Tag Archives: Research Blogging

How Wright was wrong: When is it genetic drift?

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This post was chosen as an Editor's Selection for ResearchBlogging.orgScience is often said to work in three easy steps: (1) observe something interesting, (2) formulate a hypothesis for why that something is interesting in the way it is, and (3) collect more observations to see if they also support that hypothesis. Wash, rinse, repeat, and you eventually get from Newton to Einstein, from Aristotle to Darwin.

Sewall Wright, pioneer of population genetics. (Wikimedia Commons)

Except, of course, it’s never that straightforward. Sometimes scientists come up with a hypothesis without a clear-cut example to support it, and then go looking that example. Sometimes observations that support a hypothesis turn out not to, if you look closer. And—here’s the funny thing—this can even happen with hypotheses that are, in the end, pretty much correct.

In the spirit of this month’s Giants Shoulders blog carnival, which focuses on “fools, failures, and frauds” in the history of science, I’m going to recount a case in which one of the greatest biologists of the Twentieth Century was fooled by a small desert flower. Sewall Wright was no fool or failure, and he certainly didn’t commit fraud, but he does seem to have been totally wrong about his favorite example of a particular population genetic process, one he discovered. That process, isolation by distance, is widely documented in natural populations today—but it also doesn’t seem to have worked the way Wright thought it did for Linanthus parryae.

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Double the mutualists, double the fun?

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ResearchBlogging.orgFor all living things, information is critical to survival. Where’s the best food source? Is there a predator nearby? Will this be a good place to build a nest? It probably shouldn’t be surprising, then, that lots of animals do what humans do when faced with a host of hard-to-answer questions—they take their cues from their neighbors.

Red-backed shrikes place their nesting sites near where other shrike species have set up territories. Many bird species recognize each other’s predator alarm calls, and respond appropriately. And a new natural history discovery published in the latest issue of The American Naturalist shows that treehoppers let one species of butterfly know where to find ants that will tend its larvae [$a].


The ant-tended butterfly (Parrhasius polibetes, above) looks for ant-tended treehoppers (Guayaquila xiphias, below) to know where to lay her eggs. Photos from Kaminski et al. (2010), figure 2.

The treehoppers help out the butterfly inadvertently, because both of them are dependent on a common resource: ants. Like many true bugs, treehoppers make their living sucking the sap of a host plant. This gives them a surplus of simple sugars and water, which they excrete as “honeydew” to attract ants for protection. As it happens, the larvae of the butterfly Parrhasius polibetes do the same thing—so the new study’s authors hypothesized that P. polibetes females might prefer to lay their eggs on plants where treehoppers were already present, since those would likely already have ants ready to protect butterfly larvae.

To test this, the authors set up experimental pairs of host-plant branches, one occupied by ant-tended treehoppers, and one not. They excluded ants from accessing the unoccupied branch with Tanglefoot, a water-resistant glue used in insect traps. After 48 hours, they checked the experimental plants for newly-laid butterfly eggs, and found that P. polibetes was both more likely to lay eggs, and laid more eggs at a time, on branches occupied by treehoppers.

To assess the fitness benefit of laying eggs on treehopper-occupied plants, the authors compared the survival of newly hatched P. polibetes larvae artificially introduced onto branches occupied by treehoppers to the survival of larvae introduced to branches unoccupied by treehoppers (and with ants excluded, again, using Tanglefoot). The larvae placed with treehoppers had substantially better odds of survival—about six times better.

These two experiments confound the effect of treehoppers with the effect of ants, however—so the authors performed one additional experiment. In this one, they set up paired branches with and without treehoppers, but allowed ants to reach both the occupied and unoccupied branches—and the general result from the earlier experiment held. Larvae placed near treehoppers were three times more likely to survive for the duration of the experiment even when larvae placed on a branch without treehoppers were able to attract ants on their own.

So it looks like P. polibetes is able to freeload on the treehoppers’ ant-attracting efforts, and benefits from that freeloading. What effect does that freeloading have on the treehoppers, or the ants, or the host plant? It’s hard to say based on the data presented in the current paper, but I’d guess that the treehoppers don’t lose much—in fact, they might gain from having another ant-attracting insect nearby, just as the butterfly larvae do. Similarly, it’s probably helpful for the ants to have more honeydew-producing species in the same location. It’s almost like that commercial for … what was the product?

(I’ll leave it to you, dear reader, to decide which insects correspond to which gendered pair in that video.)

I’d think, though, that this pile-on isn’t so good for the host plant, if plants already hosting treehoppers are more likely to have to deal with butterfly larvae, too. Untangling all the different ways these four species—ants, treehoppers, butterflies, host plants—exert direct and indirect natural selection on each other should keep the authors busy for a long time to come.

References

Hromada, M., Antczak, M., Valone, T., & Tryjanowski, P. (2008). Settling decisions and heterospecific social information use in shrikes. PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003930

Kaminski, L., Freitas, A., & Oliveira, P. (2010). Interaction between mutualisms: Ant‐tended butterflies exploit enemy‐free space provided by ant‐treehopper associations. The American Naturalist DOI: 10.1086/655427

Magrath, R., Pitcher, B., & Gardner, J. (2007). A mutual understanding? Interspecific responses by birds to each other’s aerial alarm calls. Behavioral Ecology, 18 (5), 944-51 DOI: 10.1093/beheco/arm063

Global warming roundup: There’s bad news, and weird news, but no really good news

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ResearchBlogging.orgRegardless of what James Inhofe thinks, global climate change is going to dramatically reshape the natural systems our civilization depends upon. Unfortunately, even as we embark on the radical experiment of turning our planet’s temperature up to 11, we’re just figuring out what results to expect. A whole series of papers released in the last week exemplify this point, showing that living communities’ response to the changing planet may often be counter-intuitive.

Temperature stress may offset trees’ ability to soak up carbon dioxide. Photo by Wade Franklin.

Let’s start with the bad news:

A study out in last week’s PLoS ONE suggests that, rather than growing more rapidly and absorbing more carbon dioxide as the planet warms, forest trees may actually grow more slowly. More carbon dioxide in the atmosphere should generally increase plants’ growth rates, since carbon dioxide is the raw material for photosynthesis. On the other hand, rising temperatures may put plants under so much stress that it offsets the benefits of more carbon dioxide.

Silva et al. examined core samples from four tree species—black spruce, red pine, red oak, and red maple—growing in Ontario forests, and found that the trees’ growth rings were narrower in more recent years, as atmospheric carbon dioxide increased. Comparison of the growth rings to carbon isotope ratios (which capture a tree’s response to temperature stress) suggested that the growth declines were due to less hospitable temperatures.

A large-scale historical study just out in Nature shows similar results for phytoplankton, microscopic photosynthetic organisms that form the base of ocean food chains. Working from historical records of ocean water transparency—phytoplankton makes water cloudy—going back to 1899, Boyce et al. found widespread declines in phytoplankton density [$a]. That’s bad news on multiple levels, implying that phytoplankton growth isn’t helping to absorb carbon dioxide, and that the oceans’ productivity is declining with its foundational food sources, not just from overfishing. (See also coverage of this result by the BBC and NPR.)

Earlier springs mean bigger marmots. Photo by Blake Matheson.

Now, the weird news:

The rule of thumb for plants’ response to climate change has been that they’ll respond to warmer temperatures by starting the growing season earlier. But a new survey of plant populations in Florida finds that as global warming progressed, most species flowered later. The authors suggest that this is because many Florida plant communities that are already adapted to warm conditions, and because climate change across much of Florida has meant not just warmer temperatures overall, but also greater seasonal variation in temperatures—areas where summer temperatures increased also tended to have decreasing winter temperatures. Faced with the possibility of more late frosts, Floridian plants are waiting till later in the spring to start flowering.

Another weird result of climate change received lots of press last week: a thirty-year study of yellow-bellied marmots in Colorado found that, as their alpine habitats grew warmer, the marmots grew bigger and more numerous [$a]. Warmer overall temperatures mean earlier spring thaws, so the marmots are emerging from hibernation earlier, have more time to grow and pack on fat reserves before hibernation in the fall, and can make more babies the next spring. Is this good or bad? Co-author Dan Blumstein’s answer to that question in an interview with NPR is worth quoting:

I don’t know if I’m worried as much as I’m intrigued by it and I want to continue following the story. … it’s only through these long-term studies that we can gain important insights into what’s happening, what’s happened and ultimately identify mechanisms through which we may be able to predict what might happen in the future.

Climate change is essentially a global gamble, with the function of ecological communities everywhere as the stakes. Even as we humans are unable to muster the will to stop it, we’re finding out daily how many changes are on the way as the planet warms.

References

Boyce, D., Lewis, M., & Worm, B. (2010). Global phytoplankton decline over the past century. Nature, 466 (7306), 591-6 DOI: 10.1038/nature09268

Ozgul, A., Childs, D., Oli, M., Armitage, K., Blumstein, D., Olson, L., Tuljapurkar, S., & Coulson, T. (2010). Coupled dynamics of body mass and population growth in response to environmental change. Nature, 466 (7305), 482-5 DOI: 10.1038/nature09210

Silva, L., Anand, M., & Leithead, M. (2010). Recent widespread tree growth decline despite increasing atmospheric CO2. PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011543

Von Holle, B., Wei, Y., & Nickerson, D. (2010). Climatic variability leads to later seasonal flowering of Floridian plants. PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011500

Before they were yucca moths

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This post was chosen as an Editor's Selection for ResearchBlogging.orgYuccas 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].

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Sex after dawn: Marriage and natural selection

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ResearchBlogging.orgThe book Sex at Dawn, by Christopher Ryan and Cacilda Jethá, has had a lot of press in the last month—it first popped up on my radar with Eric Michael Johnson’s review for SEED, and then it became unavoidable (for me, anyway) when Dan Savage devoted a whole column and podcast to it. The thesis of Sex at Dawn is that early humans were highly promiscuous, and that modern expectations of monogamy are probably not consistent with our biology. I haven’t read the book yet, but the discussion surrounding it has largely missed an important detail—human evolution didn’t stop when we invented agriculture.

In fact, we’ve evolved in response to agriculture. My capacity to digest milk proteins at age 28—most other mammals lose this ability as soon as they’re old enough for solid food—is the result of natural selection acting on my northern European ancestors. Sex at Dawn coauthor Christopher Ryan acknowledges exactly this, citing the same example, in a recent response to a question on his blog. I’m not aware of a study that documents human evolution in response to marriage customs, but conveniently enough, an article in the current issue of The American Naturalist does show that a population’s marital customs can shape its response to natural selection [$a].

Vintage postcard via chicks57.

The intensity and nature of natural selection often varies with age—it’s strongest on traits expressed prior to and during the period of life when most reproduction happens, and weaker on traits having to do with life after reproduction is mostly complete.

The new paper examines the effect of marriage on this relationship between age, reproductive activity, and the strength of natural selection. The authors are able to do this thanks to church records of births (via christenings), marriages, and deaths from four Finnish towns during the nineteenth century—a deep multigenerational dataset. The society described is probably as far away from the Sex at Dawn world of communal relationships within hunter-gatherer tribes as Western society has ever gotten—the births recorded are all in the context of monogamous marriages. How monogamous these marriages actually were is debatable; this was also a world before paternity testing. But the study follows women, who would probably have had less opportunity, and certainly had less social leeway, for affairs outside marriage.

Within that society, women’s reproductive success depended strongly on their husbands’ economic status, as approximated by whether or not they owned land. Women who married landowners married almost three years earlier, on average, than those who married non-landowners (between 24 and 25 years old, compared to 27). Women who married at an earlier age generally had more children survive to age 15, the paper’s benchmark for lifetime fitness—and this effect was stronger for women who married landowners.

Vintage wedding portrait via freeparking.

This meant that the intensity of potential natural selection acting on women in the study group peaked around their 30th birthday, declined slowly for around a decade, and then dropped off sharply. By comparison, an estimate of selection intensity based only the women’s probability of survival to a given age (i.e., without accounting for the need to marry before having children) just shows a steady decline with age. So marriage customs probably shaped the way natural selection could act on this population. (The comparison made, though, is a pretty facile one. I’d love to know what the intensity of selection looks like under other post-agricultural mating systems like, say, polygyny.)

Does that mean that these nineteenth-century Finns were evolving in response to the strictures of monogamy? Not necessarily. This study only estimates how strong selection would be on a trait relative to the age at which it’s expressed. That is, traits that reduced (or improved) a woman’s ability to bear children would be more strongly selected against (or favored), if they were expressed while she was between the ages of 30 and 40.

So the fact that marriage customs shape natural selection doesn’t mean that we’ve evolved to be better adapted to current marriage customs than we are to those of pre-agricultural hunter-gatherers. Marriage customs vary considerably among cultures, and over time—I don’t know of any culture that has maintained strict monogamy since the origin of agriculture. Even if a single culture did prefer monogamy that long, natural selection to adapt to that mating system would be working from a pool of genetic variation evolved from hundreds of generations of our earlier polyamorous lifestyle. It doesn’t matter how strongly natural selection would favor a perfectly monogamous person, if such a person doesn’t exist.

In other words, the key insight of Sex at Dawn—which is also a key insight of evolutionary biology in general—is right: What we can become is shaped by what we used to be. That’s certainly an important thing to keep in mind when considering a commitment that lasts till death do you part.

(For example, you might want to makes sure your significant other is of at least the same genus as you. I mean, talk about biological impediments.)

References

Gillespie, D., Lahdenperä, M., Russell, A., & Lummaa, V. (2010). Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity. The American Naturalist, 176 (2), 159-69 DOI: 10.1086/653668

When ecological opportunity knocks, does adaptive radiation answer?

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ResearchBlogging.orgOne of the most basic questions in evolutionary ecology is, “why are there more kinds of this kind of critter than that kind of critter?” As in, why are there more than twenty thousand species of orchids, but only one species of ginkgo? Why are there hundreds of thousands of species of beetles, but only four species of horseshoe crab? In a literature review just released online—and my first publication as lead author!—my coauthors and I assess the support for one hypothesis: that species multiply because of ecological opportunity.

Biologists interested in the origins of species diversity frequently focus on the phenomenon of adaptive radiation, the process by which a single species rapidly gives rise to many new species, each with different traits adapted to different lifestyles. Darwin’s finches, with their beaks shaped to suit to different foods [$a], are a classic case; the Anolis lizards of the Caribbean, which have repeatedly evolved into a handful of “ecomorphs” with different body sizes and shapes adapted to different perching locations [PDF], are another.

Why are there so many [insert taxon here]? Photos by Bill & Mark Bell (1 & 2), fturmog (3 & 4).

The two most influential theories of adaptive radiation—by G.G. Simpson and Dolph Schluter—have suggested that it results when a species encounters ecological opportunity. Ecological opportunity might be a newly-evolved trait, or a new habitat, or the extinction of a species’ competitors or predators. For instance, a butterfly might evolve a way to overcome the chemical defenses of an abundant plant species, or a plant introduced by humans to a new habitat might find that local pathogens aren’t as deadly to it as the ones in its native range. Ecological opportunities have the effect of granting access to new resources. We have pretty good evidence that this can allow individual populations to increase in number, and even evolve greater diversity—but is that enough to spur the rapid speciation that forms adaptive radiation?

Ecological opportunity ? adaptive radiation

We’re pretty sure about steps 1 and 3. We’re still trying to figure out step 2.

Readers in certain demographic groups may think this sounds like an underpants gnome problem. But it isn’t, exactly. The gnomes’ business model can’t get to from step 1 (collect underpants) to step 3 (profit) because they don’t have a step 2. Evolutionary ecologists, on the other hand, already have their step 3 in the phenomenon of adaptive radiation. Ecological opportunity looks like a good prospect for step 1 precisely because it suggests some plausible options for step 2.

When a population encounters ecological opportunity, the new habitat, new trait, or extinction of antagonists provides access to new resources, and relaxes natural selection on the population. This leads to three phenomena usually grouped together under the term ecological release

  • The population experiences density compensation—more individuals can live in a particular area, creating stronger competition within the population.
  • Because of this stronger competition within the population, or because there isn’t much competition from other species, members of the population venture into new habitats, or use new food resources.
  • The population becomes more diverse, either because of the relaxed selection, or because of competition-driven selection for using new habitat and new resources.

One or more of these three aspects of ecological release turn up whenever populations find new food resources, or escape predators and/or competitors. Density compensation has been widely observed in populations colonizing new habitats, especially islands; and experiments with sticklebacks and fruit flies [$a] suggest that the stronger competition resulting from density compensation can spur the population to become more diverse in its use of resources. Bacterial populations can even evolve different specialized forms—adaptive radiations in microcosm—when introduced to new food resources.

Anoles show signs of density compensation on Caribbean islands—is that the reason behind their diversification? (Pictured: Anolis oculatus.) Photo via WikiMedia Commons

But where’s the speciation?

However, the evolution of bigger, more diverse populations is not the same thing as the evolution of new species—and that’s what adaptive radiation is really all about. These changes resulting from ecological opportunity might directly promote speciation if stronger competition leads to disruptive natural selection. Similarly, the competition-driven incentive to colonize new habitats or exploit new food sources could expose some parts of the population to different forms of natural selection, eventually causing them to evolve into specialists on the new resources. Finally, even if speciation only happens when natural barriers cut off migration, maybe larger, more variable populations provide more diversity for vicariance events to divvy up.

This is all pretty speculative, though. We still don’t know how often—or how rarely—divergent natural selection contributes to making new species. One way to deal with this is to approach the question from the other direction: look backward at the history of existing species, rather than following what happens to populations immediately after ecological release.

A backward-looking approach might use statistical analyses of the evolutionary relationships between living things to identify points in time when species formed unusually fast, and try to identify the cause. Some of my coauthors from the review paper recently published an analysis of the evolutionary tree connecting all vertebrates, and found that speciation rates increased around the origins of the largest group of birds, a large portion of the lizards and snakes, and non-marsupial mammals, among others.

This is very much a starting point, but maybe by complementing similar studies with research on populations currently evolving in response to ecological opportunity, biologists can work our way closer to understanding the origins of the endless and beautiful forms of life on Earth.

References

Alfaro, M., Santini, F., Brock, C., Alamillo, H., Dornburg, A., Rabosky, D., Carnevale, G., & Harmon, L. (2009). Nine exceptional radiations plus high turnover explain species diversity in jawed vertebrates. Proc. Nat. Acad. Sci. USA, 106 (32), 13410-4 DOI: 10.1073/pnas.0811087106

Bolnick, D. (2001). Intraspecific competition favours niche width expansion in Drosophila melanogaster. Nature, 410 (6827), 463-6 DOI: 10.1038/35068555

Blumenthal, D., Mitchell, C., Pysek, P., & Jarosik, V. (2009). Synergy between pathogen release and resource availability in plant invasion. Proc. Nat. Acad. Sci. USA, 106 (19), 7899-904 DOI: 10.1073/pnas.0812607106

Grant, B., & Grant, P. (1989). Natural selection in a population of Darwin’s finches. The American Naturalist, 133 (3), 377-93 DOI: 10.1086/284924

Kassen, R. (2009). Toward a general theory of adaptive radiation: Insights from microbial experimental evolution. Annals New York Acad. Sci., 1168 (1), 3-22 DOI: 10.1111/j.1749-6632.2009.04574.x

Losos, J. (1990). Ecomorphology, performance capability, and scaling of West Indian Anolis lizards: An evolutionary analysis. Ecological Monographs, 60 (3), 369-88 DOI: 10.2307/1943062

Schluter, D. 2000. The Ecology of Adaptive Radiation. Oxford University Press. Google Books.

Simpson, G.G. 1949. Tempo and Mode in Evolution. Columbia University Press. Google Books

Svanbäck, R., & Bolnick, D. (2007). Intraspecific competition drives increased resource use diversity within a natural population. Proc. Royal Soc. B, 274 (1611), 839-44 DOI: 10.1098/rspb.2006.0198

Wheat, C., Vogel, H., Wittstock, U., Braby, M., Underwood, D., & Mitchell-Olds, T. (2007). The genetic basis of a plant insect coevolutionary key innovation. Proc. Nat. Acad. Sci. USA, 104 (51), 20427-31 DOI: 10.1073/pnas.0706229104

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 DOI: 10.1111/j.1420-9101.2010.02029.x

#evol2010 day 4: In which the race is not always to the swift, and giving up on sex isn’t a dead end

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Updated, 2010.06.30: Publish in haste, revise at leisure. I’ve gone back and added some links to original papers mentioned in the talks, and a note on another talk I meant to include (the first in the list, now).
And again, 2010.07.02: Added a specific link to the EvoDevoGeno audiocast, and to Vincent Calcagno’s professional page.

ResearchBlogging.orgThe final day of Evolution 2010 featured a fantastic series of talks in the ASN Young Investigators Symposium, and marked the premiere of the iEvoBio sister conference, which ran concurrently today. Perhaps not surprisingly, the #ievobio tag quickly outran the #evol2010 tag on Twitter.

I’m ending the conference with a final wrap-up audiocast with the crew from Evolution, Development, and Genomics, and then hopefully a quick run before the closing banquet.

A western bluebird arrives at its nest box. Photo by kevincole.

Primary literature referenced

Calcagno, V., Dubosclard, M., & de Mazancourt, C. (2010). Rapid exploiter‐victim coevolution: The race is not always to the swift. The American Naturalist DOI: 10.1086/653665

Duckworth, R., & Kruuk, L. (2009). Evolution of genetic integration between dispersal and colonization ability in a bird. Evolution, 63 (4), 968-77 DOI: 10.1111/j.1558-5646.2009.00625.x

Johnson, M., Smith, S., & Rausher, M. (2009). Plant sex and the evolution of plant defenses against herbivores. Proc. Nat. Acad. Sci. USA, 106 (43), 18079-84 DOI: 10.1073/pnas.0904695106

McGlothlin, J., Jawor, J., & Ketterson, E. (2007). Natural variation in a testosterone‐mediated trade‐off between mating effort and parental effort. The American Naturalist, 170 (6), 864-75 DOI: 10.1086/522838

#evol2010 day 3: In which butterflies self-medicate and Orr conjectures

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ResearchBlogging.orgHow do you know it’s getting to be the end of the Evolution 2010 meetings? Because I didn’t get to this until this morning, in the back rows of the SSE symposium on evolutionary prediction. But the third day of the meetings were great, with cool natural history and a great address by SSE president H. Allen Orr.

And don’t forget to check out the daily wrap-up audiocast over at Evolution, Development, and Genomics, which was just endorsed by none other than Carl Zimmer.

A monarch butterfly. Photo by mikebaird.
  • Thierry Lefevre presented evidence that female monarch butterflies infected with a microbial parasite lay their eggs on host plants with more toxins that can fight the parasite.
  • Susan Dudley presented new work on kin recognition in the small annual plant Cakile edentula, in which the plants grow less aggressively if planted next to close relatives.
  • Ian Pearse presented evidence that introduced oak species were more likely to be attacked by a native herbivore if they were more closely related to native oak species.
  • Finally, H. Allen Orr capped the day with an SSE presidential address that focused on what we know—and what we don’t—about how reproductive isolation evolves and creates new species. Orr concluded with three conjectures:
    • Extrinsic postzygotic isolation is usually due to adaptation to ecological conditions,
    • Intrinsic postzygotic isolation is usually due to adaptation to the intrinsic environment within the genome, and
    • Prezygotic isolation is usually due to sexual selection.

    The idea, of course, is to collect the data to test these conjectures. But I’d say these make pretty good sense based on what we already know.

Edit, 2010.06.30: Swapped the original photo for one that actually depicts a monarch butterfly, as discussed in the comments (thanks, Julie!).

Primary literature referenced

Dudley, S., & File, A. (2007). Kin recognition in an annual plant. Biology Letters, 3 (4), 435-8 DOI: 10.1098/rsbl.2007.0232

With conspecifics like these, who needs predators?

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Update, 19 July 2011: More than a year after this study was published, some important objections have been made about very basic assumptions of the experiment presented. Also, I’ve fixed the first link the original article.

ResearchBlogging.orgThere’s something special about islands. After moving to islands, plants adapted to rocky outcrops evolve to grow in rainforests and alpine meadows, and finches evolve to behave like woodpeckers. But why? Islands contain new food sources and habitats, they often lack predators, and they can provide more geographic barriers to generate reproductive isolation—to name just a few possibilities. A newly published ecological experiment now provides evidence that one group of island lizards diversfied because islands are crowded [$a].

There’s something about islands. Photo by Storm Crypt.

Diversification on islands may be related to density compensation, the frequently-observed principle that islands often support fewer species than mainland sites of the same area, but contain more individuals of each species—that is, island populations are usually at higher density than their mainland counterparts [$a]. Density compensation seems to arise both from lack of predators on islands, and because island populations have fewer competitor species. This may mean that, compared to mainland populations, island populations are under weaker natural selection from other species, and stronger selection from competition with other members of their own species.

A somewhat strained analogy

How could that difference in selective regimes spur diversification? Imagine two towns, one surrounded by other settlements, the other on its own in the middle of the wilderness. The town in densely-populated country is probably best off doing one thing well—to have, say, most of its inhabitants working at a factory making (to pick a product at random) sausages for trade with other towns. People living in this first town might want to start up a factory making a different product, but odds are good there’s strong competition from another town nearby, so it’s hard to get the new business off the ground—it’s really just better to invest in the existing factory.

On the other hand, the inhabitants of the town in a lightly-populated district might need more products made locally because it costs too much to import. A businesswoman in the isolated town is probably better off starting a factory that makes a product no-one is making locally—if sausages are already accounted for, there might be a market for (to pick another product at random) pharmeceuticals.

In this scenario, competition from outside exerts economic pressure to do one thing well; competition from within exerts pressure to do many different things. Both kinds of competition are present in each town, but outside competition is stronger in the town surrounded by other towns, and competition from within is stronger in the isolated town.

Anole vs. anole

The density compensation hypothesis proposes that something similar happens on islands. With fewer predators or competitor species, island populations are able to maintain higher densities of individuals. That increased density means that competition within the species becomse stronger, creating natural selection that favors individuals who can use new food resources or live in new habitats.

Density compensation seems likely to be responsible for the diversification of anole lizards on the islands of the Caribbean. In the course of colonizing Caribbean islands, anoles have repeatedly evolved into a handful of different niche specialists [PDF] called “ecomorphs,” ranging from “giant” species that live high in the forest canopy, to small species that can navigate and perch on fine twigs, and intermediate species that live on and around tree trunks. Anoles on the mainland of Central America are no less diverse than their Caribbean congeners, but they haven’t evolved mini-radiations of replicated ecomorphs—and their population densities are much lower than those of the island species.

Anolis sagrei, the brown anole. Photo from WikiMedia Commons.

If release from predators, and the ensuing increase in population density, drove the diversification of island anoles, then we might expect that natural selection from predators has less effect on the traits that differentiate the anole ecomorphs than natural selection from other anoles. Testing that hypothesis experimentally is ambitious to say the least, but that’s what the new study attempts to do.

The authors, Calsbeek and Cox, identified six very small, similar islands off the coast of the Bahamian island Greater Exuma, and introduced varying numbers of brown anoles (Anolis sagrei) onto them at the beginning of the summer. The islands were small enough that Calsbeek and Cox could selectively exclude birds by enclosing the islands in netting; by introducing predatory snakes onto some islands, they could then generate three selective regimes: no predators, birds only, and birds plus snakes. Before introducing them into these experimental setups, the authors measured each anole’s body size, hind-leg length, and running stamina, and marked each lizard so they could estimate selection acting on the three traits based on which lizards survived to be recaptured at the end of the season. (The experiments were carried out over two years, with both years’ results compiled at the end.)

The results suggest that competition makes a bigger difference for the experimental populations than predation—while the strength of natural selection acting on all three traits increased with the anoles’ population density, it didn’t change when predators were allowed access to the islands. If the levels of predation simulated on the micro-islands accurately reflect what anoles experience throughout the Caribbean, then the result is, I’d say, pretty good evidence that competition is the most important evolutionary force acting on island anoles.

I should note that, although Calsbeek and Cox’s raw result is suggestive, it’s not clear that their sample size is big enough to support all the statistical analyses they perform on the data. On balance, I think they deserve a lot of credit just for tackling this question experimentally.

References

Calsbeek, R., & Cox, R. (2010). Experimentally assessing the relative importance of predation and competition as agents of selection. Nature, 465 (7298), 613-6 DOI: 10.1038/nature09020

Givnish, T., Millam, K., Mast, A., Paterson, T., Theim, T., Hipp, A., Henss, J., Smith, J., Wood, K., & Sytsma, K. (2009). Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae). Proceedings of the Royal Society B: Biological Sciences, 276 (1656), 407-16 DOI: 10.1098/rspb.2008.1204

Losos, J. (1990). Ecomorphology, performance capability, and scaling of West Indian Anolis lizards: an evolutionary analysis. Ecological Monographs, 60 (3), 369-88 DOI: 10.2307/1943062

MacArthur, R., Diamond, J., & Karr, J. (1972). Density compensation in island faunas. Ecology, 53 (2) DOI: 10.2307/1934090

Pinto, G., Mahler, D., Harmon, L., & Losos, J. (2008). Testing the island effect in adaptive radiation: rates and patterns of morphological diversification in Caribbean and mainland Anolis lizards. Proceedings of the Royal Society B: Biological Sciences, 275 (1652), 2749-57 DOI: 10.1098/rspb.2008.0686

Not just babble: Cooperating birds talk it through

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ResearchBlogging.orgFor cooperation to work, everyone involved needs to know what the others are willing to contribute in order to decide what she will contribute. You might think that only humans can achieve that kind of back-and-forth negotiation, but a paper recently published online by Proceedings of the Royal Society suggests otherwise. In it, ornithologists decode the negotiations [$a] that allow sociable birds to share the task of watching for predators.

The southern pied babbler, Turdoides bicolor. Photo by Blake Matheson.

Pied babblers (Turdoides bicolor) are sociable South African songbirds, which live and forage for food in groups. During foraging, some adult babblers act as sentinels, perching above the ground to scan for predators, and alerting the rest of the foragers if any danger shows up. Sentinel behavior is cooperative—sentinels free the rest of the group to concentrate on feeding, but sentinels themselves cannot forage. The study’s authors, Bell et al. hypothesized that sentinels might communicate how long they’re willing to stand watch to the rest of the group, so as to prompt new birds to take up watch and given the current sentinels a break to feed.

The authors first established that how hungry a babbler is determines how long he or she is willing to stand watch, which they did by feeding the birds with meal worms immediately after they concluded a period of sentinel duty. Babblers receiving ten worms returned to duty faster than those receiving just one, and stayed on duty longer. Further feeding experiments and observations established that both foragers and sentinels called to each other less frequently when they were well fed, that sentinels called more frequently the longer they stayed on watch, and that sentinels who ultimately stayed on watch the longest also called the least frequently in their first minute on watch.

So call frequency is the babblers’ signal for how badly they want to forage—foragers hearing higher-frequency calls from sentinels should take them as a call for relief; and sentinels hearing lower-frequency calls from foragers should take them as permission to leave the watch and start foraging. To test this hypothesis, the authors played recorded calls to foragers and sentinels, and found that the birds responded as I’ve just described. The apparent babble of the babblers, then, is actually a perpetual negotiation about who should be on sentinel duty—sentinels complaining when they get hungry, and foragers telling the sentinels, “Not yet! I just need to catch a few more worms.”

Reference

Bell, M., Radford, A., Smith, R., Thompson, A., & Ridley, A. (2010). Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird. Proc. Royal Soc. B DOI: 10.1098/rspb.2010.0643