23(11), 2494-2500. DOI: 10.1111/j.1420-9101.2010.02092.x
- June 28, 2011
- 09:05 AM
- 647 views
Snake-eating opossums have evolved venom-resistant blood
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } The humble Virginia opossum can shrug off snakebites that would kill larger mammals. Photo by TexasEagle.If you were going to pick the traits of a single animal to confer on a superhero, you probably wouldn't pick the Virginia opossum. Possums are ubiquitous, scruffy, ratlike marsupials, their toothy grins giving the not entirely inaccurate impression that they don't have much going on upstairs. Until recently, the nicest thing I could think to say about them is that they eat a lot of ticks.
Blood-sucking Lyme disease vectors are only a small part of the opossum's eclectic diet, however. They also eat quite a few poisonous snakes, and this has apparently led them to evolve a trait I could call a superpower without exaggeration: opossum blood is resistant to snake venom.
This curious and useful ability was first documented by J.A. Kilmon in a 1976 paper [$a], in which Kilmon reported field observations and laboratory trials showing that opossums tolerate snakebites without visible ill effect. (If animal experimentation makes you queasy, you might want to go read something else about now. Perhaps a nice post about gerbils?)
A natural bite was observed in the field by a 160 cm eastern diamondback on an adult opossum, Didelphis virginiana. The opossum displayed no apparent distress and this suggested a remarkable tolerance by that animal to envenomation. In order to ascertain if an actual envenomation did take place, Mr. Seashole conducted field experiments by manually causing snakes to inflict actual bites on captured opossums. None of the bites caused visible signs of distress to the opossums.
Kilmon brought possums back to the lab, anesthetized them, hooked them up to heart monitors, and "inflicted" bites on them from diamondback and timber rattlesnakes, water moccasins, and at least one cobra. (Kilmon reports he used 15 snakes in total, but doesn't break that number down by species.) "None of the five opossums," he wrote, "developed observable local reactions other than trauma attributable to fang penetration and none developed observable systemic effect, exhibiting negligible alteration of heart rate and respiration."
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A timber rattlesnake—no big deal to an opossum. Photo by Tom Sprinker.Finally Kilmon injected an anesthetized opossum with enough water moccasin venom to kill five fifteen-kilogram dogs, and observed no reaction beyond a brief drop in blood pressure and small spike in pulse rate—when the possum awoke, it was "apparently healthy." Upon sacrificing and dissecting the animal, Kilmon found no evidence of organ damage.
Kilmon concludes his brief scientific report with a weird aside about the evolutionary history of opossums, which, had he been writing in 2011, would have made me think his research consisted mainly of skimming the Wikipedia entry for Didelphis virginiana. In the course of reporting the opossum's taxonomic affiliations and known diet, Kilmon notes offhandedly,
This polyprotodont marsupial is a primitive but also very successful mammal. The opossums of varying species are the only marsupials surviving in the placental world, the predominant marsupial and monotreme mammals of Australia having probably survived due to their isolation. The opossum has remained unchanged for millions of years and probably reached his peak of evolutionary specialization several millions of years ago.
I don't think he could've gotten away with that last sentence in an evolutionary biology journal. It's true that the common ancestor of opossums and placental mammals (i.e., us) diverged quite a long time ago, that opossum-like critters are known from the fossil record going back that far, and that many opossum traits are thought to be shared with early mammals. But that doesn't mean opossums "remained unchanged for millions of years." The lineage leading to modern opossums has been evolving exactly as long as the lineage leading to modern humans—and if the opossum's lifestyle hasn't led it to such evolutionary heights as the wheel, war, New York and so forth, then it also hasn't left the opossum unchanged.
As it happens, a pretty good illustration of this point is the paper that led me to Kilmon's morbid little study in the first place. Mammalogists Sharon Jansa and Robert Voss have just published a study of one blood protein that may underlie opossums' resistance to venom. The venom of pit vipers like rattlesnakes and water moccasins targets the blood clotting system—one of the unpleasant effects of a snake bite is internal hemorrhage. So Jansa and Voss examined the evolution of a venom-targeted clotting protein called von Willebrand Factor, or vWF, comparing it across the entire family of opossums, the didelphidae.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Photo by Maggie Osterberg.Since the evolutionary origin of the family, the vWF of opossum species that prey on snakes has accumulated more changes than vWF in non-snake-eating species. That's circumstantial evidence for the effect of natural selection continuously acting on vWF over millions of years. Jansa and Voss picked out several specific changes that are unique to snake-eating opossums, and found that they're associated with a region of vWF that is known to bind with one of the toxins in pit viper venom.
The authors suggest that opossums may have been engaged in a evolutionary "arms race" against snake venom toxins since they first developed a taste for rattlesnake. In other words, not only is the opossum not unchanged since the early history of mammals, one of the traits that has changed continuously since then may be the very feature that piqued Kilmon's interest.
References
Jansa, S., & Voss, R. (2011). Adaptive evolution of the venom-targeted vWF protein in opossums that eat pitvipers. PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0020997
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Jansa, S., & Voss, R. (2011) Adaptive evolution of the venom-targeted vWF protein in opossums that eat pitvipers. PLoS ONE, 6(6). DOI: 10.1371/journal.pone.0020997
Kilmon, J., Sr. (1976) High tolerance to snake venom by the Virginia opossum, Didelphis virginiana. Toxicon, 14(4), 337-40. DOI: 10.1016/0041-0101(76)90032-5
- June 22, 2011
- 12:00 PM
- 510 views
The intelligent homosexual's guide to natural selection and evolution, with a key to many complicating factors
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } San Francisco Pride, 2008. Photo by ingridtaylar.This is a cross-posting of my latest contribution to the Scientific American guest blog. Since the original went up at SciAm, P.Z. Myers has pointed out a few more complicating factors. If you read one paper to follow up on what I've written here, I'd suggest Nathan Bailey and Marlene Zuk's excellent 2009 review [PDF], which is posted in PDF format by none other than The Stranger.
June is Pride Month in the United States, and in communities across the country, lesbian, gay, bisexual, and transgendered Americans are celebrating with carnivals, parades, and marches. Pride is a rebuke to the shame and marginalization many LGBT people face growing up, and a celebration of the freedoms we've won since the days when our sexual orientations were considered psychological diseases and grounds for harrassment and arrest. It's also a chance to acknowledge how far we still have to go, and to organize our efforts for a better future.
And, of course, it's a great big party.
I'm looking forward to celebrating Pride for the first time in my new hometown of Minneapolis this weekend--but as an evolutionary biologist, I suspect I have a perspective on the life and history of sexual minorities that many of my fellow partiers don't. In spite of the progress that LGBT folks have made, and seem likely to continue to make, towards legal equality, there's a popular perception that we can never really achieve biological equality. This is because same-sex sexual activity is inherently not reproductive sex. To put it baldly, as the idea is usually expressed, natural selection should be against men who want to have sex with other men--because we aren't interested in the kind of sex that makes babies. An oft-cited estimate from 1981 is that gay men have about 80 percent fewer children than straight men.
Focusing on the selective benefit or detriment associated with particular human traits and behaviors gets my scientific dander up, because it's so easy for the discussion to slip from what is "selectively beneficial" to what is "right." A superficial understanding of what natural selection favors or doesn't favor is a horrible standard for making moral judgements. A man could leave behind a lot of children by being a thief, a rapist, and a muderer--but only a sociopath would consider that such behavior was justified by high reproductive fitness.
And yet, as an evolutionary biologist, I have to admit that my sexual orientation is a puzzle.
There's reasonably good evidence for a genetic basis to human sexual orientation--although the search [$a] for a specific "gay gene" [$a] has had mixed results [$a]. Gene variants, or alleles, associated with an 80 percent decrease in reproductive fitness should be naturally selected out of the population pretty quickly. So why aren't all humans heterosexual?
Straight people are in the overwhelming majority, but gay men, lesbians, bisexuals, and transgendered people account for a non-trivial minority--the morst recent survey I'm aware of found 7 percent of women and 8 percent of men in the U.S. identify as L, G, B, or T. We don't have remotely comparable historical data, but mention of same-sex sexuality goes back to the dawn of recorded history. If natural selection is homophobic, it's not particularly good at it.
Before I get much farther, here's a disclaimer: I'm going to consider how same-sex attraction might persist in human populations in the face of its apparent selective disadvantages. In the absence of direct data--such as systematic measures of the the total evolutionary fitness of gay men or lesbians in specific societal contexts--it's easy to make up stories about natural selection, but much harder to determine which stories reflect reality. I'll try to delineate which stories fit with what we know about how selection works, and with the little data we do have--but that's the best I can do. If there's one point I hope you take from all that follows, it's that evolution is complicated, and human evolution doubly so.
Natural selection isn't all-powerful
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Human Rights Campaign banner from Portland Pride 2008. Photo by PDX Pixels.Natural selection causes traits associated with having fewer children to become less common over time. But natural selection is not the only evolutionary process at work in natural populations. Mutation introduces new alleles even as natural selection removes them. Furthermore, the effects of random chance in small populations creates an effect called genetic drift, which can interfere with the expected operation of natural selection.
Evolutionary biology has developed an excellent understanding of how mutation, selection, and drift interact over time to shape the genetic diversity of populations. That understanding allows us to do some back-of-the-envelope calculations to see how selection might operate on a gene associated with same-sex attraction. In setting this up, I'm following the lead of the evolutionary biologist Joan Roughgarden, who makes a similar point in her book Evolution's Rainbow. Brace yourself for some math!
In an idealized population of infinite size, the balance between natural selection's effect of removing disadvantageous alleles, and mutation's effect of spontaneously re-creating them, means that the equilibrium frequency of the allele in the population should be about equal to the square root of the ratio between the mutation rate and the selective cost associated with carrying two copies of the disadvantageous allele.
A single base pair of human DNA has a chance of of mutating equal to about one in one hundred million [$a] every generation. Since there may be thousands of base pairs [$a] in a single gene, the probability of a mutation occurring somewhere in the gene is more like one in one hundred thousand. If we assume that it takes two copies of our hypothetical "gay allele" to make a person attracted to members of the same sex, and about five percent of people are attracted to members of the same sex, then mutation alone could balance a selective cost to being gay of 0.0002. That is to say, mutation-selection balance alone could explain the frequency of LGBT folks in the population if those attracted to the same sex had, on average, 0.9998 children for every child born to the average straight parent. That's pretty weak selection.
This is where genetic drift enters the picture. Most natural populations don't behave anything like the mathematical ideal assumed for the calculations in the preceeding paragraph, because most natural populations are not infinite in size. In finite populations, randomness--"mere bad luck" in the words of pioneering biologist J.B.S. Haldane [$a]--can prevent selection from operating efficiently. Smaller populations are more prone to genetic drift--the relevant numb... Read more »
Bogaert, A. (2006) Biological versus nonbiological older brothers and men's sexual orientation. Proc. Nat. Acad. Sciences USA, 103(28), 10771-4. DOI: 10.1073/pnas.0511152103
Bailey, N., & Zuk, M. (2009) Same-sex sexual behavior and evolution. Trends in Ecology , 24(8), 439-46. DOI: 10.1016/j.tree.2009.03.014
Camperio-Ciani, A., Corna, F., & Capiluppi, C. (2004) Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity. Proc. Royal Soc. B, 2217-21. DOI: 10.1098/rspb.2004.2872
Eller, E., Hawks, J., & Relethford, J.H. (2010) Local extinction and recolonization, species effective population size, and modern human origins. Human Biology, 805-24. DOI: 10.1353/hub.2005.0006
Futuyma, D.J. (2005) Celebrating diversity in sexuality and gender. Evolution, 1156-9. DOI: 10.1111/j.0014-3820.2005.tb01052.x
Gavrilets, S., & Rice, W.R. (2006) Genetic models of homosexuality: generating testable predictions. Proc. Royal Soc. B, 3031-8. DOI: 10.1098/rspb.2006.3684
Haldane, J.B.S. (2008) A mathematical theory of natural and artificial selection. Part V: Selection and mutation. Mathematical Proc. Cambridge Phil. Soc., 23(07), 838. DOI: 10.1017/S0305004100015644
Hamer, D.H., Hu, S., Magnuson, V.L., Hu, N., & Pattatucci, A.M. (1993) A linkage between DNA markers on the X chromosome and male sexual orientation. Science, 321-7. DOI: 10.1126/science.8332896
Herbenick D., Reece M., Schick V., Sanders S.A., Dodge B., & Fortenberry J.D. (2010) Sexual behavior in the United States: results from a national probability sample of men and women ages 14-94. The journal of sexual medicine, 255-65. PMID: 21029383
Pillard RC, & Bailey JM. (1998) Human sexual orientation has a heritable component. Human Biology, 70(2), 347-65. PMID: 9549243
Rahman, Q., & Hull, M.S. (2005) An empirical test of the kin selection hypothesis for male homosexuality. Archives of Sexual Behavior, 461-7. DOI: 10.1007/s10508-005-4345-6
Ramagopalan, S., Dyment, D.A., Handunnetthi, L., Rice, G.P., & Ebers, G.C. (2010) A genome-wide scan of male sexual orientation. Journal of Human Genetics, 131-2. DOI: 10.1038/jhg.2009.135
Roach, J., Glusman, G., Smit, A., Huff, C., Hubley, R., Shannon, P., Rowen, L., Pant, K., Goodman, N., Bamshad, M.... (2010) Analysis of genetic inheritance in a family quartet by whole-genome sequencing. Science, 328(5978), 636-9. DOI: 10.1126/science.1186802
Rice, G. (1999) Male homosexuality: Absence of linkage to microsatellite markers at Xq28. Science, 284(5414), 665-7. DOI: 10.1126/science.284.5414.665
Takahata, N. (1993) Allelic genealogy and human evolution. Molecular Biology and Evolution, 2-22. info:/8450756
Tenesa, A., Navarro, P., Hayes, B.J., Duffy, D.L., Clarke, G.M., Goddard, M.E., & Visscher, P.M. (2007) Recent human effective population size estimated from linkage disequilibrium. Genome Research, 520-6. DOI: 10.1101/gr.6023607
Xu, L., Chen, H., Hu, X., Zhang, R., Zhang, Z., & Luo, Z.W. (2006) Average gene length is highly conserved in prokaryotes and eukaryotes and diverges only between the two kingdoms. Molecular Biology and Evolution, 1107-8. DOI: 10.1093/molbev/msk019
- June 7, 2011
- 09:05 AM
- 730 views
Freeloading caterpillars get in the way of plant-ant mutualism
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Cecropia obtusifolia provides food for ants that come and protect it—unless caterpillars get there first. Photo by wallygroom.Imagine you need a team of security guards. To find them, you decide not to place an ad in the local paper or on Craigslist. Instead, you build an apartment complex next to your home, complete with a full-service cafeteria providing free hot meals 24 hours a day. You leave the front doors unlocked, then hope that anyone who shows up to live in the apartments will also keep an eye on your home.
If you took that strategy to protect your assets, you'd have to be crazy. But that's pretty much what ant-protected plants do all the time. They grow hollow structures called domatia, secrete nectar from special structures, and even produce tasty and nutritious "food bodies." Then they wait for ants to move into the domatia, eat the nectar and the food bodies, and hopefully chase away anything that might want to do the plant harm. The crazy thing is, it works.
Well, it mostly works.
One gap in the ant-protection mutualism is the period when an ant-protected plant hasn't grown big enough to support a whole colony of ants. In this early stage, ants won't colonize the plant, but other insects might be quite happy to take the rewards that are already being offered. That's exactly what larvae of the butterfly Pseudocabima guianalis do—they make themselves at home on unprotected ant-plants.
The ant-plant Pseudocabima caterpillars target is Cecropia obtusifolia, a shrubby Central American tree that relies on ants in the genus Azteca for protection. Azteca ants make vicious and well-coordinated bodyguards. Here's video Ed Yong posted last year, showing a bunch of the ants flushing a hapless moth into an ambush.
However, Cecropia saplings can't produce enough food to support a colony of ants until the plants grow to more than a meter tall. What's too little for thousands of ants is a feast for a Pseudocabima caterpillar, however. Each caterpillar builds a silk shelter around a region of the plant that grows food bodies, and settles in to eat. As it grow larger, the caterpillar moves into a domatium near its original shelter, covering the entrance hole with silk. Finally the caterpillar pupates inside the domatium, emerging as an adult to lay eggs on another unprotected Cecropia plant.
Eventually the Cecropia saplings grow large enough to attract ants, who run off the caterpillars. However, as the paper I linked to above describes, the caterpillars seem to be able to resist an ant colony's establishment on the plant—the silk shelters prevent ants from getting to the best sources of food. Cecropia saplings occupied by caterpillars didn't seem to suffer more herbivore damage than ant-protected plants, but they did grow more slowly over the course of several years' observations. Caterpillar-infested Cecropia plants were also more vulnerable to infection by a fungus, which the ants removed quite effectively.
Interestingly, though, caterpillar-infested plants also produced less food than those guarded by ants. This is a point of circumstantial evidence for a new model of mutualism I wrote about earlier this year, in which cheating is reduced or prevented when a host like Cecropia better mutualists help create better rewards. An ant-protected plant can divert more resources to feeding its tenants, so their work rewards itself. However, Pseudocabima caterpillars are glad to take the lower level of rewards that Cecropia plants offer up to all comers.
In other words, if you're going to give out free lunches, you can't really expect everyone who eats to pay you back.
Reference
Roux, O., Céréghino, R., Solano, P.J., & Dejean, A. (2011). Caterpillars and fungal pathogens: Two co-occurring parasites of an ant-plant mutualism. PLoS ONE, 6 : 10.1371/journal.pone.0020538
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Roux, O., Céréghino, R., Solano, P.J., & Dejean, A. (2011) Caterpillars and fungal pathogens: Two co-occurring parasites of an ant-plant mutualism. PLoS ONE. info:/10.1371/journal.pone.0020538
- May 31, 2011
- 09:05 AM
- 684 views
Passive aggression: Parasitic wasp larvae interfere with each other via their host's host plant
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A large white butterfly caterpillar weaves a cocoon around the wasp larvae infesting its body. Photo by EntomoAgricola.I'm embarrassed to admit that I've only just gotten around to picking up Carl Zimmer's book Parasite Rex. It's turned out to be a wonderful compendium of all the peculiar ways parasites evade, confound, and resist the defenses of their hosts. Some of the wildest cases Zimmer examines, though, are parasites that manipulate their hosts' behavior.
One grotesque and well-studied example is the wasp Cotesia glomerata. Female C. glomerata wasps inject their eggs into butterfly caterpillars, and when the eggs hatch, the wasp larvae eat the caterpillar from the inside, saving critical organs so the poor thing stays alive the whole time. Then, when the wasp larvae are ready to burrow out of the caterpillar and form pupae to complete their devlopment, they induce the half-dead caterpillar to spin a web around them and stand guard against predators. (In technical language, this life history makes the wasp a parasitoid, rather than a parasite.) Christie Wilcox has written up a fuller description of the whole grisly process, if you want more detail.
That sounds like a pretty incredible set of manipulations for one clutch of wormy-looking wasp larvae, but they're not all that Cotesia glomerata can do. New evidence published in Ecology Letters suggests that C. glomerata can somehow make the plants that its host caterpillar feeds on less hospitable [$a] to the larvae of another caterpillar-infesting wasp. In other words, the wasp larvae may manipulate not just their host, but their host's host.
First off, here's video of Cotesia glomerata in action. Don't watch this on your lunch break.
Now, the wasp's plant manipulations. Lots of plants have what are called induced defenses against herbivores like the butterfly larvae that host C. glomerata larvae. Induced defenses are usually protective toxins that plants produce in response to herbivore damage [PDF]. Erik Poelman and his collaborators reasoned that, since C. glomerata can manipulate it's host's behavior, the parasites might change how plants respond to herbivory by infested caterpillars.
To test this, the team first had to induce plant responses. They grew Brassica oleracea—Brussels sprouts—plants in the greenhouse, then infested them with either un-parasitized caterpillars of the cabbage white butterfly Pieris rapae, cabbage white caterpillars infected with Cotesia glomerata, or cabbage white caterpillars infected with larvae of the related wasp C. rubecula. Once the caterpillars had nibbled on the plants enough to induce defensive responses, Poelman et al. removed the caterpillars in preparation for the experiment proper.
The team then introduced parasitoid-free caterpillars and caterpillars infested with one or the other parasitoid species onto host plants that had been through one of the three induction treatments, or that had never been exposed to herbivores. They then tracked the development of the caterpillars, and whether or not the wasp larvae inside them survived.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A healthy cabbage white butterfly caterpillar feeds on a piece of broccoli stem. Photo by Sam Fraser-Smith.Larvae of C. rubecula fared more-or-less equally well no matter what kind of plant their host caterpillar fed on. But C. glomerata larvae had substantially higher mortality when their hosts fed on plants induced by caterpillars infested with the competitor species. While about 50 percent of C. glomerata larvae died if their hosts fed on plants induced by uninfested caterpillars or caterpillars infested with C. glomerata, almost 75 percent of C. glomerata larvae died when their hosts fed on plants that had previously been occupied by caterpillars infested with C. rubecula.
This impact isn't because the host caterpillars fared poorly—in fact, caterpillars developed a little faster on plants induced by rubecula-infested caterpillars. So somehow, Cotesia rubecula seems to have influenced its hosts in a way that makes their host plants less hospitable to C. glomerata.
Poelman et al. are scrupulous to point out that this effect might not be anywhere nearly as strong in nature—host plants and host caterpillars might be plentiful enough that Cotesia glomerata can simply avoid the competitor species. On top of that, any natural selection that C. rubecula could be exerting on C. glomerata via induced responses in their shared hosts' host plants is occurring at multiple removes. The effect Poelman et al. documented is probably not an adaptation for competition with C. glomerata so much as a side effect of C. rubecula's effect on its host.
So although this result shows that one parasitoid wasp can reach out and influence another through three other organisms—its own host, that host's host plant, and the other wasp's host—it's not clear how strong that impact has been over the evolutionary history of these two Cotesia species. That said, this is a pretty nifty proof-of-concept.
Reference
Agrawal, A., Conner, J., Johnson, M., & Wallsgrove, R. (2002). Ecological genetics of an induced plant defense against herbivores: Additive genetic variance and costs of phenotypic plasticity. Evolution, 56 (11), 2206-2213 DOI: 10.1111/j.0014-3820.2002.tb00145.x
Poelman, E., Gols, R., Snoeren, T., Muru, D., Smid, H., & Dicke, M. (2011). Indirect plant-mediated interactions among parasitoid larvae. Ecology Letters DOI: 10.1111/j.1461-0248.2011.01629.x
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Agrawal, A., Conner, J., Johnson, M., & Wallsgrove, R. (2002) Ecological genetics of an induced plant defense against herbivores: Additive genetic variance and costs of phenotypic plasticity. Evolution, 56(11), 2206-2213. DOI: 10.1111/j.0014-3820.2002.tb00145.x
Poelman, E., Gols, R., Snoeren, T., Muru, D., Smid, H., & Dicke, M. (2011) Indirect plant-mediated interactions among parasitoid larvae. Ecology Letters. DOI: 10.1111/j.1461-0248.2011.01629.x
- May 24, 2011
- 09:05 AM
- 741 views
Pesticides and parasites add up to an evolutionary Catch-22
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } When Daphnia evolve resistance to pesticides, they become more vulnerable to bacterial parasites. Photo by Chantal Wagner.If you haven't read Joseph Heller's classic Catch-22, cancel your plans for next weekend and spend the time with a copy from the nearest library. It's a hilarious, bracingly bleak satire of military bureaucracy, as epitomized in the titular clause governing when bomber pilots can be grounded for reason of insanity:
There was only one catch and that was Catch-22, which specified that a concern for one's safety in the face of dangers that were real and immediate was the process of a rational mind. Orr was crazy and could be grounded. All he had to do was ask; and as soon as he did, he would no longer be crazy and would have to fly more missions.
Heller conceived Catch-22 as a product of malicious middle management, but a similar situation crops up in the natural world when living things are under natural selection from conditions that favor contradictory traits. Biologists most commonly call these tradeoffs.
Over the course of evolution, tradeoffs set up "choices" that natural selection must make—a population can adapt to one alternative set of conditions, or another, or settle on a middle ground. A trivial example is that elephants have long ago "chosen" not to fly (Dumbo notwithstanding) in the course of evolving large, un-aerodynamic bodies suitable for massive-scale herbivory. A more relevant example is a new finding that the evolution of pesticide resistance creates vulnerability to parasites [$a].
The US Environmental Protection Agency estimated [PDF] that in 2006 and 2007 (the latest years for which reports are online) we used upwards of five billion pounds of pesticides to kill unwanted plants, insects, fungi, and other organisms worldwide. Once they're sprayed, we don't have much control over where pesticides end up—rain runoff takes them into lakes, ponds, and the ocean. In those bodies of water, critters at the base of the food chain are the first to feel the effects—critters like the tiny, translucent crustacean Daphnia magna.
Of course, those critters may be able to evolve resistance to the pesticides contaminating their environment—but that resistance may come at a cost.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Pesticide application, via the most picturesque method available. Photo by Scott Butner.Anja Coors and Luc De Meester had already found a hint of this cost [$a] in an experiment using a single clonal line of Daphnia, in which Daphnia exposed to both sublethal concentrations of the widely-used insecticide carbaryl and a parasitic bacterium fared much worse than Daphnia exposed to only carbaryl or bacteria.
In the new study, Coors, De Meester, and three collaborators expand on that initial observation by determining whether Daphnia become more vulnerable to parasites as they evolve resistance to carbaryl, and whether this costly evolution could occur in natural populations. The coauthors took samples of Daphnia from natural populations in four separate lakes and exposed them to carbaryl over several generations—then sampled the resultant evolved populations and tested their vulnerability to the bacterium. Compared to Daphnia left unexposed to carbaryl, the evolved populations were more resistant to the pesticide—and were also more badly hurt by bacterial infection.
It's hard to say how general this particular result is to the many, many other species that, like Daphnia, must cope with pesticides and other pollutants humans have introduced into the environment. Evolution to resist one pesticide leads to lowered resistance to infection in one aquatic crustacean; in other species, facing different chemicals, maybe such costs are different or lesser or nonexistent. But living things are not infinitely pliable as they evolve in response to the many and rapid changes we're making in the world. To slow the extinction crisis going on around us, we need to avoid trapping other living things in Catch-22.
References
Coors, A., & De Meester, L. (2008). Synergistic, antagonistic and additive effects of multiple stressors: predation threat, parasitism and pesticide exposure in Daphnia magna.Journal of Applied Ecology, 45 (6), 1820-8 DOI: 10.1111/j.1365-2664.2008.01566.x
Jansen, M., Stoks, R., Coors, A., van Doorslaer, W., & de Meester, L. (2011). Collateral damage: Rapid exposure-induced evolution of pesticide resistance leads to increased susceptibility to parasites. Evolution DOI: 10.1111/j.1558-5646.2011.01331.x
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Coors, A., & De Meester, L. (2008) Synergistic, antagonistic and additive effects of multiple stressors: predation threat, parasitism and pesticide exposure in Daphnia magna. . Journal of Applied Ecology, 45(6), 1820-8. DOI: 10.1111/j.1365-2664.2008.01566.x
Jansen, M., Stoks, R., Coors, A., van Doorslaer, W., & de Meester, L. (2011) Collateral damage: Rapid exposure-induced evolution of pesticide resistance leads to increased susceptibility to parasites. Evolution. DOI: 10.1111/j.1558-5646.2011.01331.x
- May 10, 2011
- 09:05 AM
- 573 views
When does a beneficial mutation fail to benefit?
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Beneficial mutations, according to Hollywood, include the superpowered ability to make San Francisco Bay foggy. Photo via Comics Contiuum.Every time a cell divides is an opportunity for mutation, creating new genetic variation that may be beneficial, may be harmful, or may make no difference at all. In sexually reproducing species, the fate of a useful new mutation is relatively straightforward. If it overcomes the vicissitudes of genetic drift, the mutation will spread through the population as recombination swaps it into different genetic backgrounds, so that on average the mutation spreads or disappears on its own merits.
In asexual species, though, things are less straightforward. This is because new mutations are stuck with the genetic backgrounds in which they first appear—whether they spread of disappear depends not only on the fitness benefits they might provide, but on how beneficial the variation in the rest of the genome is, too. A new beneficial mutation in an asexual population is like a race car driver who can't change cars—she might be an ace at the wheel, but if she's stuck in a Yugo, she's probably not going to win.
So what happens to a new beneficial mutation in an asexual population is largely dependent on random factors: genetic drift and mutation. That randomness means that in order to know how new useful mutations behave in general, the only robust solution is to watch lots of new useful mutations in lots of otherwise identical populations.
In other words, it's a question best approached using experimental evolution. That brings us to a study just released in advance of print by the journal Genetics, in which a team headed by Greg Lang uses some clever methods to track the origin and fate of beneficial mutations in yeast.
The first clever thing about the project is that its authors knew in advance where to expect a beneficial mutation. Yeast cells reproduce both sexually and asexually—if the experimental populations are maintained in conditions that keep them reproducing asexually, mutations that turn off the costly cellular machinery necessary for sexual reproduction provide a measurable benefit.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Electron micrograph of budding yeast cells. Image from Microbe World.By using a strain of yeast engineered to produce fluorescent protein in the course of sexual reproduction, the authors could check for the presence of permanently asexual mutants by taking a sample from the population, prompting it to mate and measuring the sample's total fluorescence. Lower fluorescence would mean that more cells had lost the ability to reproduce sexually; if samples from a population were to become less and less fluorescent over time, the beneficial mutation would be spreading through the population.
Lang and his coauthors then set up the kind of experiment that you can only do with single-celled critters: they started 592 populations of yeast evolve for 1,000 generations of asexual reproduction. Each population started out from a single genetic strain, so differences between populations started from the same strain were purely due to differences in the random processes of mutation and drift. (The full experimental design used two different strains of yeast, and kept the population size at either 100,000 or 1,000,000 cells, for a total of four treatments.)
You might expect that the loss-of-sex mutation would reliably emerge and spread until it dominated each replicate population. In fact, that only occurred in a small fraction of the replicates. In many more cases, the loss-of-sex mutation showed up and started to spread, but was then overwhelmed by yeast that could still reproduce sexually—presumably because other, more beneficial mutations had arisen elsewhere in the population. This phenomenon, clonal interference, is widely expected to happen in competition among clonal strains.
What determined the success or failure of the loss-of-sex mutation? The authors found a considerable range of variation in the rate at which loss-of-sex strains increased in the experimental populations, suggesting that variation elsewhere in the genome contributed to the fitness of the yeast strain carrying the loss-of-sex mutation. Since every replicate population started as a genetically identical clone, that meant that mutations built up quite early in the course of experimental evolution. That variation corresponded to differences in the fitness of strains within the population—and the success or failure of the loss-of-sex mutation depended on whether it turned up in a strain that was already pretty fit to begin with.
Without recombination to mix up the genome, a beneficial mutation is bound to genetic variants at many, many other loci that may boost the benefits from that mutation, or cancel them out. In a clonal population, each genome succeeds or fails as a unit—a single useful mutation simply cannot do it alone.
References
Lang, G., Botstein, D., & Desai, M. (2011). Genetic variation and the fate of beneficial mutations in asexual populations. Genetics DOI: 10.1534/genetics.111.128942
Lang, G., Murray, A., & Botstein, D. (2009). The cost of gene expression underlies a fitness trade-off in yeast. Proc. Nat. Acad. Sciences USA, 106 (14), 5755-60 DOI: 10.1073/pnas.0901620106
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Lang, G., Botstein, D., & Desai, M. (2011) Genetic variation and the fate of beneficial mutations in asexual populations. Genetics. DOI: 10.1534/genetics.111.128942
Lang, G., Murray, A., & Botstein, D. (2009) The cost of gene expression underlies a fitness trade-off in yeast. Proc. Nat. Acad. Sciences USA, 106(14), 5755-60. DOI: 10.1073/pnas.0901620106
- May 3, 2011
- 09:05 AM
- 628 views
Released from predators, guppies reshape themselves—and their environment
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A (domestic) male guppy. Photo by gartenfreuden.Consider a population of guppies living in the Aripo River in Trinidad. They have a happy existence, as far as guppies can be happy, but their lives are shaped by the constant threat of larger, predatory fish. The river runs clear over a colorful gravel bed, and guppies who stand out against that background are eaten quickly. Even guppies whose coloration helps them blend in have to be ready to make a break for it if a predator shows up. All in all, a guppy's chances of surviving to mate depends most on its ability to hide from bigger fish, and to swim quickly when it can't hide.
Then one fine day a biologist comes along, scoops up a couple hundred guppies, and moves them to a pool in a tributary of the river. The pool is separated from the mainstream by a series of waterfalls, so larger fish can't swim up—the guppies are now free from their most dangerous predators. They can be fruitful and multiply. In this new habitat, camouflage and evasive maneuvers don't matter so much. What does matter is finding enough food to make some babies in the midst of a whole bunch of other guppies who are also not particularly worried about predators.
John Endler started the experiment I've just described back in 1976 to see whether guppies' coloration helps them hide from predators [PDF]. The guppies he moved to a predator-free stream have continued to evolve, though, and three decades later, new studies are showing how release from predators changed the guppies—and how those changed guppies could be changing the living community around them.
Since the 1976 introduction, Endler and other biologists have tracked the Aripo River guppies' response to the change in natural selection he created. Release from predators is considered one of the classic sources of ecological opportunity that can free a population to evolve new traits and behaviors, and explore new ways of making a living. At the same time, a sudden lack of predators means that competition within the population can become stronger.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Points of measurement for guppy body and head shape, illustrated on a stained specimen. Image from Palkovacs et al, fig. 1.In one study just published by PLoS ONE, Eric Palkovacs and two colleagues compared the body shape of guppies from the experimental population with guppies from the source stream. (Endler had noted changes in body shape along with changes in coloration in his original paper.) First, Palkovacs and his coauthors gauged how rapidly female guppies taken from each site snapped up standardized food. Then they killed the test fish, treated them with stain, and measured their body and head shape. Fish from the site with lower predation ate faster, and they had bigger mouths and deeper bodies than fish from the site with more predators.
Palkovacs and his coauthors also observed that the guppy populations at the experimental site were denser—without predators thinning them out, the fish are probably most limited by their food supply. A study published last year in PNAS suggests that this denser guppy population might reshape its own environment. The paper's authors created artificial ponds stocked with algae and small invertebrates, then introduced guppies from the high-predation source site or from the low-predation experimental site. They also controlled for the differences in guppy population density associated with predator pressure, maintaining the fish at either the high density observed with low predation, or the lower density observed with high predation.
Where the guppies came from made a significant difference in the artificial ecosystems, and these differences were in some cases exaggerated by the increased population density caused by predator release. Guppies from the "released" site ate less selectively than guppies from the site experiencing higher predation, who favored invertebrates over algae. As a result, guppies from the released site were associated with less algae growth, and higher invertebrate population density. Probably because they ate more plant matter, guppies from the released site also excreted less nitrogen, reducing the nutrient's availability for plant growth.
These results echo a study I discussed last year, which used a very similar approach to show that speciating sticklebacks can change their environment. It's another reminder that evolutionary change can feed back to change the environmental conditions that prompted change in the first place—that natural selection operates in the midst of continuous change.
References
Bassar, R., Marshall, M., Lopez-Sepulcre, A., Zandona, E., Auer, S., Travis, J., Pringle, C., Flecker, A., Thomas, S., Fraser, D., & Reznick, D. (2010). Local adaptation in Trinidadian guppies alters ecosystem processes. Proc. Nat. Acad. Sciences USA, 107 (8), 3616-21 DOI: 10.1073/pnas.0908023107
Endler, J. (1980). Natural selection on color patterns in Poecilia reticulata. Evolution, 34 (1), 76-91 DOI: 10.2307/2408316
Palkovacs, E., Wasserman, B., & Kinnison, M. (2011). Eco-evolutionary trophic dynamics: Loss of top predators drives trophic evolution and ecology of prey. PLoS ONE, 6 (4) DOI: 10.1371/journal.pone.0018879... Read more »
Bassar, R., Marshall, M., Lopez-Sepulcre, A., Zandona, E., Auer, S., Travis, J., Pringle, C., Flecker, A., Thomas, S., Fraser, D.... (2010) Local adaptation in Trinidadian guppies alters ecosystem processes. Proc. Nat. Acad. Sciences USA, 107(8), 3616-21. DOI: 10.1073/pnas.0908023107
Endler, J. (1980) Natural selection on color patterns in Poecilia reticulata. Evolution, 34(1), 76-91. DOI: 10.2307/2408316
Palkovacs, E., Wasserman, B., & Kinnison, M. (2011) Eco-evolutionary trophic dynamics: Loss of top predators drives trophic evolution and ecology of prey. PLoS ONE, 6(4). DOI: 10.1371/journal.pone.0018879
- April 26, 2011
- 09:05 AM
- 661 views
Deprived of pollinators, flowers evolve to do without
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Who needs pollinators? Not monkeyflowers—at least not after a few generations of evolution. Photo by Brewbooks.The loss of animal pollinators poses a potentially big problem for plants. However, many plant species that rely on animals to move pollen from anther to stigma have the capacity to make due if that service goes undone—and, as a new study released online early by the journal Evolution demonstrates, such plants can rapidly evolve to do without pollinators [$a] if they must.
The paper's authors, Sarah Bodbyl Roels and John Kelly, demonstrate this using a simple greenhouse experiment with the monkeyflower Mimulus guttatus, a wildflower native to western North America, and a member of a genus rapidly developing into a major model system for studying the evolution of ecological isolation and floral evolution.
Mimulus species vary in their reliance on animal pollinators—some grow minimalistic flowers, with the anther so close to the stigma that pollen transfers without any assistance. In natural populations, M. guttatus is usually pollinated by bees, but individual plants vary in the distance between anther and stigma, and this variation has a genetic basis. So a population of M. guttatus deprived of pollinators would have the raw material to evolve a solution—natural selection would favor plants that are better able to self-pollinate. As the population evolved to be more self-fertilizing, it might also evolve to look more like self-pollinating Mimulus species, losing the bright petals that attract pollinators.
To see whether this could actually happen, Bobdyl Roels and Kelly challenged an experimental population of Mimulus guttatus to do without pollinators, and tracked its response.
The authors raised seeds derived from a natural wild population of Mimulus guttatus in greenhouses under two trial conditions: control populations were provided with hives of bumblebees to pollinate them when their flowers were ready for servicing; and experimental populations were left to produce what seed they could without pollinators. The authors collected the seeds produced by each population, and planted them to form the next generation.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A bumblebee digs for nectar in flowers of Mimulus moschatus. Photo by Mollivan Jon.Early on in the experiment, the experimental populations deprived of pollinators fared badly. Without pollinators, the average plant produced two seeds or fewer by the end of the generation, compared to eight or ten seeds per plant in the population provided with bees. By the fifth generation, however, this was starting to improve—plants in both populations without pollinators were producing more seeds, and one of the two experimental populations produced nearly as many seeds as the control plants.
Examining the traits of plants produced by this final generation (actually, the grand-offspring of the fifth generation, to control for effects of inbreeding), the authors found that the average distance between the pollen-producing anther and the pollen-receiving stigma had shrunk significantly in plants from the experimental population. Across all the treatments, plants with a shorter distance between stigma and anther produced more self-pollinated seeds. There was no evolved change in other floral measurements, however—plants in the no-pollinators treatment had petals as big and showy as plants evolved with bumble bees.
In a natural population of Mimulus guttatus, the drop-off in seed production created by loss of pollinators should have much the same effect as in this experiment, creating a strong selective advantage for individual plants that can make more seeds on their own. The fact that the experimental plants did not evolve reduced petals could mean that in the cushy conditions of a greenhouse, there wasn't much need to stop spending resources making showy flowers. Or maybe, when the major source of natural selection is the need to make any seeds at all, selection to save resources on flower production is relatively weak and correspondingly slow-acting.
As the authors point out, one of many changes humans are making to natural communities around the world is to disrupt pollination relationships. In a sense, experiments like theirs are being carried out worldwide, on hundreds of plant species—and each species will adapt, or fail to adapt, in its own way.
Reference
Bodbyl Roels, S., & Kelly, J. (2011). Rapid evolution caused by pollinator loss in Mimulus guttatus. Evolution DOI: 10.1111/j.1558-5646.2011.01326.x
... Read more »
Bodbyl Roels, S., & Kelly, J. (2011) Rapid evolution caused by pollinator loss in Mimulus guttatus. Evolution. DOI: 10.1111/j.1558-5646.2011.01326.x
- April 5, 2011
- 09:05 AM
- 654 views
How can you tell if a plant is carnivorous? Feed it!
by Jeremy Yoder in Denim and Tweed
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A Venus flytrap closes on an unfortunate spider. Photo by cheesy42.Plants that eat animals offend our trophic sensibilities. Those of us who can move independently are supposed to eat those of us who can make sugar from sunlight—that's just the way the food chain works, right?
Well, not really. From a certain perspective, plants prey on animals all the time, using the sneaky strategy of just waiting us out—when we animals stop moving for good, we're fertilizer. And there are quite a few plants that aren't so patient. Venus flytraps, sundews, and pitcher plants have been recognized as carnivores since before Charles Darwin devoted a book to their ecology and anatomy. They all have structures—fly-trapping leaves, or sticky hairs, or deep pitfalls full of water—that are uniquely good at catching wayward insects. All of them also grow in particularly nutrient-poor soils, such as bogs, where the nitrogen from trapped insects makes a big difference.
The vast majority of plants lack either adaptations for trapping, or the same kind of need for nitrogen—they either don't grow where they can't get the stuff, or they hire symbiotic bacteria to help fix it. Yet there is a third category of plants, which are not exactly carnivorous, but which might just "eat" the occasional stray fly anyway. Many plants have hairy surfaces that can catch insects, or leaf structures that trap water and create pitfalls—and some of these plants can take advantage of the critters caught in these proto-traps.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Sticky purple geranium can trap insects on its sticky leaves, and seems to get some nutrition out of them. Photo by jby.One such plant is the sticky purple geranium (Geranium viscosissimum), which grows on dry Palouse hillsides around my current hometown of Moscow, Idaho. As its name implies, sticky purple geranium is sticky—its leaves are velvety with tiny glandular hairs, which leave a gummy residue on your hands if you brush against them. These hairs make it difficult for small insect herbivores to get to the leaves—but they also trap some of those insects.
Back in 1999, a biologist in my department at the University of Idaho, George Spomer (who left the department before my arrival), showed that sticky purple geranium leaves would digest a protein film pressed against them, somewhat like the leaves of a sundew. When Spomer placed protein labeled with carbon-14 on geranium leaves, he found elevated levels of carbon-14 elsewhere in the plant, suggesting that geranium leaves could absorb protein as well as digest it [$a].
Spomer demonstrated that the plants he studied could digest and absorb insects caught on their leaves, but his data can't tell us whether that ability is of any particular use to a geranium growing in a natural population—whether, that is, geraniums actually need the nutrients they might get from trapped insects. A more recent study of another possibly carnivorous plant gets closer to answering that question.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Water collected in the leaves of a teasel plant forms a death trap for insects, and a source of nitrogen for the plant. Photo by HermannFalkner/sokol.The plant in this second study is fuller's teasel, Dipsacus fullonum, a widespread European wildflower that has been introduced into North America. The leaves of many teasel plants form catchments (pictured above) that can collect water and form a makeshift pitfall, which catches and drowns small insects. It has been speculated that, like sticky purple geranium, fuller's teasel can absorb nutrients from these catchments full of rotting insect corpses. British biologists Peter Shaw and Kyle Shackleton set out to test this hypothesis not by tracking protein from trapped insects, but by determining whether teasel plants benefit from the trapping.
To do this, Shaw and Shackleton experimentally manipulated the number of insects trapped in the catchments formed by teasel plants' leaves. In one treatment, they watched experimental plants and removed insects as soon as they were trapped; in the other, they "fed" the experimental plants an extra bluebottle maggot at set intervals. They compared both treatments to a group of plants that were left un-manipulated as a control. The "fed" plants didn't necessarily grow bigger or produce more seeds, but they did produce more seeds as a proportion of their total biomass. That is, fuller's teasel plants that trap more insects can devote more of their resources to making seeds.
Does this make fuller's teasel carnivorous? Maybe, but probably not in the same sense that a Venus flytrap is. Teasels tend to grow in better soil than carnivorous plants do in general—they like open fields and stream banks, in my experience. Furthermore, we don't have any evidence that teasels actively attract insects, as most carnivorous plants do. On balance, it seems far more likely that what Shaw and Shackleton found is not carnivory as we usually know it, but plants making sure that a handy source of nitrogen doesn't go to waste.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Fuller's teasel relies on insects for pollination—but does it also rely on them for nutrition? Photo by gynti_46.
References
Darwin, C. (1875.) Insectivorous plants. Google Books link.
Shaw, P., & Shackleton, K. (2011). Carnivory in the teasel Dipsacus fullonum — The effect of experimental feeding on growth and seed set. PLoS ONE, 6 (3) DOI: 10.1371/journal.pone.0017935
... Read more »
Shaw, P., & Shackleton, K. (2011) Carnivory in the teasel Dipsacus fullonum — The effect of experimental feeding on growth and seed set. PLoS ONE, 6(3). DOI: 10.1371/journal.pone.0017935
Spomer, G. (1999) Evidence of protocarnivorous capabilities in Geranium viscosissimum and Potentilla arguta and other sticky plants. Int. J. Plant Sci., 160(1), 98-101. DOI: 10.1086/314109
- March 29, 2011
- 09:05 AM
- 699 views
Moths that pass in the night: Reproductive isolation due to pickiness, or just bad timing?
by Jeremy Yoder in Denim and Tweed
On a summer night in a Florida corn field, a female armyworm moth emerges from her underground cocoon and spreads her wings to dry in the humid air. Over the next few weeks, she will fly miles away in search of a mate, and a likely-looking patch of host plants on which to lay her eggs.
Her brief adult life will be shaped in many ways by the life she led as a larva, feeding on domestic corn. She could easily find other grasses to feed her offspring, but she'll probably seek out another cornfield. She may encounter armyworm males who were raised on many other grasses, but the odds are that the males she accepts as mates will also have grown up eating corn. This is so likely to be the case that it has left a mark on the genetics of her species [PDF].
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } At night in a cornfield, moths mate nonrandomly. Photo by K e v i n.Yet it isn't clear how much of this isolation between armyworms from corn (the "corn strain") and armyworms from other grasses (called the "rice strain") arises because moths from the different host plants actively prefer mates from their own larval food plant, or because they just don't encounter moths from the other food plants as frequently. Like many moths, armyworms of both sexes deploy pheromones to attract and woo mates—so maybe armyworms from the same food plant smell better to each other. On the other hand, corn-strain armyworms do more of their mate searching early in the evening (although they'll keep hunting all night), while rice-strain armyworms wait to search till the last few hours of nighttime.
Disentangling which of these two sources of isolation—preference versus timing—maintains the genetic differences between host plant strains of the armyworm takes some careful experimental work. As in many biological questions, the answer might well be not one or the other, but a little of both [$a].
In a study published in the latest issue of The American Naturalist, a team of entomologists at the Max Planck Institute for Chemical Ecology took on the question of what keeps the armyworm host strains separated. They performed two mating experiments with laboratory-reared moths of both sexes from both strains.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Fall armyworm adult. Photo via Wikimedia Commons.First was a "no-choice" experiment in which moths were kept in a chamber with a single member of the opposite sex from their own strain, or the other strain. The test was repeated over three nights in a row. On the first night, females from the corn strain were less likely to mate with males of the rice strain than males of their own, and when they did accept rice-strain males, it wasn't till later in the night. The second and third nights, though, corn-strain females mated about equally with males of both strains. Rice-strain females mated with males of both strains at about equal frequency all three nights, although they did so late in the night.
In the second round of experiments, moths were introduced into flight cages with one member of the opposite sex from each of the two host strains, so they could choose between them. To control for the differences in timing of mate searching between the two strains, the team repeated the experiment twice—in one version, the choosing moth had the entire length of the night to pick a mate, and in the other, the moths were only put into the same cage for the last four hours of the night, when the grass strain prefers to mate.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Fall armyworm larva. Photo by agrilifetoday.In the all-night experiment, corn-strain males and females were both more likely to choose a mate from their own strain than the other. Rice-strain moths of both sexes mated with moths of both strains about equally—but rice-strain females were less likely to choose any mate at all. On the other hand, when the research team waited till the end of the night to introduce the test moths to their possible mates, rice-strain moths of both sexes mated much more frequently overall, and rice-strain females strongly preferred rice-strain males. Corn-strain males were basically indiscriminate in the late-night experiment, and corn-strain females were also less choosy.
In short, when mating during their usual activity periods, females of both strains were choosy about their mates; but when offered mates at the wrong time, they didn't discriminate as much. The authors suggest that these mistimed matings were less discriminating because they were more likely to be initiated by the males, who showed relatively weak preferences even during their own usual mating times.
So the genetic differentiation between armyworm host strains is probably due to both timing and mate choice, and the two isolating factors affect males and females differently. Females, particularly rice-strain females, are quite picky about mating with a male of their own strain. Males, on the other hand, seem mainly to be prevented from pursuing females of the other strain by the fact that their respective schedules don't line up. As the study's authors conclude, all these individual rejections and missed connections, added up across entire armyworm populations, bring these moths a little bit closer to speciation.
References
Prowell, D., McMichael, M., & Silvain, J. (2004). Multilocus genetic analysis of host use, introgression, and speciation in host strains of fall armyworm (Lepidoptera: Noctuidae). Annals Entomol. Soc. America, 97 (5), 1034-44 DOI: 10.1603/0013-8746(2004)097[1034:MGAOHU]2.0.CO;2
Schöfl, G., Dill, A., Heckel, D., & Groot, A. (2011). Allochronic separation versus mate choice: Nonrandom patterns of mating between fall armyworm host strains. The American Naturalist, 177 (4), 470-85 DOI: 10.1086/658904... Read more »
Prowell, D., McMichael, M., & Silvain, J. (2004) Multilocus genetic analysis of host use, introgression, and speciation in host strains of fall armyworm (Lepidoptera: Noctuidae). Annals Entomol. Soc. America, 97(5), 1034-44. DOI: 10.1603/0013-8746(2004)097[1034:MGAOHU]2.0.CO;2
Schöfl, G., Dill, A., Heckel, D., & Groot, A. (2011) Allochronic separation versus mate choice: Nonrandom patterns of mating between fall armyworm host strains. The American Naturalist, 177(4), 470-85. DOI: 10.1086/658904
- March 28, 2011
- 12:40 PM
- 714 views
In which I try to explain why "heritability" is not quite the same thing as "heritable"
by Jeremy Yoder in Denim and Tweed
Robert Kurzban responds in the ongoing "adaptive" homophobia kerfuffle (henceforth, the O.A.H.K.) with continued confusion about how biologists identify possible adaptations and test to see whether natural selection is responsible for them. He notes that one effect of natural selection is to remove heritable variation in traits under selection, so that many traits which are probably adaptations—arguably, sometimes the best-adapted traits—actually have zero heritability.
This is true. But it's important to note that a trait having zero heritability, or no genetic variation, is not the same thing as that trait not being heritable, or having no genetic basis. If the trait has zero heritability, the observed variation in the trait may not be heritable, but the trait still may be. Kurzban's confusion over this distinction may be a fault of the terminology, as was pointed out to me in a couple independent conversations following the last round of the O.A.H.K.
That aside, reduced heritable variation in a trait—relative to appropriate standards for comparison, like other traits in the same species or the same trait in closely related species—is sometimes used to infer that selection has acted on that trait in the past. This is what my lab has done in the case of Joshua tree and its pollinators, which Kurzban cites. This sort of approach provides only indirect evidence of natural selection's activity—but it's often the best you can do when your focal species isn't amenable to growing in a lab or greenhouse within the span of a single grant cycle.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } The two varieties of Joshua tree, because apparently these are part of the discussion now. Photo by jby.The comparison to other traits or to other species is the critical point here. Without it, you can't determine whether a lack of genetic variation is due to strong selection, or due to the fact that there is no genetic basis for the trait. In isolation, the observation that there is no heritable variation for a single trait or behavior in a single species doesn't tell you much except that natural selection cannot currently be acting on the observed range of variation in that trait. If there's no genetic basis for the trait at all, then it cannot have been under selection in the past, either.
Forming hypotheses versus testing them
Regarding Kurzban's broader point about how biologists identify adaptations:
Futuyama’s textbook, which Yoder cites for the discussion of heritability, indicates the following: “Several methods are used to infer that a feature is an adaption for some particular function” (p. 261), and lists the criteria that evolutionary psychologists rely on, including complexity, evidence of design, experiments, and so on. From the material I quoted in my prior post, it seems to me that by indicating the two kinds of evidence that are necessary for inferring a feature is an adaptation, Yoder is rejecting Futuyama’s claim that one can infer adaptation from its form, complexity, and so on.
Here Kurzban is confusing how we initially infer that a trait or behavior might be an adaptation with how we actually demonstrate that a trait or behavior is an adaptation. Forming a hypothesis is not the same thing as testing it, as Jon Wilkins explained so well. If Kurzban is accurately representing evolutionary psychology's standards of evidence, then he's confirmed Wilkins's accusation that evo psych usually doesn't go beyond the step of forming a plausible hypothesis to collecting the data that can test it.
Demonstrating that an adaptive hypothesis is well supported by data is, as I've previously said, a lot of work—usually enough for more than one scientific article. Depending on what is easiest to do, building the case that a trait is an adaptation might start with a paper that merely demonstrates a trait's function—but that trait has not been conclusively shown to be an adaptation until we know that its demonstrated function is selectively important, and that the trait itself has a genetic basis.
While familiar to anyone who reads the evolutionary biology literature, this maybe isn't so obvious to non-biologists. This may be because popular science accounts don't always differentiate between hypotheses with good scientific support and those with none. Walk through a zoo or a natural history museum, and you'll read nothing but adaptive hypotheses all day—but you'll rarely see good, deep discussion of how well they're supported.
This is why, since I started graduate school, I've became rather tiresome company on trips to museums and zoos. But one of the great things about popular writing by working scientists (from my perspective as a scientist) is that it lets specialists explain exactly such finicky details of our fields directly to the public. Doing so clearly and accessibly is challenging, to be sure, but naïve, uncritical endorsements of unsupported hypotheses—about the adaptive values of human behavior, or anything else—are available in just about every major media outlet. If scientists don't do better than that in our own science communication, what value do we have to add to the discussion?
And now something new: relevant data
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Your reaction to this image might be in your genes, but the evidence is that it can change, too. Photo source unknown, presumed public domain.Which brings us back to evaluating Gordon Gallup's "adaptive" homophobia hypothesis. Kurzban also points to evidence (ye gads! data!) that natural selection actually could have something to work with in the case of attitudes towards homosexuals. A 2008 Australian twin study, which finds a genetic component of variation in responses to a questionnaire about attitudes towards homosexuality.
This is indeed, as Kurzban suggests, preliminary data in support of the idea that natural selection could operate on homophobia. As Neuroskeptic pointed out in the comments on my last O.A.H.K. post, it also means that natural selection could be operating on tolerance of homosexuals. It's an interesting and important question, actually, why the authors of that study chose to frame their results as showing the heritability of intolerance, rather than the heritability of tolerance.
However, as I noted all the way back at the beginning of the O.A.H.K., we also know that homophobic attitudes can change considerably over the course of an individual's lifetime. It's hard to say how survey responses taken at a single point in time relate to what natural selection would actually have to work with, if homophobic attitudes or lack thereof somehow shape an individual human being's expected reproductive fitness. Even if there is some solid genetic basis to homophobia, we still don't have data that can rigorously determine whether or how natural selection might act on that variation.
References
Gods... Read more »
Godsoe, W., Yoder, J.B., Smith, C.I., Drummond, C., & Pellmyr, O. (2010) Absence of population-level phenotype matching in an obligate pollination mutualism. Journal of Evolutionary Biology, 23(12), 2739-2746. DOI: 10.1111/j.1420-9101.2010.02120.x
Verweij, K., Shekar, S., Zietsch, B., Eaves, L., Bailey, J., Boomsma, D., & Martin, N. (2008) Genetic and environmental influences on individual differences in attitudes toward homosexuality: An Australian twin study. Behavioral Genetics, 38(3), 257-265. DOI: 10.1007/s10519-008-9200-9
- March 23, 2011
- 09:05 AM
- 649 views
In which several evolutionary psychologists still don't understand evolution
by Jeremy Yoder in Denim and Tweed
Jesse Bering has responded to criticism—by me, Jon Wilkins, and P.Z. Meyers, among others—of his post about Gordon Gallup's hypothesis that fear of homosexuals is favored by natural selection, in the form of an interview with Gallup. The result is informative, but probably not in the way intended.
To recap: Gallup proposed that homophobia could be adaptive if it prevented gay and lesbian adults from contacting a homophobic parent's children and—either through actual sexual abuse or some nebulous "influence," making those children homosexual. In support of this, he published some survey results [$a] showing that straight people were uncomfortable with adult homosexuals having contact with children.
I pointed out that all Gallup did was document the existence of a common stereotype about homosexuals—he presents no evidence that believing this stereotype can actually increase fitness via the mechanism he proposes, or that it is heritable.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Homophobia. And, um, everyone-else-phobia, too. Photo by yksin.So now Gallup and Bering have responded, although they have not, I think, improved their case. There's a lot for me to address here, so I'll try to break it up into sections, and follow the order of the interview.
In which Gordon Gallup is not a homophobe
In the response post, Gallup (and Bering, who contributes quite a lot to the argument in his role as interviewer) takes issue with the collective objections of working biologists, but manages not to actually address those objections. Bering starts the conversation on the moral high ground:
BERING: Let’s address the elephant in the room. It’s embarrassing for me to even ask this of you, since the answer is so obviously "no" to me. Is your theory a justification of your own homophobia?
GALLUP: A lot of people think that if a person has a theory it’s a window unto their soul. I have lots of theories. (See CV (pdf).) I have a theory of homophobia, I have a theory of homosexuality, and I have a theory of permanent breast enlargement in women, just to mention a few. So that would make me a homophobic, homosexual who is preoccupied with women’s breasts.
Neither I, nor any of the other critics I've seen have called Gallup a homophobe. He may be uniquely bad at understanding how societal homophobia nullifies his interpretation of his survey results, but that doesn't make him a homophobe. Thanks for clearing that up, though, guys.
Gallup then demonstrates that he either hasn't actually read any of the latest criticism, or has missed the point entirely:
... It is interesting how my critics tip-toe around the fact that my approach is based on a testable hypothesis, and how they go out of their way to side-step the fact that the data we’ve collected are consistent with the predictions. Whether it is politically incorrect or contrary to prevailing social dogma, is irrelevant. In science, knowing is preferable to not knowing. Minds are like parachutes, they only function when they’re open. If I were a homosexual, I’d want to know about these data.
I certainly didn't tiptoe around the testability of Gallup's hypothesis—I wrote that (1) the data he presented do not test his hypothesis, and (2) the data we do have regarding the probable fitness benefits of homophobia and its heritability contradict his hypothesis. I'm entirely prepared to revise my conclusions given new data, but Gallup doesn't have any.
In which at least one of us doesn't understand heritability
In his next question to Gallup, Bering accuses me of "bungling" the definition of heritability, linking to evolutionary psychologist Rob Kurzban, who says that my brief definition of heritable as "passed down from parent to child more-or-less intact" is wrong because heritability is actually "the extent to which differences among individuals are due to differences in genes."
Wow, dude. You are aware that what you just said means exactly the same thing as what I originally said, right?
Let's go to the textbooks that Kurzban says I'm contradicting. Here's the passage on heritability from Douglas Futuyma's gold-standard undergraduate textbook Evolution (page 209):
One way of detecting a genetic component of variation, and of estimating VG [trait variation attributable to genetic differences] and h2 [the proportion of total trait variation explained by genetic variation], is to measure correlations* between parents and offspring, or between other relatives. For example, suppose that in a population, the mean value of a character in the members of each brood of offspring was exactly equal to the value of that character averaged between their two parents (the MIDPARENT MEAN) (Figure 9.20A). So perfect a correlation clearly would imply a strong genetic basis for the trait. [Bold text and bracketed notes mine; otherwise sic.]
The asterisk in that quote leads to a footnote pointing out that regression, rather than correlation, is more typically used. This is the definition of heritability that I learned in my undergraduate and graduate courses. It's also the definition I've just helped teach to a class of third- and fourth-year undergraduate biology students in my capacity as a teaching assistant on a course in population biology.
In non-statistical terms (the kind I try to use on this blog), a positive regression between a parent's traits and those of their offspring means, in fact, that the parent's traits are passed on to their offspring, um, more-or-less intact.
Parent-offspring regression is widely used to estimate heritability [PDF], but you can also do similar analyses using trait measurements for siblings, or multiple generations on a pedigree. In all of these cases, known parental or sibling or familial relationships are proxies for genetic similarity—you can estimate heritability without knowing anything about specific genes. (In fact, sometimes biologists use genetic data to reconstruct pedigree relationships, then estimate heritability from the pedigree.) As implied in the quote from Futuyma's textbook, this approach is statistically equivalent to showing that there is a significant portion of trait (phenotype) variation explained by genetic variation—which is where Kurzban seems to have become confused.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Wild parsnip, mostly here to break up the wall of text. Photo by Bas Kers.Here's a specific example near and dear to my field of study, species interactions: To determine whether parsnip webworms could be under natural selection to resist nasty chemicals produced by their food plant, the wild parsnip, May Berenbaum and Arthur Zangerl estimated the genetic component of variation [$a] in the worms' capacity to choose food with lower levels of the toxins, and to tolerate the toxins they did eat. To do this, they raised webworm larvae of known parentage in the lab, and tested them on controlled diets. Their actual statistical analysis tested for an effect of the worms' sibling relationships (parentage) on their ability to avoid toxins and sur... Read more »
Arden, N., & Spector, T. (1997) Genetic influences on muscle strength, lean body mass, and bone mineral density: A twin study. Journal of Bone and Mineral Research, 12(12), 2076-2081. DOI: 10.1359/jbmr.1997.12.12.2076
Berenbaum, M., & Zangerl, A. (1992) Genetics of physiological and behavioral resistance to host furanocoumarins in the parsnip webworm. Evolution, 46(5), 1373-84. DOI: 10.2307/2409943
Young, K. (2004) How the horned lizard got its horns. Science, 304(5667), 65. DOI: 10.1126/science.1094790
Campbell, D. (1996) Evolution of floral traits in a hermaphroditic plant: Field measurements of heritabilities and genetic correlations. Evolution, 50(4), 1442-53. DOI: 10.2307/2410882
Gallup, G. (1995) Have attitudes toward homosexuals been shaped by natural selection?. Ethology and Sociobiology, 16(1), 53-70. DOI: 10.1016/0162-3095(94)00028-6
Mousseau, T., & Roff, D. (1987) Natural selection and the heritability of fitness components. Heredity, 59(2), 181-97. DOI: 10.1038/hdy.1987.113
- March 22, 2011
- 09:05 AM
- 684 views
Parasitism of a different color
by Jeremy Yoder in Denim and Tweed
The common cuckoo is such a lazy parent that brood parasitism—laying its eggs in the nests of other birds—is built into its biology.
No bird will willingly adopt cuckoo chicks, which usually out-compete, and sometimes kill, their adoptive siblings. Given any hint that one of the eggs in her nest isn't hers, a bird will eject the intruder. So cuckoos have evolved eggs that mimic the coloring of their hosts' eggs—dividing the species into "host races" that specialize on a single host species, and lay eggs that mimic that host's.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Cuckoo eggs (indicated by arrows) in the nests of three different host species. Illustration via The Knowledge Project.As you can see from this illustration, the match is often extremely good—the cuckoo egg is really only obvious when the hosts' eggs are visibly smaller. In fact, a new study by Mary Caswell Stoddard and Martin Stevens shows that a this matching is often even better than it looks to the human eye [$a].
Birds see the world differently than humans—where we have three kinds of color-sensitive cells in our eyes, they have four. This allows them to see colors in the ultraviolet range, which is invisible to us. Birds' eyes also have an additional class of sensory cell that may help them perceive and discriminate among textures. So to study the match between cuckoo and host eggs, Stoddard and Stevens first had to figure out what each egg looked like to a bird.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A reed warbler feeds the cuckoo chick that has taken over its nest. Photo via Wikimedia Commons.To do this, they developed a mathematical model of each host species' vision. The model estimated how similar two eggs should look to a bird, given raw data about what colors of light the eggs reflect and the specific colors the bird can detect. Using the model, Stoddard and Stevens could then calculate the "overlap" between the colors and patterning of a host egg and the egg of a cuckoo specializing on that host species.
Stoddard and Stevens then applied the vision model's measure of similarity to museum specimens of eggs from the cuckoo-parasitized nests of eleven European bird species. They found that cuckoo eggs matched their hosts' quite well overall, but the match was best for cuckoos specialized on especially vigilant hosts. For each host, the authors looked up studies of egg rejection behaviors to calculate the probability that each species would eject eggs that didn't look like their own. Species with higher ejection probabilities were parasitized by cuckoo host races whose eggs were better mimics.
That suggests host rejection behavior exerts strong natural selection on cuckoos, which makes sense given that successfully fooling a host is essential to cuckoo reproduction. In light of evidence that cuckoos can also exert selection on their hosts, it looks as though brood parasitism is a truly coevolutionary interaction between cuckoos and their hosts—one that can cause both to evolve greater diversity.
Reference
Stoddard, M., & Stevens, M. (2011). Avian vision and the evolution of egg color mimicry in the common cuckoo. Evolution DOI: 10.1111/j.1558-5646.2011.01262.x
... Read more »
Stoddard, M., & Stevens, M. (2011) Avian vision and the evolution of egg color mimicry in the common cuckoo. Evolution. DOI: 10.1111/j.1558-5646.2011.01262.x
- March 10, 2011
- 04:57 AM
- 533 views
An adaptive fairytale with no happy ending
by Jeremy Yoder in Denim and Tweed
The evolution of human traits and behaviors is, as I've noted before, a contentious and personal subject. This is enough of a problem when there's some data to inform the contentiousness. In the absence of meaningful data, it's downright dangerous.
Take, for instance, Jesse Bering's recent post about the evolution of homophobia, which Steve Silberman just pointed out to me.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A grim fairy tale indeed. Photo by K Wudrich.When evolutionary biologists say a trait or behavior is "adaptive," we mean that the trait or behavior is the way we see it now because natural selection has made it that way. That is, the trait or behavior is heritable, or passed down from parent to child more-or-less intact; and having it confers fitness benefits, or some probability of producing more offspring than folks who lack the trait. Lots of people, including some evolutionary biologists, speculate about the adaptive value of all sorts of traits—but in the absence of solid evidence for heritability or fitness benefits, such speculation tends to get derided as "adaptive storytelling."
Evolutionary biology wasn't always so rigorous, once upon a time. Then Stephen Jay Gould and Richard Lewontin buried adaptive storytelling under an avalanche of purple prose in their landmark 1979 essay "The Spandrels of San Marco" [PDF]. Norman Ellstrand made a similar point with better humor in a satirical 1983 article for the journal Evolution proposing adaptive explanations for why children always start life smaller than their parents [PDF]. Nowadays, when evolutionary biologists want to, say, argue that horned lizards' horns are an adaptation for defense against predators, they have to demonstrate the claimed fitness benefit [PDF].
Evolutionary psychologists, however, seem not to have gotten the memo.
Bering's post focuses on a series of studies by the evolutionary psychologist Gordon Gallup. Gallup was interested in the question of whether there might be an adaptive explanation for homophobia—which, given the fact that many (although far from all) human cultures treat homosexuality as a taboo—is a fair question for research. He hypothesized that treating homosexuality as taboo helped to prevent homosexual adults from contacting a homophobic parent's children, which would reduce, however slightly, the prospects of those children growing up to be homosexual, and ensure more grandchildren for the homophobe.
Gallup supported this adaptive hypothesis with ... evidence that straight people were uncomfortable about homosexuals coming into contact with children [$a]. Here's the opening sentence of that paper's abstract:In a series of four surveys administered either to college students or adults, reactions toward homosexuals were found to vary as a function of (1) the homosexual’s likelihood of having contact with children and (2) the reproductive status (either real or imagined) of the respondent.If you've noticed that this doesn't mention evidence of heritability or a fitness benefit to homophobia, that's not because I left it out—that's because Gallup's work contains no data to support either.
What this amounts to is arguing that homophobia is an adaptation favored by natural selection because homophobia is a thing that exists.
Could a complex behavior like homophobia have a genetic basis? Sure. Homosexuality itself is a complex behavior, and it certainly does have some genetic basis. However, the fact that attitudes toward homosexuality have shifted as far and as fast as they have in the last few decades suggests that any genetic effects underlying homophobia are pretty easy to overcome. Behaviors can be inherited culturally, too, since human children learn from their parents. But—note, again, lots of change in the last thirty years or so—cultural inheritance is more fleeting and malleable than biological inheritance.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Careful, Red Riding Hood—that wolf might be gay. Photo by crackdog.What about Gallup's proposed fitness benefit for homophobes? Well, that would require homophobia to, you know, actually prevent homosexuality. Gallup's argument there hangs on two distasteful assertions. First, that gay men are more likely to be pedophiles, and second, that boys sexually abused by gay men are themselves more likely to grow up gay. In spite of Gallup's assertions otherwise [$a], we have strong evidence from multiple studies that gay men are no more likely to be sexually attracted to children than straight men.
And there is, to my knowledge, no evidence to suggest that abuse can cause homosexuality. Bering cites a recent study that does document an association between childhood abuse and later homosexuality in men. However, the study's authors point out that, "The reason for the connection between childhood sexual abuse and same-sex partnerships among men is not clear from our findings."... gay men tend, on average, to be more gender non-conforming as boys (Bailey & Zucker, 1995). This tendency could increase their appeal or conspicuousness to sexual predators, which might make them more likely to be victims of abuse (B. Mustanski, personal communication, February 11, 2008). Similarly, it is possible that boys who are developing and exploring a same-sex sexual orientation are more likely to enter situations where they are at risk for being sexually abused (Holmes & Slap, 1998). [In-text citations sic]Why on Earth would Bering dredge up Gallup's adaptive fairytale a decade and a half after it was published, if it was baseless to begin with and no new evidence supports it? Well, according to Bering, because he's a hard-nosed scientist who isn't afraid to consider uncomfortable possibilities.Sometimes, science can be exceedingly rude—unpalatable, even. The rare batch of data, especially from the psychological sciences, can abruptly expose a society’s hypocrisies and capital delusions, all the ugly little seams in a culturally valued fable. I have always had a special affection for those scientists like Gallup who, in investigating highly charged subject matter, operate without curtseying to the court of public opinion.Of course, says Bering, Gallup's work isn't conclusive, but it sure would be interesting if someone tested it.
Except, when Gallup was forming his hypotheses about the evolutionary benefits of gay-hating—he first proposed the idea in a 1983 article—he was hardly thumbing his nose at public opinion. He was, in fact, giving natural selection's approval to the prevailing ugly stereotypes about gay men. And, as any competent evolutionary biologist would recognize, he did it without a shred of relevant evidence.
References
... Read more »
Ellstrand, N. (1983) Why are juveniles smaller than their parents?. Evolution, 37(5), 1091-4. DOI: 10.2307/2408423
Gallup GG Jr, & Suarez SD. (1983) Homosexuality as a by-product of selection for optimal heterosexual strategies. Perspectives in Biology and Medicine, 26(2), 315-22. PMID: 6844119
Gallup, G. (1995) Have attitudes toward homosexuals been shaped by natural selection?. Ethology and Sociobiology, 16(1), 53-70. DOI: 10.1016/0162-3095(94)00028-6
Gallup, G. (1996) Attitudes toward homosexuals and evolutionary theory: The role of evidence. Ethology and Sociobiology, 17(4), 281-284. DOI: 10.1016/0162-3095(96)00042-8
Gould, S., & Lewontin, R. (1979) The Spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proc. Royal Soc. B, 205(1161), 581-98. DOI: 10.1098/rspb.1979.0086
Wilson, H., & Widom, C. (2010) Does physical abuse, sexual abuse, or neglect in childhood increase the likelihood of same-sex sexual relationships and cohabitation? A prospective 30-year follow-up. Archives of Sexual Behavior, 39(1), 63-74. DOI: 10.1007/s10508-008-9449-3
Young, K., Brodie, E.D., Jr., & Brodie, E.D., III. (2004) How the horned lizard got its horns. Science, 304(5667), 65. DOI: 10.1126/science.1094790
- March 8, 2011
- 09:05 AM
- 735 views
One snout to rule them all: Does migrating help weevils win the arms race of coevolution?
by Jeremy Yoder in Denim and Tweed
Natural selection and gene flow have a sort of love-hate relationship. Natural selection acts, on average, to make a population better fit to its environment. Gene flow, the movement of individuals and their genes, can counter the optimizing effect of selection if it introduces less-fit individuals from somewhere a different environment. On the other hand, not all new immigrants are necessarily less fit—sometimes they're better suited to their new environment than the locals.
This gets more complicated, and more interesting, when the environment in question is another living species. Then, the question is not just how movement of one species changes its response to natural selection, but how movement of the other species changes the nature of that natural selection. That's the focus of the latest study of a Japanese weevil species and its favorite food plant. The two species are locked in a coevolutionary arms race—but who wins the arms race in any given location depends on the gene flow each species is receiving from elsewhere [$a].
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Male and female camelia weevils, caught at an indelicate moment. Evidently he doesn't find her much longer rostrum intimidating. Photo from Toju et al. (2011), figure 1.These are camelia weevils, Curculio camelliae. As their name suggests, they like to eat camelias, at least when they're young. Specifically, weevil larvae eat camelia seeds, which are protected by a thick layer called a pericarp. To deal with camelia pericarps, the weevils have evolved prodigious proboscises, or rostrums, which female weevils use to drill through the pericarp so they can lay their eggs inside. Note that the female in the picture above is the one with the rostrum longer than the rest of her body.
Camelias can reduce their risk of losing seeds to weevil larvae by evolving thicker pericarps; weevils can make sure they're able to feed their young by evolving longer rostrums. Both species are constrained by costs, though—the cost of producing more pericarp tissue, or carrying around a Pinocchio-grade snout. These costs vary somewhat with climate—camelias grow thinner pericarps in cooler conditions [$a]. This means the arms race won't proceed equally far in all camelia populations, and introduces the possibility that the way in which weevils and camelias (well, camelia seeds and pollen) move across the landscape may very well determine which species has the upper hand.
.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A female weevil drills into a camelia fruit. Photo from Toju et al. (2011), figure 1.The new study sets out to see whether gene flow among populations of the two species determines how far the arms race proceeds in each population. Rather than directly track weevils and camelia seeds, the authors use genetic markers for each species—the more migrants move between two weevil (or camelia) populations, the more similar those two populations' genetics will be. The populations in question were seven sites on a small island at the south end of the Japanese archipelago, and presumably relatively free from the influence of immigration from the larger islands.
It looks like the movement of weevils, but not camelias, affects how the arms race proceeds. As the genetic difference between weevils at two different sites increased, the difference in how far the arms race had proceeded—that is, how long the local rostrums were, and how thick the local pericarps—increased too. That suggests weevils may be prevented from evolving rostrums of the optimum length for their local camelias by the arrival of less-than-optimal migrants. On the other hand, there was no statistically significant relationship between the genetic similarity of camelia populations and their place in the arms race.
This is where the relationship between selection and gene flow gets complicated, though. Even given the relationship between weevil gene flow and how far the arms race seems to have proceeded, the genetic differences between weevil populations were consistent with very low actual rates of migration. A female weevil arriving in a population of camelias with pericarps too long for her rostrum isn't going to contribute many offspring to the next generation of weevils at that site. So it's not impossible that what we're seeing is selection constricting gene flow rather than gene flow slowing down selection.
Alternatively, weevils from a population with super-long rostrums should be able to lay eggs in any population of camelias they meet. In fact, an analysis that uses the genetic data to estimate rates of immigration and emigration suggests that one of the weevil populations with the longest snouts contributes more migrants to the other sites than it receives from each of them. In arms-race coevolution, size is all that matters—and so the weevils with the longest snouts may be winners no matter where they go.
Reference
Toju, H. (2008). Fine-scale local adaptation of weevil mouthpart length and camellia pericarp thickness: Altitudinal gradient of a putative arms race. Evolution, 62 (5), 1086-102 DOI: 10.1111/j.1558-5646.2008.00341.x
Toju, H., Ueno, S., Taniguchi, F., & Sota, T. (2011). Metapopulation structure of a seed-predator weevil and its host plant in arms race coevolution. Evolution DOI: 10.1111/j.1558-5646.2011.01243.x
... Read more »
Toju, H. (2008) Fine-scale local adaptation of weevil mouthpart length and camellia pericarp thickness: Altitudinal gradient of a putative arms race. Evolution, 62(5), 1086-102. DOI: 10.1111/j.1558-5646.2008.00341.x
Toju, H., Ueno, S., Taniguchi, F., & Sota, T. (2011) Metapopulation structure of a seed-predator weevil and its host plant in arms race coevolution. Evolution. DOI: 10.1111/j.1558-5646.2011.01243.x
- March 1, 2011
- 09:05 AM
- 867 views
Pollinating birds leave plants in the lurch
by Jeremy Yoder in Denim and Tweed
Plants' ancient relationship with animal pollinators is pretty crazy, when you think about it. On the one hand, it gives plants access to mates they can't go find on their own, and it's more efficient than making scads of pollen and hoping the wind blows some onto another member of your species. On the other hand, it can leave a plant totally dependent upon another species for its reproduction.
This catch is probably why lots of plants still use wind pollination strategies, or reserve the option to pollinate themselves if animals don't do the job for them. Avoiding complete dependence on animal pollinators is likely to become more important in the modern era, as human disruption of the environment amplifies the inherent risk of entrusting your reproduction to another species [$a], a study in the latest issue of Science shows.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A flower of Rhabdothamnus solandri, waiting for pollinators who may never show up. Photo by Tonyfoster.Sandra Anderson and her coauthors examined the health of populations of Rhabdothamnus solandri, a forest shrub native to the North Island of New Zealand. The flowers of R. solandri are classic examples of the pollination syndrome associated with birds—bright red-orange, with long nectar tubes. Rhabdothamnus solandri is incapable of self-pollinating, because its The flowers attract three native bird species, the tui, the bellbird, and the stitchbird. Thanks to human activity, all three of these birds "functionally extinct" in most of the range where R. solandri grows.
The bellbird and the stitchbird were eliminated from much of the North Island in the Nineteenth Century as European colonists cleared forests for farmland and introduced cats, rats, and dogs that preyed on the native fauna. Tuis have persisted, but tend to stay in the upper forest canopy—possibly to avoid rat predation—and don't visit lower-growing shrubs. However, all three birds are still living as they did before Europeans arrived on two island nature preserves just a few kilometers off the North Island's shores. This creates an inadvertent experiment in pollinator loss, allowing Anderson et al. to compare R. solandri populations on the mainland with those on the preserve islands to see how the plant gets on without its pollinators.
The short answer is: not well.
.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } The three principle pollinators of R. solandri, the tui, the bellbird, and the stitchbird. Only the Tui is still common in most R. solandri habitat. Photos by kookr, angrysunbird, and digitaltrails.To test whether R. solandri's reproduction is limited by pollen supply (as opposed to water or nutrients), the authors compared flowers that were either enclosed to prevent pollinator access, left open to natural pollination, or pollinated artificially. On the islands, plants left open set about as much fruit as plants pollinated by hand—but on the mainland, plants pollinated by hand set much more fruit than those left open. Mainland plants also produced smaller fruits, with fewer seeds per fruit, than island plants. The enclosed flowers set very little fruit, so it seems clear that pollen is the limiting factor for island and mainland R. solandri populations, and mainland populations aren't getting enough.
The age structure of island and mainland R. solandri populations bears this out. Anderson et al. surveyed the island and mainland sites and counted the number of "adult" shrubs in a given area relative to recently sprouted seedlings. Island and mainland sites had similar densities of adult shrubs, but mainland sites had much lower densities of seedlings. It looks very likely that R. solandri populations on the North Island mainland are in decline as a direct result of losing pollinator services.
As Cagan Sekercioglu points out in an invited commentary [$a], this study demonstrates that species' ecological roles can be strongly compromised even if they don't go extinct. Tuis and bellbirds are not considered particularly endangered, and the stitchbird is classified as "vulnerable," the lowest level of "threatened" under the system used by the International Union for the Conservation of Nature. Yet these birds' local losses and adaptation to human activity have left R. solandri without adequate pollination services. Conserving biodiversity requires more than preventing extinction—but it can be quite a bit harder to preserve important relationships between species such as this one.
References
Anderson, S., Kelly, D., Ladley, J., Molloy, S., & Terry, J. (2011). Cascading effects of bird functional extinction reduce pollination and plant density. Science, 331 (6020), 1068-1071 DOI: 10.1126/science.1199092
Sekercioglu, C. (2011). Functional extinctions of bird pollinators cause plant declines. Science, 331 (6020), 1019-20 DOI: 10.1126/science.1202389
... Read more »
Anderson, S., Kelly, D., Ladley, J., Molloy, S., & Terry, J. (2011) Cascading effects of bird functional extinction reduce pollination and plant density. Science, 331(6020), 1068-1071. DOI: 10.1126/science.1199092
Sekercioglu, C. (2011) Functional extinctions of bird pollinators cause plant declines. Science, 331(6020), 1019-20. DOI: 10.1126/science.1202389
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