Here’s a nicely gruesome image for the week of All Hallows’ Eve.
“I dreamed I was in a dark room,” said Jane, “with queer smells in it and a sort of low humming noise. Then the light came on … I thought I saw a face floating in front of me. … What it really was, was a head (the rest of a head) which had had the top part of the skull taken off and then … as if something inside had boiled over. … Even in my fright I remember thinking, ‘Oh, kill it, kill it. Put it out of its pain.’ … It was green looking and the mouth was wide open and quite dry. … And soon I saw that it wasn’t exactly floating. It was fixed up on some kind of bracket, or shelf, or pedestal—I don’t know quite what, and there were things hanging from it. From the neck, I mean. Yes, it had a neck and a sort of collar thing round it, but nothing below the collar; no shoulders or body. Only these hanging things. … Little rubber tubes and bulbs and little metal things too.”
—Jane describes the disembodied Head in That Hideous Strength
Before he started The Chronicles of Narnia, C.S. Lewis tried his hand at science fiction. Lewis’s Space Trilogy—Out of the Silent Planet, Perelandra, and That Hideous Strength—is like H.G. Wells dunked in (by modern American standards) gentle British Christianity. As in Narnia, Lewis wrote the Space Trilogy with a thesis in mind. The villains of Lewis’s imagined universe are materialistic scientists. In the first two books, the protagonist fights the scientists to preserve prelapsarian conditions among the intelligent inhabitants of Mars and Venus, respectively. The third book returns to Earth, where the evil scientists are plotting to take over the planet in the service of a demon-possessed disembodied head kept alive by technology that would’ve put Frankenstein off his lunch.
Lewis derived the scientists’ ideology, and one of the evil scientist characters in particular, from the writings and person of the evolutionary geneticist J.B.S. Haldane—which is not surprising, since Haldane was something of the Richard Dawkins of his day, a visible public advocate for the scientific worldview. What is surprising, though, is that Lewis may have had a perfectly good reason to connect Haldane to an artificially resurrected head: five years before the publication of That Hideous Strength, Haldane had narrated a film depicting just such an experiment.
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
If you’re a bird, brood parasitism seems like a cushy reproductive strategy—lay your eggs in someone else’s nest, then sit back and let the inadvertent foster parents raise your kids for you. But what if they don’t raise you kids quite right? Could brood parasite chicks raised by parents of another species grow up a bit … confused? According to a recent study of brood-parasitic ducks, they can indeed [$a].
Redheads (above) sometimes lay eggs in the nests of canvasback ducks (below)—but redhead chicks raised by canvasbacks may not know what species they are. Photos by Nick Chill and meantux.
The new study examines redheads, a species of North American duck which frequently lays its eggs in the nests of other duck species, particularly the canvasback duck, which occupies much of the same range. Redheads are facultative brood parasites—in years when conditions produce lots of resources, female redheads lay eggs in other ducks’ nests as a supplement to their own nests; and in poorer years, they may lay only parasitic eggs [PDF]. Canvasback ducks, on the other hand, will lay eggs in the nests of other canvasbacks (which is not uncommon in birds [$a]), but don’t parasitize other species.
This sets up a nice behavioral experiment. In birds, species recognition may be due to varying degrees of nature and nurture—a male bird may recognize females of his own species by genetically-transmitted instinct, but he may also have to learn socially important songs or other behaviors from his parents and other adults. You might expect that redhead chicks have evolved to recognize their own species regardless of who raised them, while canvasbacks might be confused if they grow up around another species. So the authors experimentally transferred just-hatched male redhead chicks into canvasback broods, and male canvasback chicks into redhead broods, and compared their social development to male chicks raised by their own species.
A female redhead spurns the advances of a cross-fostered male canvasback. Photo from Sorenson et al. (2010), figure 2.
When the chicks had grown up, the authors offered the cross-fostered males access to females of both species, and recorded their interactions. It turned out that the brood parasitic rednecks were just as prone to species-confusion as the canvasbacks. Males of both species preferred to associate with females of the species with which they were raised, and directed almost all of their courting effort—displays of neck-arching and special calls—toward the species that fostered them. In fact, many of the cross-fostered males successfully formed mated pairs with females of the other species.
So why hasn’t redhead parasitism of canvasback nests broken down the reproductive isolation between these two species? The authors don’t have a clear answer, but note that the rate of observed hybrid couplings are much lower in natural populations than in their experimental setting. Social learning is a complicated thing, and life in larger, natural populations of the two species might not be well replicated in this study.
References
Petrie, M., & Moller, A. (1991). Laying eggs in others’ nests: Intraspecific brood parasitism in birds. Trends in Ecology & Evolution, 6 (10), 315-20 DOI: 10.1016/0169-5347(91)90038-Y
Sorenson, M., Hauber, M., & Derrickson, S. (2010). Sexual imprinting misguides species recognition in a facultative interspecific brood parasite. Proc. Royal Soc. B, 277 (1697), 3079-85 DOI: 10.1098/rspb.2010.0592
Sorenson, M. (1991). The functional significance of parasitic egg laying and typical nesting in redhead ducks: an analysis of individual behaviour. Animal Behaviour, 42 (5), 771-96 DOI: 10.1016/S0003-3472(05)80122-8
Somewhat like cooperation between members of the same species, mutually beneficial interactions between different species should be prone to fall apart when one species evolves a way to cheat the other. Biologists who study mutualism (myself included) have long believed that the solution to cheating is to punish cheaters—but a new model suggests that the benefits gained from playing nice might be enough to deter cheating [PDF].
I knew I had to write about this one when I saw that the authors use their model to propose a new explanation for the dynamics of my own favorite mutualism, between yuccas and yucca moths. (And, yes, it’s also an excuse to reference Eddie Izzard. I’m only human.)
Cake: definitely preferable to death. Photo by 3liz4.
The new analysis by Weyl et al. applies an economic modeling framework to species interactions in which one species provides some benefit to another, and then itself receives a benefit that at least partially derives from the help initially provided. To take one example the authors cite, many ant species colonize acacia plants, which grow structures in which the ants can nest (or domatia), and often produce nectar or other food rewards for the ants. The ant colony defends the plant from insect herbivores, with the consequence that the plant can devote more energy to growth, including new domatia and new leaves to fuel nectar production via photosynthesis.
In many such interactions, it’s been thought that each species can only keep the other from cheating—taking the benefits of the relationship without returning the favor—by actively punishing such behavior. Weyl et al. argue that instead of punishment, cheaters might be deterred if their refusal to play their role results in reduced payback from the other partner.
In the ant-acacia example, ant-tended plants kill off branches that lose a lot of leaves to herbivores, which can happen if the ants cheat by slacking off on their protection duties. But this isn’t punishment as such, say Weyl et al. Plants that aren’t protected by ants also kill off damaged branches, to conserve resources. Instead, because ant domatia tend to be located on the youngest, most herbivore-vulnerable shoots of ant-tended plants, lazy ants harm themselves by allowing herbivores to trigger a response that the plant would make whether or not it hosted ants.
An ant domatium on a “whistling thorn” acacia tree. Photo by Alistair Rae.
It sounds a bit passive-aggressive on the plant’s part, doesn’t it? But let’s look at the example that caught my attention: yuccas and yucca moths. Yucca moths are the sole pollinators of yuccas, and lay their eggs in pollinated yucca flowers; as a pollinated flower develops into a fruit, the eggs hatch, and the new-born larvae eat some of the seeds inside. Moths have good incentive to cheat on yuccas by laying lots of eggs in a single flower or not providing much pollen, but yuccas abort flowers that receive too many moth eggs, or not enough pollen [PDF]. Those of us who study yuccas have tended to interpret this as punishment, since killing off a pollinated flower also kills off any seeds a yucca might have produced via that flower.
However, as Weyl et al. note, yuccas abort flowers in response to damage to the floral ovules [PDF] (the tissue that will become seeds when pollinated), not to the presence of moth eggs per se. Moths generally damage the ovules a bit when laying eggs inside the flowers; but damage without eggs has the same effect. If floral abortion were punishment, say Weyl et al., it would occur as a result of moth eggs alone, not damage to the ovules in general.
In other words, the mutualists analyzed by this new paper are kept honest not by the threat of punishment (death) but the possibility that cheating will result in reduced rewards (less cake). It’s a clever inversion of perspective, and I’ll be very interested to see whether new empirical studies can back it up.
References
Marr, D., & Pellmyr, O. (2003). Effect of pollinator-inflicted ovule damage on floral abscission in the yucca-yucca moth mutualism: the role of mechanical and chemical factors. Oecologia, 136 (2), 236-43 DOI: 10.1007/s00442-003-1279-3
Pellmyr, O., & Huth, C. (1994). Evolutionary stability of mutualism between yuccas and yucca moths. Nature, 372 (6503), 257-60 DOI: 10.1038/372257a0
Weyl, E., Frederickson, M., Yu, D., & Pierce, N. (2010). Economic contract theory tests models of mutualism. Proc. Nat. Acad. Sci. USA, 107 (36), 15712-6 DOI: 10.1073/pnas.1005294107
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
There’s already been a lot of blogospheric discussion of the BBC’s recent declaration that “Darwin may have been wrong” based on a recently-published paleontology paper. I hadn’t paid it much attention, because while sloppy science journalism irritates me, it’s not quite in my wheelhouse, expertise-wise. Then I actually got around to reading the paper, and it turns out that it’s directly related to some of my own work—and the conclusion that led to the sensationalistic sub-headline doesn’t make any sense.
Coauthors Sahney, Benton, and Ferry analyze the fossil record of four-limbed vertebrates—tetrapods—to show that in general, as more species evolve, they also evolve to fill a wider variety of ecological roles [$a]. Ecological roles are here defined by combinations of body size, diet, and habitat. (Sahney et al estimate there are 207 such combinations possible, though only 75 are “occupied.”) That’s a straightforward and mostly unsurprising result—the number of tetrapod species increases as tetrapods evolve new ways to make a living. But then we get to the conclusions of the paper, and things get weird. Sahney et al. conclude that because diversification is associated with finding unoccupied ecological roles, competition is mostly unimportant in the diversification of tetrapods: “Given the unrestricted access tetrapods have to ecospace, perhaps there is little need for competitive interactions to shape diversification.” In other words, if diversification happens by finding ways to make a living that aren’t already occupied, competition isn’t important.
Except that the very reason species diversify following an ecological opportunity like the development of a new ecological role is the lack of competition the new role provides. As my coauthors and I documented in a recently published literature review, competition shapes the kind of diversification documented by Sahney et al. in two ways: first, by its absence following the evolution of a new lifestyle; then in spurring an adaptive radiation as new species evolve to partition up the newly-available “ecospace.”
What makes this doubly odd is that Sahney et al. refer to another kind of ecological opportunity, the extinction of competitors, as a good example of competition-driven diversification. But a central insight of the literature on ecological opportunity is that diversifying because a whole bunch of ecological roles have just opened up is not fundamentally different from diversifying after a new mutation makes a never-before-seen ecological role possible. Think of it like starting a new business: to avoid competition, you could either sell an existing product in a place where no one else sells that product, or you can invent a product no one else offers. Both approaches give you a market all to yourself, and both are defined by competition.
It’s hard for me to understand why Sahney et al. don’t make this conceptual connection—which, for what it’s worth, has its roots in The Origin of Species.
References
Sahney, S., Benton, M., & Ferry, P. (2010). Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters, 6 (4), 544-7 DOI: 10.1098/rsbl.2009.1024
Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J., Schenk, J.J., Spear, S.F., & Harmon, L.J. (2010). Ecological opportunity and the origin of adaptive radiations Journal of Evolutionary Biology, 23 (8), 1581-96 DOI: 10.1111/j.1420-9101.2010.02029.x
Two of the most diverse groups of living things on Earth are flowering plants and the insects that make their living from flowering plants. Biologists have long thought that the almost incessant, intimate interactions between plants and plant-eating insects might be the evolutionary cause of each group’s spectacular diversity. On a smaller scale, this means that we’re interested in the reasons that specific insects and plants interact in the first place—what evolutionary trails leads one insect species to specialize on a single host while others eat pretty much any plant they land on.
A new study of one group of plant-eating insects suggests that the kind of interaction between insects and their host plants also determines how specific those interactions are. Examining a group of moths that, like the yucca moths I study, pollinate their host plant and then eat some of its seeds, the authors of the new study find that related, non-pollinating moths use more host plant species than the pollinators [$a]. I think it makes a particularly nice companion piece to my post about the evolutionary origins of yucca moths, because it provides an example of one or two other things biologists can deduce from phylogenies—and, as we’ll see, some things they can’t.
Epicephala: like a yucca moth without the snappy name
The moths in question are in the genus Epicephala, and they have an obligate pollination relationship with trees in the genus Glochidion, a diverse group of plant species found in southern Asia. That is, female moths carry pollen between Glochidion flowers in special mouthparts, deliberately apply pollen to the flower, and then lay eggs in the flower so that, when it develops into a fruit, her larvae can eat some of the seeds inside. Epicephala species are highly specialized, with most species only using one species of Glochidion [$a]. That’s a higher degree of specialization than what’s seen in yucca moths, in fact.
Pollinating moths (genus Ephicephala, left) use fewer host plant species than related non-pollinating moths (genus Caloptilia, right). Photos by CharlesLam and Bettaman.
The family of which Epicephala is a member happens to include other moths that interact with Glochidion, but only as herbivores: species in the genera Caloptilia and Diphtheroptila, whose larvae all eat Glochidion leaves. Do these antagonistic moths use more, or fewer, species of the host plant than the mutualistic Epicephala? Kawakita and his coauthors set out to answer that question by reconstructing the phylogenies of Caloptilia and Diphtheroptila. Finding species in evolutionary trees
Most biologists agree that two groups of organisms are separate species if there is no gene flow between them. A consequence of genetic isolation between species is that, if they’re isolated long enough, they become monophyletic within phylogenies. That is, all the individuals within each species share a common ancestor that is not shared with any other species. You can see this by contrasting two monophyletic species (on the left in the figure below) with two groups that turn out to be paraphyletic—some individuals of the red species are more closely related to individuals of the blue species than to other individuals of their own species.
Monophyletic and paraphyletic groupings. Image by jby.
The reasoning behind this is a bit subtle. Paraphyletic groups might still be separate species—they just haven’t been isolated long enough to become monophyletic. As a good example, I’m a coauthor on a recent study that did this kind of analysis on non-pollinating “bogus” yucca moths that use three different yucca species. In that case, the moths were paraphyletic with respect to which yucca species they used, but more analysis showed that there is currently very little gene flow between moths using different hosts [PDF].
In the case of the Glochidion-using Caloptilia and Diphtheroptila, Kawakita et al. found something more complicated. Each genus broke up into several monophyletic groupings, or clades of genetically similar individuals—but in most cases each clade included moths collected from at least two different Glochidion species. Kawakita et al. note that the clades also correspond to differences in the moths’ wing coloration, larval feeding behavior, and genitalia, and conclude that each clade is a different species. That would mean that the two antagonist genera tend to use multiple host plants.
Interesting question, but is this the way to answer it?
Except I’m not sure I buy this usage of phylogenies to define species. Kawakita et al. have shown that within the clades they call species, the individuals all have very similar genetics, but only for the two commonly-used genetic markers from which the phylogenies are reconstructed. It’s not impossible that within each clade the moths might be adapted to individual host plant species, and reproductively isolated by that adaptation—and this could have happened recently enough that not many genetic differences would have built up in the two markers.
To really answer the question Kawakita et al. have posed would require a study of each clade in the two antagonist genera at a much finer scale. The question of how specialized Caloptilia and Diphtheroptila are hinges on how many species are in each genus, and that’s better addressed by examining population genetics, not ancient relationships among these genera.
Reference
Drummond, C., Xue, H., Yoder, J., & Pellmyr, O. (2009). Host-associated divergence and incipient speciation in the yucca moth Prodoxus coloradensis (Lepidoptera: Prodoxidae) on three species of host plants. Heredity, 105 (2), 183-96 DOI: 10.1038/hdy.2009.154
Kawakita, A., & Kato, M. (2006). Assessment of the diversity and species specificity of the mutualistic association between Epicephala moths and Glochidion trees. Molecular Ecology, 15 (12), 3567-81 DOI: 10.1111/j.1365-294X.2006.03037.x
Kawakita, A., Okamoto, T., Goto, R., & Kato, M. (2010). Mutualism favours higher host specificity than does antagonism in plant-herbivore interaction. Proc. Royal Soc. B, 277 (1695), 2765-74 DOI: 10.1098/rspb.2010.0355
The oven in my apartment needs a serious deep-cleaning. A really serious deep-cleaning. To the point that, when I want to do some baking, smoke is more or less inevitable. As a result, I’ve developed the habit of responding to the apartment’s smoke alarm by reaching up and un-mounting it from the ceiling, which completely disables it. If a fire were to start somewhere else in the apartment while I’m baking, I’d probably be in trouble.
That’s more or less the idea behind an approach to agricultural pest control proposed in a paper just released online at PNAS: if you saturate insect pests with a predator warning signal, they become used to the signal, and more vulnerable to predators [$a]. Aphids are the target pest—they form huge, clonal swarms to literally suck the life out of plants, as described very nicely in this BBC Nature video.
As the video notes, those clonal swarms are vulnerable to all sorts of predators, most famously ladybird beetles. So when attacked, the aphids emit an alarm pheromone to warn the rest of the clone. But it’s possible to habituate aphids to the alarm pheromone—if they’re surrounded by it long enough, they won’t respond to it by running away. The new study’s authors proposed genetically engineering crop plants to produce the alarm pheromone, to automatically produce that habituation.
To see if this would work, they raised aphids on a line of Arabidopsis thaliana (the white lab mouse of the plant world) that had been engineered to produce the alarm pheromone. And, indeed, habituated aphids were much less likely to be repelled by the alarm pheromone—and were even in some cases attracted to it. Perhaps the most telling test involved leaving habituated and non-habituated aphids on an experimental plant with ladybird beetles introduced—habituated aphids were less likely to survive 24 hours with the beetles.
This is a pretty clever approach to pest control, but there’s an obvious caveat. I don’t see any reason why aphids couldn’t evolve a way around this attempt to swamp out their own alarm signals—the paper notes that different aphid species have different responses to the particular alarm pheromone tested, so engineering one pheromone into crop plants doesn’t leave the aphids without evolutionary options. Unless it’s very cleverly designed, any pest-control strategy creates strong natural selection—and the better the strategy is, the stronger the selection is—to evolve resistance. Alarm-pheromone-producing crops might be another tool for pest control, but they won’t be the last one we need.
A ladybird beetle makes short work of some aphids. Photo by kenjonbro.
See also this press release from Cornell University, which discusses the paper’s results.
Corrected, 17 Aug 2010, 2305h: Fixed the photo of the ladybird with aphids, which was meant to be full-width, and added a jump. Why do I keep forgetting those?
Reference
de Vos, M., Cheng, W., Summers, H., Raguso, R., & Jander, G. (2010). Alarm pheromone habituation in Myzus persicae has fitness consequences and causes extensive gene expression changes. Proc. Nat. Acad. Sci. USA DOI: 10.1073/pnas.1001539107
Nature News is reporting some interesting results presented as a paper at a meeting of the International Society for the Psychology of Science & Technology last week: articles published in the journal Science with longer “Works Cited” sections are themselves more frequently cited.
A plot of the number of references listed in each article against the number of citations it eventually received reveal that almost half of the variation in citation rates among the Science papers can be attributed to the number of references that they include. And — contrary to what people might predict — the relationship is not driven by review articles, which could be expected, on average, to be heavier on references and to garner more citations than standard papers.
The same authors did a similar analysis of papers published in the journal Evolution and Human Behavior over 30 years, and found similar results [PDF]. Here’s the relevant figure from that paper:
The lack of a “review effect” is surprising, but I don’t think this overall result is. Academia, as much as we might describe it as cutthroat, also runs on reciprocal altruism. Authors notice when their papers are cited, and are more likely to cite papers that build on or relate to their own work. I’d be interested to see the network of citation underlying the pattern Webster et al. have found—I suspect that there’s a lot of clustering around disciplines and sub-disciplines and sub-sub-sub-disciplines that contributes to all this mutual back-scratching citing.
Updated, 15 August 2010, 2126h: Fixed the link to the original Nature News article, which turns out not to be access-restricted.
Reference
Webster, G.D., Jonason, P.K., & Schember, T.O. (2009). Hot topics and popular papers in evolutionary psychology: Analyses of title words and citation counts in Evolution and Human Behavior, 1979-2008. Evolutionary Psychology, 7 (3), 348-348 Other: http://www.epjournal.net/filestore/ep07348362.pdf
Here’s a nicely gruesome image for the week of All Hallows’ Eve.
