Category Archives: science

Female birds stop singing when they move north

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ResearchBlogging.orgA study in this week’s Proceedings of the Royal Society B suggests that the sexually dimorphic pattern of birdsong we’re used to in temperate latitudes — with males singing elaborately and females usually not — evolves because female birds stop singing when their species move to more northerly latitudes [$-a]. Why this is, however, remains an open question.

The study’s authors reconstruct the evolution of home range (temperate versus tropical) and sexual song dimorphism (both sexes singing versus only males singing) in the New World blackbirds, the family that includes orioles, cowbirds, and red-winged blackbirds (pictured). The reconstruction reveals a strongly significant association between the evolution of male-only singing and transitions from tropical to temperate breeding ranges. The authors discuss this transition in a few key groups, including North American red-winged blackbirds (Agelaius phoeniceus) and their sister species, the Cuban red-shouldered blackbird (A. assimilis):

Females of A. assimilis are nearly indistinguishable from conspecific males in song structure and song rate and are also similar in plumage and body size … whereas females of A. phoeniceus differ considerably from conspeicific males in these traits …. it is clear that the changes in female song and plumage must have occurred quite rapidly. [In-text citations omitted.]

As clear as the observed pattern is, however, there doesn’t seem to be a good general explanation for it. The authors point to cases where female singing is lost within tropical-breeding lineages, which might help disentangle the effects of latitude and other evolutionary forces generating the observed pattern. In these cases, loss of female song is associated with colonial nesting and polygynous breeding, whereas singing by both sexes is associated with year-round pairing.

The temperate-breeding blackbirds tend to be migratory, with males often arriving at the breeding range ahead of females to establish nest sites and territories. In these cases singing by the males serves to attract females and to announce ownership of territory. Could that migration-induced division of labor lead females to give up singing? I’m just an amateur birder, but it sounds plausible to me.

Reference

Price, J., Lanyon, S., & Omland, K. (2009). Losses of female song with changes from tropical to temperate breeding in the New World blackbirds Proceedings of the Royal Society B: Biological Sciences, 276 (1664), 1971-80 DOI: 10.1098/rspb.2008.1626

Talking systematics

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Over at dechronization, Luke Harmon has started a series of interviews with leading systematic biologists. The first two are with Jack Sullivan, the editor-in-chief of Systematic Biology, and Joe Felsenstein, widely considered the godfather of modern phylogenetics. They’re both well worth reading, and excellent examples of why scientists should blog: in what other venue would you see personal interviews with either of these guys?

(Full disclosure: Jack is on my doctoral committee, and I’ve audited his systematics class, which uses Felsenstein’s authoritative textbook.)

Un-bear-able (ha) predation creates variable natural selection

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ResearchBlogging.orgNatural selection is a fact of life. As Steven Jay Gould put it, it’s an “inescapable conclusion” arising from the “undeniable facts” that (1) populations of living things have inheritable variation in many traits; and (2) produce a surplus of offspring. But populations often experience selection from multiple sources, and in conflicting directions. The cover article for this month’s issue of Evolution suggests that bears may be creating ongoing selection in wild salmon populations, but the strength, and outcome, of that selection varies from stream to stream [$-a].

Selective agent at work (Flickr: Dr.DeNo)

Salmon are famously anadromous — they hatch in freshwater streams and swim out to sea, only to return to the stream of their birth to spawn before they die. Male salmon are generally better off if they’re bigger, both to maximize stored energy for the return to their spawning site, and to better compete for mates when they arrive. Natural selection for larger bodies, however, is checked by bears, who preferentially target large, fatty fish. Yet bear predation varies from stream to stream: in narrower streams, where salmon are easier to catch, bears can fill up on big, newly-arrived fish; but in wide streams, bigger fish can more easily evade bears, so bears tend to target older, weaker fish instead.

Continue reading →

Ants trim trees for more living space

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ResearchBlogging.orgIn the natural world, cooperative interactions evolve not as expressions of altruism, but as careful “negotiations” between interacting species. Each player may benefit from the relationship, but each stands to benefit from trying to cheat the other. In this month’s issue of The American Naturalist, we see a prime example: mutualistic ants sterilize their host plants [$-a] to get the most out of the interaction.



Cordia nodosa flowers (top)
and ant domatia (bottom)
Photos by Russian_in_Brazil.

The ant species Allomerus octoarticulatus is part of a classic protection mutualism with the tropical tree Cordia nodosa, in which the plant grows structures called domatia that provide shelter for a colony of ants, and nutrient rich “food bodies” for the ants to feed on. The ants, in turn, patrol the plant and drive off herbivores. This mutually beneficial relationship also sets up a conflict of interest. The tree must divide its resources between providing food and shelter for its resident ant colony — growing new domatia and fruiting bodies — and its own reproductive efforts — growing flowers and fruit. The ants, naturally, would prefer for the host tree to spend as much energy as possible on them.

Indeed, Allomerus octoarticulatus has been observed killing the flowers of its host trees. This is what led the new paper’s author, Megan Frederickson, to conduct a simple experiment on C. nodosa, asking whether such pruning prompts the tree to grow more domatia. She experimentally removed flowers from trees occupied by a species of ants that don’t engage in flower pruning to see if pruned trees grew more domatia — and pruned trees grew more domatia over the course of four months than trees that were allowed to flower and produce fruit.

Ant-hosting plants need not be totally subject to the whims of their protectors, however — this kind of regulation works both ways. A study published last year in Science found that ant-hosting Acacia trees cut back on support for their resident ant colonies [$-a] when herbivores are removed and ant protection is no longer needed. (I wrote about this study back when it was released.) It seems likely that flower-pruning ants are exerting strong selection on Cordia nodosa to circumvent this behavior — a new tree variant that can overcome pruning, or make life uncomfortable for pruning ants, should have a large selective advantage.

In the absence of such a mutation, as Frederickson points out, Allomerus octoarticulatus is creating a tragedy of the commons by reducing the long-term viability of its host tree’s populations in exchange for the short-term benefit of more living space. As it stands, Cordia nodosa can only reproduce when it hosts non-pruning ant species, which are a minority in the populations Frederickson studied. Only time, and further study, can determine whether this mutualism might break down altogether.

References

Frederickson, M. (2009). Conflict over reproduction in an ant-plant symbiosis: Why Allomerus octoarticulatus ants sterilize Cordia nodosa trees. The American Naturalist, 173 (5), 675-81 DOI: 10.1086/597608

Palmer, T., Stanton, M., Young, T., Goheen, J., Pringle, R., & Karban, R. (2008). Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna Science, 319 (5860), 192-5 DOI: 10.1126/science.1151579

Evolution-proof insecticide?

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ResearchBlogging.orgIn this week’s issue of PLoS Biology, an essay describes the perfect means for controlling malaria-carrying mosquitoes: an “evolution-proof insecticide.” By taking advantage of the life history traits of both mosquitoes and the malaria parasite, Read et al. argue it should be possible to create an insecticide that will cut malaria transmission without selecting for resistance in the mosquitoes.

Malaria remains a major public health problem in much of the world – according the World Health Organization, a child dies of the disease every 30 seconds, and the cost of malaria may cut economic growth by as much as 1.3% in countries with high infection rates. In the absence of a vaccine, the best approach for malaria management is to control the mosquitoes that transmit the malaria parasite. This is usually done with insecticides, but these have a limited useful lifespan, as they create strong selective pressure for mosquito populations to evolve resistance.


Photo by LoreleiRanveig.

As Read et al. point out, it’s not that we need to kill off mosquitoes as such; we just need to stop them from transmitting malaria. If this can be accomplished without strongly reducing the mosquitoes’ fitness, it would reduce or eliminate selection for resistance. Malaria typically needs a long time to incubate inside a mosquito before it becomes transmissible to humans, and, in what Read et al. call “one of the great ironies of malaria,” this incubation time is longer than most mosquitoes live. That is, the mosquitoes who successfully transmit malaria are the small proportion of the population who live long enough to incubate the parasite.

Here’s where evolutionary biology interacts with the life history of malaria parasites in a highly convenient way: an insecticide that selectively targets older mosquitoes will have a smaller impact on the mosquito population’s fitness. This is because most of a female mosquito’s fitness – the total number of offspring she produces – is concentrated in her first one or two egg-laying cycles. Her fitness can increase if she survives to complete more cycles, but it’s pretty rare that she does. From natural selection’s point of view, that first of eggs counts much more than possible future batches, because they’re not very likely.

For that hypothetical female mosquito to transmit malaria, she has to bite an infected human in the course of feeding to fuel one egg-laying cycle, then incubate the malaria parasites for an additional two to six cycles. Therefore, say Read et al., an insecticide that doesn’t harm mosquitoes until they complete their first few egg-laying cycles is the “evolution-proof” solution – the only offspring it “steals” from the affected mosquitoes were pretty improbable anyway, and it prevents the malaria parasites from incubating long enough to successfully infect a new human host.

As it happens, the evolution-proof insecticide might not be a chemical agent, but a biological one. A paper I discussed back in January suggested that infecting malaria-carrying mosquitoes with the parasitic Wolbachia bacterium could control mosquito populations [$-a] by, yes, reducing their total lifespan to something less than the malaria parasite’s incubation time. In short, it looks like the goal of a malaria-free world is not as improbable as it used to be.

References

McMeniman, C., Lane, R., Cass, B., Fong, A., Sidhu, M., Wang, Y., & O’Neill, S. (2009). Stable introduction of a life-shortening Wolbachia infection into the mosquito Aedes aegypti Science, 323 (5910), 141-144 DOI: 10.1126/science.1165326

Read, A., Lynch, P., & Thomas, M. (2009). How to make evolution-proof insecticides for malaria control PLoS Biology, 7 (4) DOI: 10.1371/journal.pbio.1000058

The easter-egg hunt for scholarly PDFs

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The Open Source Paleontologist has few good suggestions on digging up electronic copies of scholarly papers when you’re outside the walled garden of institutional subscriptions. Google Scholar is getting better at identifying online PDF copies related to its results, but given that it doesn’t seem to know about my doctoral advisor’s considerable archive (he posts PDFs of all his pubs, as more and more academics do), I have to wonder what it’s missing. I’ve started going there to check for PDFs before resorting to an [$-a] tag, though.

Speciation changes ecosystem

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ResearchBlogging.orgWe know that ecosystem processes can act on organisms to help create reproductive isolation and speciation – now, a new paper released online in advance of publication in Nature shows that speciation can change the ecosystem [$-a].

The study’s authors are a group of University of British Columbia scientists, including Luke Harmon (who occasionally blogs at Dechronization) and Simone Des Roches, who have since come to my department at UI as a faculty member and doctoral student, respectively. They focus on the case of ecological speciation in sticklebacks (Gasterosteus aculeatus), which have repeatedly into evolved two reproductively isolated, ecologically different forms [$-a] after colonizing North American freshwater lakes from the ocean about 10,000 years ago. One of the two forms is “limnetic,” living in open water near the surface and feeding on plankton; the other is “benthic,” living on lake bottoms and feeding on invertebrates.


A stickleback
Photo by frequency.

Harmon et al. reasoned that the presence of these two different fish must have a substantial effect on lake food webs. To test this hypothesis, they set up mesocosms – big cattle tanks seeded with a standard mix of sediment, plankton, and invertebrates – and introduced either (1) sticklebacks of the “generalized” type ancestral to the benthic and limnetic types, (2) either the benthic or limnetic type alone, or (3) both the benthic and limnetic types together. They found that the fish species present in the mesocosm strongly affected the plankton species diversity – limnetic-type nearly eliminated one of their preferred prey species – and on measures of total ecosystem productivity and metabolic activity.

Perhaps the most important effect was on dissolved organic content (DOC) and light transmission in the water column. Mesocosms containing both fish types had about the same amount of (non-living) organic material as those containing the generalist ancestor, but the two-species treatment changed the DOC composition to make the water column more transparent to light. In a real lake, this effect could significantly change the productivity and composition of the aquatic plant community, which would in turn reshape the rest of the food web.

The buzzword for this phenomenon is “ecosystem engineering,” which the ESA blog puts front-and-center in its discussion of this paper. I think Harmon et al.‘s result is most interesting as the closing of a feedback loop between the ecosystem and a population undergoing speciation. It’s evidence that a speciation event can actually alter the conditions that created it in the first place – which might prevent future speciation events, or create opportunities for new ones.

Reference

Harmon, L., B. Matthews, S. Des Roches, J. Chase, J. Shurin, & D. Schluter (2009). Evolutionary diversification in stickleback affects ecosystem functioning Nature DOI: 10.1038/nature07974

Vines, T., & D. Schluter (2006). Strong assortative mating between allopatric sticklebacks as a by-product of adaptation to different environments Proc. R. Soc. B, 273 (1589), 911-6 DOI: 10.1098/rspb.2005.3387

Carnival of Evolution #10 at The Oyster’s Garter

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The tenth issue of the The Carnival of Evolution is now live at The Oyster’s Garter, complete with a whimsical framing narrative. Topics range from the relationship between stress and testosterone levels to an essay on the species problem that complements my own contribution, to the suprising usefulness of half a wing.

That “mystery of mysteries”: What makes a species?

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ResearchBlogging.orgIn a special issue of Philosophical Transactions of the Royal Society on speciation, James Mallet argues that the Biological Species Concept is at odds with Charles Darwin’s original ideas about what a species is – and that current research supports Darwin [$-a].

When The Origin of Species was first published, biologists mostly thought species were easy to recognize – they looked different from each other, and they couldn’t successfully interbreed with each other. This view was a problem for Darwin’s ideas about gradual evolution by natural selection, since gradual divergence shouldn’t give rise to nice, discrete species. In fact, as Darwin argued, different groups of organisms exhibit a whole spectrum of reproductive isolation, from complete interfertility to total isolation – and the degree of isolation is not easy to predict based on how similar organisms look. In Darwin’s description, species are just labels that humans put on clusters of similar-looking organisms.

By the mid-Twentieth Century, evolutionary biologists favored what is commonly called the Biological Species Concept (BSC), defining species as non-interbreeding populations of living things. Research on speciation has accordingly focused on the ways that evolution creates reproductive isolation between populations. Mallet argues that this amounts to an abandonment of Darwin’s insights, and that by focusing on isolating mechanisms, biologists have returned to viewing species as distinct, “real” entities, missing much of the evolutionary process as a result.

I’m not sure I believe the distinction that Mallet makes between Darwin’s description of species and the BSC; they seem to me more different in their emphasis than in their fundamentals. Darwin was interested in demonstrating that species arise gradually, as accidents of adaptation to different environments – and, as Mallet says, he was trying to overcome the then-predominant view that species were real, discrete entities instead of the names that humans assign to clusters of similar organisms. Research motivated by the BSC generally takes this view as well, but it’s interested in the processes that create such clusters, and can prevent them from merging into nearby clusters by interbreeding.


Two types of Joshua tree
Photo by jby.

Research on the evolution of isolating mechanisms necessarily focuses on cases where isolation is incomplete, somewhere between complete speciation and free interbreeding. A prime example is my lab’s research on the two pollinator-associated types of Joshua tree, Yucca brevifolia. It’s not clear that the two types are reproductively isolated – preliminary genetic data suggests they’re not [PDF] – even though they’re pollinated by different moth species, and classified as separate subspecies, the taller Y. brevifolia brevifolia and the short, bushy Y. brevifolia jaegeriana. They may be on the way to becoming different species, but they’re not there yet. Two other examples out of the endless forms available: marine snails that choose mates by their slime trails, and wildflowers that would interbreed if only they could survive each other’s habitat.

As Mallet concludes in the more empirical part of his review, this is what we see across the diversity of life: a continuum of reproductive isolation between populations, not a granular world of neatly divided, obviously different species. Rather than over-simplifying this reality, the Biological Species Concept gives us a framework through which to understand it.

References

Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. First ed. London: John Murray. Full text on Google Books.

Mallet, J. (2008). Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation Phil. Trans. R. Soc. B, 363 (1506), 2971-86 DOI: 10.1098/rstb.2008.0081

Smith, C., W. Godsoe, S. Tank, J. Yoder, & O. Pellmyr (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

The ever-expanding science blogosphere

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Just before I left for the field, I happened to see a familiar-looking article title in the ResearchBlogging.org feed, associated with a familiar-sounding new blog. Turns out, it was familiar for good reason: Coevolvers is the newly-launched blog for the Palouse Coevolution Study Group, a journal club of UI and WSU scientists who study the ecology and evolution of species interactions.


Photo from Coevolvers.

I’ve been involved since I started grad school here, and was lucky enough to be able to contribute (a very little bit) to the group’s 2007 review on studying geographic mosaics of coevolving species, which is freely available online. The blog will never be able to capture the club’s, um, robust give-and-take interactions, but it’s a great way to see what we’re reading and what we think about it.