Scientists love it when the real world validates our more theoretical predictions. It helps, of course, if those predictions are rooted in the real world to begin with. This is more or less what happened in my own research, with results reported in two just-published scientific papers. In the first, which I discussed last week, my coauthor and I showed that some kinds of species interactions can reduce the diversity of the interacting species [PDF]. Today, I’m turning to the second, in which my coauthors and I found exactly this predicted pattern in one such species interaction, the pollination mutualism between Joshua tree and yucca moths.
The new paper, published this month in the Journal of Evolutionary Biology, examines the phenotypic variation of two forms of Joshua tree and the two different moth species that pollinate it. The data show that although the Joshua trees pollinated by different moths are very different from each other, those pollinated by the same moth species are extremely similar [PDF].
Two forms of Joshua tree pollinated by different moth species, seen here side by side, don’t vary much among themselves. (Flickr: jby)
This is a nice confirmation of the theory paper because it strongly suggests that coevolution between mutualists like Joshua tree and its pollinators works the way the theoretical model assumes it does, with natural selection favoring individuals who best match their partners in the other species.
Biologists have long thought that coevolutionary interactions between species help to generate greater biological diversity. This idea goes all the way back to The Origin of Species, in which Darwin proposed that natural selection generated by competition for resources helped cause species to diverge over time:
Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking at the inhabitants of any small spot or at naturalised productions.
—Darwin (1859), page 128.
In the twentieth century, this idea was extended into suggestions that coevolution between plants and herbivores or flowers and pollinators helped to generate the tremendous diversity of flowering plants we see today. In general, biologists have found that strong coevolutionary interactions are indeed associated with greater diversity.
Species interact in a lot of different ways, as antagonists, competitors, and mutualists. Do all these interactions shape diversity the same way? (Flickr: jby)
One step toward determining how often coevolution promotes diversification would be to identify what kinds of coevolutionary interaction are more likely to generate diversity. This is precisely the goal of a paper I’ve just published with Scott Nuismer in this month’s issue of The American Naturalist. In it, we present a single mathematical model that compares a wide range of species interactions to see how they shape diversification, and that model shows that coevolution doesn’t always promote diversity [PDF].
Correction, 22 December 2010: Vincent Lynch, author of the second paper discussed in this post, notes in the comments that he didn’t actually conclude that female orgasm was an adaptation. I’ve corrected the post accordingly.
Whether or not a trait is an adaptation, shaped by natural selection for a specific function, can be a surprisingly contentious question in evolutionary biology. When the trait in question belongs to human beings, though, “contentious” reaches a whole new level—because when evolutionary biologists consider humans, their conclusions get personal.
Among the myriad traits and behaviors of Homo sapiens evolutionary biologists might choose to study, few can be as personal as the female orgasm. The adaptive function of male orgasm is about as clear-cut as possible—it’s a mechanistic necessity for uniting a sperm with an egg. But while female orgasm is enjoyable (or so I am told; this is as lousy a point as any to admit that my expertise in this phenomenon is purely academic), it isn’t necessary for fertilization. No man can be a father without having had at least one orgasm, but a woman could conceivably give birth to a huge family without having any.
To explain the existence of female orgasm in an evolutionary context, then, biologists have two options: (1) discover a way in which female orgasm shapes reproductive success indirectly, or (2) conclude that female orgasm isn’t an adaptation. Possibilities advanced for the first option range from the benefits of closer bonding with a mate—sex is, after all, about more than mere reproduction—to suppositions that the contractions associated with orgasm help draw semen into a woman’s reproductive tract.
The argument in support of non-adaptive female orgasm takes a developmental perspective: that female orgasm is really male orgasm, as experienced in a female developmental context. That is, women have orgasms for the same reason men have nipples—because the anatomies of both sexes are constrained by their origins in the same underlying developmental program. If this is the case, natural selection would work to optimize male orgasm, without necessarily affecting female orgasm—and that suggests a way to test whether female orgasm is an adaptation.
Natural selection removes less-fit versions of traits from a population—making that trait less variable within the population under selection. Traits that don’t affect survival or reproductive success, on the other hand, are free to accumulate variation via mutation. So non-adaptive traits can be identified by comparing their variation to traits with known adaptive functions.
Who cares what natural selection thinks, anyway? Photo by JorgeMiente.es.
Psychologist Kim Wallen and philosopher of science Elisabeth Lloyd (who had advanced the hypothesis that female orgasm is non-adaptive in a 2005 book) made just such a comparison in a 2008 study. Variation in female orgasm would be challenging to measure, so they used the clitoris as an anatomic proxy. This let them use the penis—which shares a developmental origin with the clitoris and is presumably under natural selection associated with male sexual function—as an adaptive standard for comparison. In comparison to (flaccid) penis length, Wallen and Lloyd found that clitoris length was indeed more variable [$a]. As a second control, the authors also compared variation in clitoris and penis length to variation in the length of women’s vaginas, understanding that this trait, unlike the clitoris, is important for female reproductive success. Vaginal length turned out to be about as variable as penis length, and much less so than clitoris length.
There are several objections to be made to Wallen and Lloyd’s analysis, and many were made in a response [$a] by evolutionary biologist Vincent Lynch. Lynch objected to the use of length as the focal measure for the size of the clitoris, and showed that clitoral volume was about as variable as penile volume. (I would add that the study of social insects Wallen and Lloyd cite as a precedent for their analysis isn’t actually focused on variation, but on the symmetry of traits under consideration, which is not quite the same thing.) More critically, though, Lynch points out that there isn’t any known relationship between clitoral size and ability to achieve orgasm—so the data don’t have the bearing on the question that Wallen and Lloyd assigned in the first place. Lynch concluded that female orgasm is an adaptation after all—a more conservative interpretation of his result is that we can’t answer the question by measuring clitorises.
Understanding the evolution of human sexual behaviors can help us to figure out how best to navigate the tricky business of a sexual relationship with another person—an approach most recently exemplified in the book Sex at Dawn. But we also tend to view evidence that natural selection favors a particular trait or behavior as a kind of approval, or as evidence of what is “natural.” That’s silly. Whether or not they help to make more babies, orgasms are fun, and they’re a wonderful part of our most intimate expression of affection and love. In some respects, that’s all we need to know.
References
Crespi, B., & Vanderkist, B. (1997). Fluctuating asymmetry in vestigial and functional traits of a haplodiploid insect. Heredity, 79 (6), 624-30 DOI: 10.1038/hdy.1997.208
Lynch, V. (2008). Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm. Evolution & Development, 10 (4), 396-7 DOI: 10.1111/j.1525-142X.2008.00248.x
Wallen K, & Lloyd EA (2008). Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evolution & development, 10 (1), 1-2 DOI: 10.1111/j.1525-142X.2007.00207.x
Species interactions are probablypretty important, in the evolution of life. There are all sorts of studies showing that the fitness and evolutionary history of individual species depends upon interactions with pollinators, symbiotes, foodplants, herbivores, parasites, predators, and competitors. Most of these studies focus in on a single interaction—but what living thing interacts with only one other organism? Coevolution, when it happens, happens in a community context.
Adding even a second interaction into the scientific picture can be difficult, but it may also dramatically change the evolutionary outcome, as seen in a new study of evolution in the protozoan communities living in purple pitcher plants. Individually, competitors and predators are significant agents of natural selection—but together, they seem to counterbalance each other [$a].
The purple pitcher plant, Sarracenia purpurea. Photo by petrichor.
Carnivorous pitcher plants grow funnel-shaped leaves that collect water to form a pitfall trap for hapless insects, which provide a source of nitrogen in swampy, nutrient-poor habitats. One species’ pitfall is another’s ideal habitat, however, and pitchers also play host to diverse micro-communities [PDF] of protozoans, bacteria, and even mosquito larvae. By recreating—and experimentally manipulating—these communities in the laboratory, the new study’s author, Casey terHorst, was able to disentangle the individual and combined effects of two different kinds of species interaction within pitcher plant pitfalls.
TerHorst focused on a protozoan species in the genus Colpoda, a widespread single-celled critter found in moist soil and standing water. In pitcher plants, Colpoda makes a living feeding on bacteria that break down insects trapped by the pitfall—and they themselves are prey for the larvae of the mosquito Wyeomyia smithii.
An example of genus Colpoda, the group of ciliates studied (but probably not the same species). Photo by PROYECTO AGUA** /** WATER PROJECT.
To determine the individual and combined effects of competition and predation on Colpoda, terHorst allowed experimental populations of the protozoan to evolve for 20 days (about 60-120 Colpoda generations) with either (1) no competitors or predators, (2) competition from another bacteria-eating protozoan, (3) predation by mosquito larvae, or (4) competition and predation. At the end of the experimental period, he sampled each evolved Colpoda population and measured a number of traits, including the size of Colpoda cells and their speed. Larger Colpoda cells are thought to be better competitors but more vulnerable to predators; faster ones should be better able to evade predation.
Individually, predators and competitors had significant effects on Colpoda evolution. In the presence of mosquito larvae, Colpoda evolved smaller, faster cells than it did alone. Unexpectedly, competitors also caused Colpoda to evolve smaller cells, though not faster ones. TerHorst suggests that this is because competition also favored more rapid reproduction by Colpoda, which meant that individual cells grew less before dividing.
Most interestingly, though, Colpoda evolving in the presence of both predators and competitors looked quite a lot like Colpoda that evolved alone. This is apparently because the mosquito larvae ate both Colpoda and its competitor—the mosquitoes acted to relieve some competitive pressure on Colpoda at the same time they ate fewer Colpoda because they had two prey species to pursue. In fact, the mosquitoes preferred to eat the competitor species, since it tended to hang out in the open while Colpoda hid among the plastic beads lining the base of the artificial habitat.
Thus the indirect effects of the predator offsetting competition, and of the competitor drawing away predation, canceled out the natural selection each imposed on Colpoda individually. Species interactions in a community context, even a simple one like this, are far from straightforward.
References
Buckley, H., Burns, J., Kneitel, J., Walters, E., Munguia, P., & Miller, E. (2004). Small-scale patterns in community structure of Sarracenia purpurea inquilines. Community Ecology, 5 (2), 181-8 DOI: 10.1556/ComEc.5.2004.2.6
terHorst, C. (2010). Evolution in response to direct and indirect ecological effects in pitcher plant inquiline communities. The American Naturalist, 176 (6), 675-85 DOI: 10.1086/657047
The 20th edition of the Carnival of Evolution is now online at Byte Size Biology—where the compilation of evolution-themed online writing is given a sporting spin. Check it out!
Using specific compounds to cure disease seems like a fairly advanced behavior—it’s necessary to recognize that you’re sick, then know what to take to cure yourself, then go out and find it. You might be surprised to learn, then, that one of the best examples of self-medication behavior in a non-human animal isn’t another primate species, or even another vertebrate. It’s none other than monarch butterflies. Female monarchs infected with a particular parasite prefer to lay eggs on host plants that help their offspring resist the parasite [PDF].
Most natural monarch butterfly populations are infected, at varying rates, with the protozoan parasite Ophryocystis elektroscirrha. Monarch larvae become infected when they eat parasite spores laying on the leaves of their food plants; the parasites reproduce inside the growing larvae form more spores while the larvae undergoes metamorphosis. Infected adults emerge from their chrysalises covered in O. elektroscirrha spores, which they spread to their mates and to their own offspring.
Monarch caterpillars are well-known to eat milkweeds, which defend themselves by producing organic compounds in a class called cardenolides—literally “heart poisons.” These deter lots of insect herbivores, but monarch caterpillars have evolved physiological mechanisms to store up cardenolides without suffering ill effects, which in turn makes each caterpillar, and its later adult phase, toxic to predators. (Lots of specialist herbivores evolve tolerances to, or even preferences for, their host plants’ defensive chemistry.)
It turns out that cardenolides are also bad for monarchs’ parasites. In an experiment published in 2008, de Roode et al. raised monarch caterpillars on two milkweed species that produced differing amounts of cardenolides, Asclepias curassavica and A. incarnata. They found that infected caterpillars fed the more toxic A. curassavicasuffered fewer ill effects of infection [PDF].
This result is remarkable enough on its own. It suggests that the effects of infection by Ophryocystis elektroscirrha might vary in natural monarch populations depending on something separate from the monarch-parasite interaction itself—the toxicity of the locally-available milkweed species. But what if monarchs could choose more toxic milkweed to fight infection?
This possibility of self-medication by monarchs is the focus of the latest result in the monarch-parasite system. In the new study, a team of researchers at Emory University and the University of Michigan offered infected and uninfected monarch caterpillars leaf cuttings from both of the milkweed species used in the 2008 experiment. However, infected caterpillars showed no greater preference for the more toxic milkweed.
Caterpillars might not be well-suited to self-medication anyway; they’re not very mobile, and so stuck with the host plant patch in which they hatch. Adult female monarchs, on the other hand, can fly—and seek out a patch of parasite-fighting plants on which to lay their eggs. In a second experiment, the team offered infected and uninfected adult females the opportunity to lay eggs on a single plant of each milkweed species, placed at opposite ends of a flight cage. And, indeed, infected female monarchs looked out for the best interest of their offspring, laying a larger proportion of their eggs on the more toxic plant.
This sort of trans-generational self-medication raises some very interesting questions, particularly, how do infected monarchs know they’re infected? How does local diversity of milkweed species in natural populations alter the coevolution of monarchs with Ophryocystis elektroscirrha? There’s still a lot to learn about this fascinating behavior, which may be happening in backyards across North America.
To conclude, here’s a great video produced by Emory University, in which Principal Investigator Jaap de Roode talks about monarchs in general, and the new discovery in particular.
References
Bradley, C., & Altizer, S. (2005). Parasites hinder monarch butterfly flight: implications for disease spread in migratory hosts. Ecology Letters, 8 (3), 290-300 DOI: 10.1111/j.1461-0248.2005.00722.x
de Roode, J., Pedersen, A., Hunter, M., & Altizer, S. (2008). Host plant species affects virulence in monarch butterfly parasites. Journal of Animal Ecology, 77 (1), 120-6 DOI: 10.1111/j.1365-2656.2007.01305.x
de Roode, J., Yates, A., & Altizer, S. (2008). Virulence-transmission trade-offs and population divergence in virulence in a naturally occurring butterfly parasite. Proceedings of the National Academy of Sciences, 105 (21), 7489-94 DOI: 10.1073/pnas.0710909105
Lefèvre, T., Oliver, L., Hunter, M., & De Roode, J. (2010). Evidence for trans-generational medication in nature. Ecology Letters DOI: 10.1111/j.1461-0248.2010.01537.x
Image borrowed from Wikipedia under fair use rationale.
My reasons aren’t going to surprise anyone who has even a passing familiarity with gay rights history:
Familiarity breeds acceptance. This is mainly a political argument. It’s widely accepted (and supported by ongoing public opinion surveying) that people who personally know GLBT folks are overwhelmingly more likely to support treating GLBT people like full citizens. The psychology isn’t hard to understand—it’s easy to hate the nebulous, faceless, unknown Gays; it’s rather harder to hate your son, or your niece, the nice neighbors who let you borrow their lawnmower, or (I hope) the guy who writes that one not-entirely-terrible science blog you check every so often.
Gotta give’em hope. And an example. This is more personal. I grew up without knowing any out gay people, which was, to put it mildly, not helpful. I was, to paraphrase the Onion headline, The Only Homosexual in the World; I didn’t have any of the support, or visible examples, that would’ve helped me think critically about my sexual orientation or imagine a future in which I was out, and happy about it. (Which I very much am, these days.) By being open about my orientation, maybe I can help someone else figure out his (or hers) in a way I couldn’t, and even show that, as confusing and frequently miserable as growing up gay is, it gets better.
And if there’s one impression I hope to give a confused, lonely (and presumably nerdy) gay kid reading D&T, it’s that it did get better for this formerly confused, lonely (and unquestionably nerdy) gay kid. And a large part of how it got better, for me, has to do with going into science. Evolutionary biology has turned out to be a good field for me, in this personal respect. When I started my first genuine biology-related internship, I was surrounded for the first time by people who didn’t talk about gays in the hushed, scandalized tones I’d heard through a lot of my childhood and schooling. Biologists are as human as the next ape descendent, but they’re also a generally open-minded bunch who tend to be more interested in the quality of your work than what you do after you leave the lab. And, for what are probably obvious reasons, evolutionary biology doesn’t attract the sort of people who hold doctrinaire conservative religious positions on any subject.
Evolutionary biology is also a pretty good academic discipline for me because evolutionary biology has something to say about sexual minorities, just as it has something to say about humans in general. Humans are biological beings, and we’re part of an animal kingdom that exhibits a wide array of sexual behaviors, as elaborately documented by the evolutionary biologist Joan Roughgarden in her book Evolution’s Rainbow. Exactly how to explain this diversity, particularly in the case of humans, is still quite controversial [$a]—but it’s a question for which I have some expertise, and one I’d like to weave into the writing I do for D&T in the future.
Most evolutionary biologists believe that the easiest means for two populations to become reproductively isolated—a first step to splitting into different species—is a physical barrier to movement. Mountain ranges, deep river valleys, or the sheer distance between an island and the mainland—the opportunities for allopatric speciation are all over the place. Unless, of course, you remember that the planet’s largest habitat is the ocean, and there aren’t such obvious physical barriers out at sea.
How do fish and other marine organisms form new species, then? Maybe they’re more likely to speciate as a result of natural selection that varies among otherwise connected marine habitats. For instance, a new study of rockfish finds evidence that this new species in this group usually form by adapting to conditions found at different oceanic depths [$a].
Two rockfish species, Sebastes atrovirens and Sebastes chrysomelas. Photos by brian.gratwicke.
The rockfish genus Sebastes contains several dozen species, but many of them occur in about the same regions of the Pacific ocean. Rather than being separated by physical distance, the group has diversified into different ecological niches, from the intertidal zone down to depths of 600 meters. The new study’s author, Travis Ingram, wanted to determine whether these habitat differences or geographic distance has more often been the cause of rockfish speciation, which he did using two major analyses.
In the first, Ingram asked whether pairs of rockfish species were more or less likely to occupy the same latitudes, and the same depth ranges, as they diverged over time. Allopatric speciation would lead to closely-related rockfish species occupying separate latitude ranges, but Ingram found the opposite. On the other hand, closely-related rockfish species are less likely to live at the same depth in the ocean—so depth, not geographic distance, seems to be important in rockfish speciation.
Ingram’s second analysis takes advantage of the general principle that traits associated with forming new species should change relatively rapidly at about the same time as speciation events, rather than at a uniform rate over time. Traits that undergo this speciational evolution can be distinguished from traits that don’t based on the relationship between trait values of related species. The idea is to compare the trait values for pairs of species drawn from the group of interest—if the differences in trait values are more strongly correlated with the number of speciation events that have occurred since the pair of species last shared a common ancestor than with the raw time since that common ancestry, the trait has probably evolved in speciational fashion.
This is the pattern Ingram found in the depths occupied by different species of rockfish. Changes in depth range occupied by rockfish were associated with speciation events, rather than evolving steadily over time. How these changes could have contributed to reproductive isolation is another question—different depth habitats present rockfish with different kinds of predators and prey, but also with different light environments for visual mating signals. One or more of these environmental differences could create the sort of divergent natural selection that can lead to reproductive isolation and speciation.
Reference
Ingram, T. (2010). Speciation along a depth gradient in a marine adaptive radiation. Proc. Royal Soc. B : 10.1098/rspb.2010.1127
The cover article for last week’s issue of Nature promised to be the last word in a long-running scientific argument over the evolution of cooperation—but it really just rejiggers the terms of the debate. Instead of solving the problem of how cooperative behavior can evolve, the new paper presents a model of maternal enslavement [$a]. These are not, it turns out, quite the same thing.
Group selection versus kin selection
Let’s start with some background. Unselfish, cooperative behavior has long been a puzzle in evolutionary biology, because natural selection should never favor individuals who make significant sacrifices for the benefit of others. Sure, an unselfish individual might expect those she helps to reciprocate later; but a population of the unselfish would be easily overrun by those who don’t reciprocate.
There have historically been two answers to the problem of the selfish out-competing the unselfish. The first case is basically an extension of logic we all learned in kindergarten: cooperative groups can do things that uncooperative groups can’t. Like, for instance, start a neighborhood garden.
Under this model, neighborhoods of cooperative, garden-making people are nicer places to live, and their inhabitants can collectively out-compete other neighborhoods that can’t get it together to start a community garden. In evolutionary terms, this is group selection—even if individuals sacrifice to build the garden, the group as a whole benefits. Unfortunately, this breaks down if the new garden attracts selfish people to move to the neighborhood, buy up all the cheap real estate, and open Urban Outfitters franchises.
There’s another possibility, though. What if unselfish behavior isn’t always truly unselfish? For instance, if you help your relatives, you’re actually helping some of your own genes. You share half your genes with your siblings, a quarter of your genes with half-siblings, an eighth of your genes with first cousins, and so on. This means that Michael Bluth might be on to something.
Evolutionarily speaking, it doesn’t matter if Michael spends all his time helping his feckless family, as long those efforts help someone in the family (G.O.B., most likely) reproduce and perpetuate some of the genes that Michael shares with him or her. This idea was advanced by W.D. Hamilton in two 1964 papers, one mathematical [PDF], and one more focused on real-world examples [PDF]; we now know it as kin selection. It doesn’t hold up so well for maintaining the kind of complex society humans have today, where we interact with lots of completely unrelated people—but it might have got the ball rolling toward the wheel, war, New York and so forth by selecting for cooperative behaviors within small tribes back at the dawn of history.
The group selection versus kin selection debate has gone back and forth for decades, and the new paper is a shot across the bow of kin selection. The authors, Martin Nowak, Corina Tarnita, and E.O. Wilson, aim to do two things: first, prove that kin selection is wrong; and second, describe an alternative explanation. For the first, they argue that kin selection only applies in narrow circumstances, that those circumstances never show up in nature, and that empirical studies just don’t support the model. Johnny Humphreys makes some reasonable objections to these arguments, and so do several folks interviewed by Carl Zimmer, and I’ll refer you there rather than try to improve on them.* I’m more interested in the second part: the alternative explanation.
Enslaved by Mom
No individual fitness for you—you’re cogs in the Superorganism. Photo by jby.
Nowak et al. propose to explain the evolution of unselfishness as it applies to eusociality—organisms like ants or bees or naked mole rats, in which colonies of (closely related) individuals defer most or all of their opportunities to reproduce, in order to support one or a few individuals that reproduce a lot. As Johnny points out in his critique, it’s not clear that eusociality is the same thing as unselfishness at all, even though it’s historically cited as an example of unselfishness [$a]. The new model that Nowak et al. develop actually makes the difference between eusociality and unselfishness even clearer. Under their model, it’s not that worker ants give up reproductive opportunities to help their mother, the Queen, reproduce—it’s that the Queen takes away their reproductive opportunities.
The key insight of the new model is that, in evolving from a non-social insect to a eusocial one, the natural selection that matters affects not the individuals evolving into workers, but the individual who would be Queen. Consider an insect similar to the probable ancestor of ants: females build nests, provision them with food, and lay eggs inside. Nowak et al. propose that a female who evolved the ability to lay “worker” eggs—females that grow up not to found their own nest, but to help in their mother’s—would have greater fitness than females without such helpful offspring.
Aside from the probability of evolving “worker” eggs (which is not a small issue, I think), this shift in perspective from the fitness of the worker to the fitness of the Queen makes all sorts of sense to me. I’ve often wondered why myrmecologists don’t treat ant colonies as single organisms, rather than collections of cooperating individuals.
But this approach also seems to sidestep the key question biologists hope to answer with kin selection and group selection models—these models aim to explain how individuals can come together to cooperate, but Nowak et al. have built a model that looks more like enslavement. I can’t learn anything about how unselfish behavior can spontaneously evolve in a population by looking at a population that has had unselfishness imposed upon it. To indulge in one last especially geeky pop culture reference, it’d be like trying to learn about market economics by studying The Borg.
Nowak, Tarnita, and Wilson might have come up with a very good model for the evolution of eusociality; but if so, it means that eusociality is a bad model for the evolution of cooperation as we usually conceive it.
———— * I will, however, note that Nowak et al. do something I’ve never seen in a scholarly paper before—in dismissing empirical studies of kin selection, they defer substantive discussion to the Supplementary Information. There are, in fact, 43 pages of SI for this 6-page paper, including two major mathematical models and the discussion of empirical kin selection studies. This is a problem, but one that is beyond the scope of this already-long post.
References
Axelrod, R., & Hamilton, W. (1981). The evolution of cooperation. Science, 211 (4489), 1390-1396 DOI: 10.1126/science.7466396
Hamilton, W.D. (1964). The genetical evolution of social behaviour. I. Journal of Theoretical Biology, 7 (1), 1-16 DOI: 10.1016/0022-5193(64)90038-4
Hamilton, W.D. (1964). The genetical evolution of social behaviour. II. Journal of Theoretical Biology, 7 (1), 17-52 DOI: 10.1016/0022-5193(64)90039-6
Nowak, M., Tarnita, C., & Wilson, E. (2010). The evolution of eusociality. Nature, 466 (7310), 1057-62 DOI: 10.1038/nature09205
Yuccas and yucca moths have one of the most peculiar pollination relationships known to science. The moths are the only pollinators of yuccas, carrying pollen from flower to flower in specialized mouthparts and actively tamping it into the tip of the pistil. Before she pollinates, though, each moth lays eggs in the flower—the developing yucca seeds will be the only thing her offspring eat. How does such a specialized, co-adapted interaction evolve in the first place? My coauthors and I attempted to answer this question in a paper just published in the Biological Journal of the Linnean Society, by reconstructing the ecology of yucca moths before they were yucca moths [PDF].
