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
My Sunday morning reading includes a trenchant essay by Jacob Weisberg at Slate, which gathers together President Obama’s disappointing performances on immigration, freedom of religion, and gay marriage under the rubric of moral cowardice:
Obama has had numerous occasions to assert leadership on values issues this summer: Arizona’s crude anti-immigrant law, the battle over Prop 8 and gay marriage, and the backlash against what Fox News persists in calling the “Ground Zero mosque.” These battles raise fundamental questions of national identity, liberty, and individual rights. When Lindsey Graham argues for rewriting the Constitution to eliminate the birthright citizenship clause of the 14th Amendment, or Newt Gingrich proposes a Saudi standard for the free exercise of religion, they’re taking positions at odds with America’s basic ideals. But Obama’s instinctive caution has steered him away from casting these questions as moral or civil rights issues. On none of them has he shown anything resembling courage. [links sic]
To Weisberg’s list, I’d also add the need for comprehensive, carbon-limiting energy legislation. Treating undocumented immigrants like human beings, Muslim and gay Americans like citizens, climate change as a genuine impending human-created disaster—these are all inherently moral positions. Liberals have long been sick of watching that morality overruled by the weird, selfish, other-hating morality of contemporary American conservatism. I voted for Barack Obama (and I think lots of us did) because he seemed likely to articulate liberal beliefs in explicitly moral language, and do it with conviction.
Remember his campaign speech on race? With his feet to the media fire over his apparently scandalous association with Jeremiah Wright, Obama acknowledged the subtleties and complications of our national racial history, without losing sight of basic principles of right and wrong:
The profound mistake of Reverend Wright’s sermons is not that he spoke about racism in our society. It’s that he spoke as if our society was static; as if no progress has been made; as if this country—a country that has made it possible for one of his own members to run for the highest office in the land and build a coalition of white and black; Latino and Asian, rich and poor, young and old—is still irrevocably bound to a tragic past.
That’s the Barack Obama I wanted to be President. I could’ve sworn I voted for that one. But it doesn’t seem to be the guy who ended up in office.
Candidate Obama at a rally in Pittsburgh, 21 April 2008. Photo by BarackObama.com.
Pretty sure that violates the five second rule. Some well-preserved specimens suggest that wooly mammoths ate their own dung, at least occasionally. (Brian Switek)
You rock, rocks. A newly discovered Burgess Shale outcrop is already yielding freaky new Cambrian Explosion-era fossils. (Wired Science)
Not versed in the social graces. A study of domesticated wolves finds that they’re much slower than domestic dogs to look for guidance from humans. (The Thoughtful Animal)
Afraid you’ve picked up bed bugs during fieldwork? Try baking your clothes in your car. Seriously. (dechronization)
How is a scarab like a mantis shrimp? Scarabs might also be able to see circularly polarized light. (Arthropoda)
So a sense of humor and good conversation does help? Male house finches can compensate for less-attractive plumage by expanding their social circle. (A Scientific Nature)
I know you’re out there. I can smell you eating. Predators can detect the chemical interaction of insect herbivores’ saliva and plant compounds. (Bioblog, original article in Science [$a])
In which a tenured faculty member is literally worse than some baboons. Sexual coercion—and lack thereof—in our evolutionary relatives illuminates a sadly human case study. (The Primate Diaries in Exile)
And now, via Everyday Biology, They Might be Giants on photosynthesis, with a chorus of anthropomorphic insects.
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
Major, embarrassing update, 2010.09.20: So it turns out that the Slate article from which I learned that Jefferson was the FAITH portrait was pretty much dead wrong. In fact the image is of Samuel Adams, and the source is the same painting in the Adams Wikipedia article. (Although, Shepard Fairey-ized, he still looks like Jefferson to me.) Jefferson makes so little sense for a portrait of FAITH that not even Glenn Beck is stupid enough to try and make him one. I’ve pulled the image from Flickr to prevent propagation of a false meme. Oy.
So there’s this guy who’s really popular with folks who hold political opinions mostly in opposition to mine. It’s come to my attention (probably late, I know) that he’s been waving around images of Thomas Jefferson, George Washington, and Benjamin Franklin, à la Shepard Fairey’s Obama posters, with the words “Faith,” “Hope,” and “Charity” appended to each, respectively. Now, I might quibble with “Hope” for Washington (what about “Courage?”) and “Charity” for Franklin (I’m sure Ben would’ve preferred “Thrift”), but I really take issue with assigning “Faith” to Jefferson. If you don’t know why this is both silly and (for a non-faith-inclined person such as myself) irritating, I have two words for you: Jefferson Bible.
So, with a little help from the wonderful open-source image software Inkscape, I fixed it.
It’s the hot new pigment this season. A just-discovered form of chlorophyll allows the algae that produce it to photosynthesize using infrared light. (Wired Science)
One, two, three … many? Studies of monkeys, babies, and chickens suggest that the ability to count small numbers is innate, and separate from the ability to count larger numbers. (The Thoughtful Animal)
Can you hear me now? On the Galapagos islands, marine iguanas listen for the alarm calls of mockingbirds to know if a predator is approaching. (The Thoughtful Animal)
Crocodile tears from the Adaptationist Programme. Crying confers fitness advantages by eliciting empathetic responses. Or something like that. (NPR)
Long-term forecast: 60% chance of dueling results. Remember all that oil in the Gulf of Mexico that hadn’t magically disappeared? Analysis of DNA microbial DNA sampled in that oil plume just found lots of oil-eating bacteria. (Deep Sea News, Wired Science, NPR; original peer-reviewed article in Science [$a])
Climate’s changing, with or without you. As temperatures warm throughout the Mojave Desert, Joshua tree, my favorite woody monocot, may disappear from 90% of its present range. (Voltage Gate)
I start another semester as Teaching Assistant for Mammalogy next week, so here’s David Attenborough discussing mammalian dentition, with reference to an ancient omnivore I’d never heard about up to now.
Trying out Digg.com, which is apparently rolling out a new UI. It has serious competition from incumbent systems Facebook and Twitter, but I’ll give it a week, at least.
I just built my first course webpage, for the Mammalogy lab I’ll be leading for this semester’s teaching assistantship. It pulls together a bunch of resources I developed for the same lab last year—photos of lab specimens taken by students (thanks to a little extra credit for inducement) and Anki decks. Now I need to get started on the slides for my first week’s lecture …
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.
