Category Archives: evolution

Ecological complexity breeds evolutionary complication

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Photo by futureatlas.

ResearchBlogging.orgCross-posted from Nothing in Biology Makes Sense!

It is a truth universally acknowledged in evolutionary biology, that one species interacting with another species, must be having some effect on that other species’ evolution.

Actually, that’s not really true. Biologists generally agree that predators, prey, parasites, and competitors can exert natural selection on the other species they encounter, but we’re still not sure how much those interactions matter over millions of years of evolutionary history.

On the one hand, groups of species that are engaged in tight coevolutionary relationships are also very diverse, which could mean that coevolution causes diversity. But it could be that the other way around: diversity could create coevolutionary specificity, if larger groups of closely-related species are forced into narower interactions to avoid competing with each other.

Part of the problem is that it’s hard to study a species evolving over time without interacting with any other species—how can we identify the effect of coevolution if we can’t see what happens in its absence? If only we could force some critters to evolve with and without other critters, and compare the results after many generations …

Oh, wait. That is totally possible. And the results have just been published.

A team of evolutionary microbiologists has performed exactly the experiment I outlined above. The study’s lead author is Diane Lawrence, a Ph.D. student in the lab of Timothy Barraclough, who is listed as senior author.

For the experiment, the team isolated five bacterial species, of very different lineages, from pools of water at the bases of beech trees—ephemeral pockets of habitat for all sorts of microbes that break down woody debris, dead leaves, and other detritus. They cultured the bacteria on tea made from beech leaves, in vials containing either a single species, or all five species, and let them evolve for eight weeks—several dozens of bacterial generations. In a particularly clever twist on standard experimental evolution methods, they also used nuclear magnetic resonance (NMR) to identify the carbon compounds in sterilized tea that had been “used up” by the bacterial cultures, and compared the compounds in fresh beech tea to determine what the bacteria were eating.

The base of a beech tree. Photo by -nanio-.

And, maybe not surprisingly, the bacterial species’ evolution with company turned out to be quite a bit from their evolution alone. Left alone, most of the species evolved a faster growth rate. This is a common result in experimental evolution, because the process of transferring evolving bacteria to fresh growth medium—”serial transfers” that were performed fifteen times over the course of the experimetn—can create natural selection that favors fast-growing mutants. But, grown all together in the same tube, species that had evolved faster growth rates in the solo experiment evolved slower growth instead.

To find out what had evolved in the multi-species tubes, the team tested the growth of the bacterial species on beech tea that had been used to grow one of the other species, then sterilized. The original, ancestral strains of bacteria generally had negative effects on each others’ growth—they lived on similar compounds in the beech tea, and so their used tea wasn’t very nourishing for the other species. The same thing occurred with the strains that had evolved alone, only stronger, which makes sense in light of the increased growth rates, which would’ve depleted the growth medium faster.

But the interactions among the strains of the different bacterial species that had evolved together was strikingly different. Many of them actually made the tea more nutritious for other species in the evolved community. That is, some of the bacteria had evolved the capacity to eat the waste products of another species that was evolving with them. Using the NMR method to track changes in the presence of different carbon compounds in the tea before and after use provided confirmation that the co-evolved species were using, and producing, complementary sets of resources.

In short, the evolving community didn’t simply become more diverse—it evolved new kinds of mutually beneficial relationships between species that began as competitors.

Beech leaves—yum? Photo by Colin-47.

That evolutionary shift toward mutual benefit had a significant impact on the bacterial community as a whole, too. Lawrence et al. assembled new communities of bacteria extracted from the end-point of the group evolution experiment, and compared their carbon dioxide production, a proxy for overall metabolic activity, to that of a community assembled from bacteria extracted from the end point of the solo-evolution experiments. The community of co-evolved bacteria produced significantly more carbon dioxide, suggesting they were collectively able to make more use out of the growth medium.

So that’s a pretty nifty set of results, I have to say. But I’m also left wondering what it tells us more generally. In both Lawrence et al.‘s paper, and in accompanying commentary by Martin Tucotte, Michael Corrin, and Marc Johnson, there’s a fair bit of emphasis on the unpredictability of the result. Lawrence et al. write, in their Discussion section,

The way in which species adapted to new conditions in the laboratory when in monoculture—the setting assumed for many evolutionary theories and experiments—provided little information on the outcome of evolution in the diverse community.

And, as Corrin et al. note,

These results imply that predictions constructed from single-species experiments might be of limited use given that most species interact with many others in nature.

So … evolution went differently under different conditions? That isn’t exactly a shocking revelation. The fact that this is one of the study’s major conclusions is a symptom of how little experimental work has actually tested the effects of multiple species on evolution. One experiment I’ve discussed here previously, focused on the joint effects of predators and competitors on microbes that live in pitcher plant pitfalls, similarly emphasized the fact that it wasn’t possible to predict the evolutionary effects of predators and competitors together based solely on their individual effects. Work in this line of inquiry is hanging at the point of establishing that complex conditions lead to complex results.

What I’d really like to know—and I think all the authors of both the paper and the commentary would agree with me on this—is how we can begin to make general predictions about community evolution beyond, “it depends what we put in at the start.” It may be that we’ll need a lot more studies like this current one before we can start to identify common processes, and more interesting trends.◼

References

Turcotte, M., Corrin, M., & Johnson, M. (2012). Adaptive evolution in ecological communities. PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001332

Lawrence, D., Fiegna, F., Behrends, V., Bundy, J., Phillimore, A., Bell, T., & Barraclough, T. (2012). Species interactions alter evolutionary responses to a novel environment. PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001330

Nothing in Biology Makes Sense: Making sense of polar bears’ origins

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Polar bear. Photo by ucumari.

This week at the collaborative blog Nothing in Biology Makes Sense, contributor Noah Reid goes in-depth on the recent study pinpointing the historical origins of polar bears, and why it’s taken the most recent systematic methods to correctly pinpoint them.

From 2008 to 2010, a series of algorithms were published that could take data from multiple genes and infer the history of whole populations, a drastic improvement over previous methods that could only identify the history of single genes (e.g. mtDNA). With these methods in mind, a group of researchers gathered data from 14 nuclear genes for multiple extant brown and polar bear populations (Hailer 2012). … the new data analyzed with the new method suggested that polar bears diverged far earlier than previously thought (around 600 thousand years ago) and that they were no longer closely related to the southeast Alaska population, but rather to the common ancestor of all brown bear populations.

For more details, including a nice brief explanation of why it can be important to use multiple genes in reconstructing relationships among species, go read the whole thing.◼

Evolution is undirected, political evolution doubly so

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Pioneering gay rights activist Frank Kameny, at a White House event to commemorate his work. Photo from the White House photo stream.

To everyone wringing their hands over the Obama Administration’s weird collective dance around the question of marriage equality: You do know that evolution is an inherently undirected, frequently random, ultimately goal-less process, right? In which case, the President’s description of his position as “evolving” is an exceptionally apt fit for the inefficient waffling, contradictory signals, and even reversal of previous positions that we’ve observed over the last several years.

See, doesn’t that make you feel better?

Yeah, me neither.

How about this as a kicker instead: evolutionary changes that appear to be directionless over short periods of time may eventually turn out to be part of much longer-term trends.◼

And another thing …

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Regarding that adaptive fairytale about the “runner’s high”—over at Distributed Ecology, Ted Hart points out that it doesn’t make much sense in phylogenetic context, either.

What would be really interesting is to see where this trait maps across the phylogeny. Is it a conserved trait that was selected for in some ancestor? That would point to the fact that maybe it has nothing to do with running. The authors are mute about phylogeny, but eCB’s could alternatively be the ancestral character state, and really the interesting question is why did ferrets evolve the loss of this state? On the other hand maybe the trait evolved multiple times, and that also is really interesting to ask how that happened. But either phylogenetic scenario undermine the central thesis of Raichlen.

You’ll want to read the whole thing, natch.◼

Dr. Pangloss runs a marathon

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Runners in the 2009 New York Marathon. Photo by Whiskeygonebad.

ResearchBlogging.orgThis just came over Twitter (hat tip to @DLiancourt): NPR is running a story claiming that the “runner’s high” some of us feel after a good workout is an adaptation to prompt us to keep fit, or something.

When people exercise aerobically, their bodies can actually make drugs—cannabinoids, the same kind of chemicals in marijuana. [University of Ariona anthropologist David] Raichlen wondered if other distance-running animals also produced those drugs. If so, maybe runner’s high is not some peculiar thing with humans. Maybe it’s an evolutionary payoff for doing something hard and painful, that also helps them survive better, be healthier, hunt better or have more offspring.

So, in a study [$a] pubished in The Journal of Experimental Biology, Raichlen tested this adaptive hypothesis by comparing the levels of these “endogenous cannabinoids” in the blood of humans, dogs, and ferrets after running on a treadmill. The idea being that the ancestors of dogs, like ours, made a living by running—chasing down prey—while ferrets don’t.

So it’s kind of nice to see that Raichlen and his coauthors did, indeed, find that humans and dogs both had higher levels of endogenous cannabinoids in their blood after a run, and the ferrets didn’t. That’s a useful evolutionary data point: it suggests that whatever physiological system prompts endogenous cannabinoid production in connection with exercise dates to (at least) the common ancestor of dogs and humans, and that its preservation in both species may be linked to our shared ability to run long distances.

But it really doesn’t show that this cannabinoid response is an adaptation to reward us for putting in our daily miles.

To really show that the runner’s high is an adaptation, of course, we’d need data that showed (1) observed variation in the runner’s high response has a genetic basis, and (2) people who had get stronger runner’s highs have more babies. But even apart from that, the understanding of what is good for us today—getting off our butts and going for a run—isn’t particularly that well connencted to the lives of our proto-human ancestors. Does Raichlen really think that early humans (or dogs, or any other animal that chases down prey) would just sit around and go hungry if we didn’t have a cannabinoid payoff at the end of the hunt?

And then, in the text of the very same NPR article, an orthopedic surgeon is quoted saying that the “runner’s high” can actually be a problem:

[Dr. Christina] Morganti treats runners for injuries and she says they’re the worst patients. “The treatment is to stop running,” she says. “They won’t. They don’t want to. A lot of the behavior is not unlike the patients we have who are seeking drugs. It’s really similar. It’s an addiction.”

So … a physiological response that prompts some of us to run even when running is likely to exacerbate injury is a good thing? How, exactly, would giving yourself shin splints lead to greater reproductive fitness if you’re making a living hunting gazelles on the savannah? I’m going to go out on a limb and say it wouldn’t.

Here’s an alternative hypothesis, which I freely admit is no better supported by the available data: the endogenous cannabinoid response isn’t a “reward” for running. Instead, it helped our ancestors tolerate the stress of running when they needed to, by letting them ignore minor pains and press on after that one elusive, tasty antelope. For our ancestors, dinner was the reward for running, not the cannabinoids. In the modern world, where we don’t run for our dinners, we’ve re-purposed the pleasant persistance of those cannabinoids as a motivation to replace that original life-or-death need.

Whatever the actual evolutionary origins of the “runner’s high,” the idea that it’s an adaptive reward for exercise is nothing more than adaptive storytelling filtered through the lens of our modern, very unnatural, lives. Don’t get me wrong—I love to run, and in fact I’m a month away from my fourth marathon. But I’m not going to pretend that I’ll be running those 26.2 miles because natural selection wants me to.◼

Reference

Raichlen, D., Foster, A., Gerdeman, G., Seillier, A., & Giuffrida, A. (2012). Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the ‘runner’s high’ Journal of Experimental Biology, 215 (8), 1331-6 DOI: 10.1242/jeb.063677

The living rainbow: The selective benefit of a fa’afafine in the family

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ResearchBlogging.orgOne of the key evolutionary puzzles of same-sex sexuality, as it manifests in modern, Western human societies, is that those of us attracted to members of our own biological sex don’t make a lot of babies. I’ve already spent a lot of pixels on the question of how genes for same-sex attraction might persist in human populations in the face of that selective cost—but a paper just published in PLoS ONE adds some evidence in favor of one popular hypothesis: that gene variants that make men more likely to be gay could also make their straight relatives more fertile.

The new paper presents data from Samoa, where the traditional culture has long had a place for men who are attracted to other men, in the role of fa’afafine—literally, men who “live in the manner of women.” Samoan boys who show interest in feminine activities are recognized by their families as members of this “third gender,” which is more like the modern Western conception of transgender identity than what we call “gay.” Fa’afafine often present and dress like straight women, and as adults, they generally have relationships with straight-identified men. But fa’afafine aren’t exactly “transgendered” as we understand that concept in the West—they don’t have the sense that their bodies don’t match their gender identity.

The fact that Samoan culture accommodates and accepts same-sex sexuality makes it an especially interesting context for testing hypotheses about the evolution of queer sexuality, including the idea that relatives of fa’afafine might be more fertile than people with no fa’afafine in the family. The study’s coauthors surveyed Samoan fa’afafine and straight men, asking how many children their grandmothers, aunts, and uncles had had. And they found that grandmothers of fa’afafine—both maternal and paternal grandmothers—had more children than grandmothers of the straight-identified men they interviewed.◼

Reference

VanderLaan, D., Forrester, D., Petterson, L., & Vasey, P. (2012). Offspring production among the extended relatives of Samoan men and fa’afafine. PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0036088

Carnival of Evolution, May 2012

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Photo by NS Newsflash.

This month’s issue of the Carnival of Evolution, which collects online writing about Darwin’s dangerous idea and all its variously modified descendents, is online over at John S. Wilkins’s blog Evolving Thoughts. Highlights include, but are not limited to, an attempt to trace the origin of the phrase “social Darwinism,” discussion of how sloths and turtles evolved to move slowly, and whether the diet of early humans was more healthy than ours. Go now and read the whole thing.◼

The living rainbow: In budgies, same-sex courting isn’t practice for wooing the ladies

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Budgies in their natural habitat. Photo by David Cook Wildlife Photography (kookr).

I’ve just set myself up a Google Scholar alert for papers on the evolution of same-sex mating behavior. The plan is, I’ll post some brief notes on anything interesting that shows up in my inbox. First up: bisexual budgies!

Male budgerigars—or parakeets, to those of us in the States—live in female-dominated social groups when they’re not caged in a petstore. In these groups, apparently, it’s quite common for pairs of males to engage in behaviors that look a lot like what males do when courting female budiges. It’s been hypothesized that this same-sex courting is practice for the real, reproductive deal. If that were the case you’d expect that male budgies who put in more time practicing with other males would have better luck with females later on.

However, when Puya Abbassi and Nancy Tyler Burley of the University of California Irivine compared the frequency with which individual male budgies engage in same-sex courting to their later success with females, they found a negative relationship—males that had more same-sex interactions were less likely to find female mates [$a]. The authors propose that the same-sex interactions are actually males assessing each others’ social status. That would square with Abbassi and Burley’s observations if low-status males, who are less likely to get lucky in the mating game, spend a lot more time sorting out relative rankings amongst themselves—and this is what the authors suggest may be going on.◼

Reference

Abbassi, P., & Burley, N. (2012). Nice guys finish last: same-sex sexual behavior and pairing success in male budgerigars Behavioral Ecology DOI: 10.1093/beheco/ars030

Carnival of Evolution, March 2012

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Erodium cicutarium. Photo by jby.

I completely failed to submit anything to the most recent edition of the Carnival of Evolution, but fortunately I was the exception, not the rule—the monthly compendium of online writing about the complications and implications of evolutionary biology is online over at Synthetic Daisies. This edition features a nifty phylogenetic organizing framework, and a crossword puzzle. Go check it out. ◼

Nothing in Biology Makes Sense: Making sense of “stinkbird” gut microbes

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A hoatzin. Photo by Carine06.

I was off the grid last week, so I missed Sarah Hird’s latest post at the group blog Nothing in Biology Makes Sense!, discussing a cool new study of the microbes in the guts of hoatzins, a species of wonderfully weird birds.

The hoatzin has an enlarged crop for the purpose of fermentation (see figure below). A “crop” is an anatomical structure in throat of some animals (including most birds) that primarily stores food. In the hoatzin, however, it does much, much more. Foregut fermentation is a digestive strategy where microbes living in or before the stomach break down vegetation for their host. Microbes are required by foregut fermenters because only the microbes are capable of breaking down the cell wall of plants, a barrier that confines most of the nutrients found in plant cells. The hoatzin is the only bird to use foregut fermentation and is the smallest known foregut fermenter.

To learn what the new study reveals about the diversity of microbes in hoatzin foreguts, go read the whole thing, including the evolving plans for follow-up experiments in the comments. ◼