Tag Archives: Research Blogging

When the going gets tough, C. elegans gets sexy

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ResearchBlogging.orgThe trouble with sex, from an evolutionary perspective, is that it’s expensive. Not just in terms of the efforts a sexually-reproducing organism has to go through to secure a mate; every offspring produced by sexual reproduction bears half the genome of each of its parents, compared to an asexual offspring, which bears a complete copy of its only parent’s genome. So, in terms of natural selection, an asexual critter gains twice as much reproductive fitness for each offspring it produces — asexual critters should overrun sexual competitors.


C. elegans tagged with gfp.
Photo by derPlau.

And yet they don’t. Sex is widespread in the animal kingdom, and common in the plant kingdom (although many plants can switch between sexual and asexual reproductive strategies). Many explanations have been proposed for this quandary; most of them have to do with the idea that sometimes it’s useful to mix your genome with someone else’s. The current front-runner hypothesis is that sex basically helps to separate useful genes from damaging ones [PDF], making sexual offspring more fit, on average. A different (but not mutually exclusive) possibility is that by mixing up genomes, sex can help generate the genetic variation necessary for a population to evolve in response to environmental stress. This might explain a discovery reported in this month’s issue of Evolution: that stressful conditions trigger the normally hermaphroditic nematode Caenorhabditis elegans to begin reproducing sexually [$-a].

The study’s authors subjected three experimental lineages of C. elegans to stress — starvation — triggering the worms to produce semi-dormant larvae called “dauer.” They then relieved the stress by transferring the population to a new food source. Some experimental treatments were kept well-fed after one period of dauer; others were repeatedly starved. Two of the three experimental lines responded to repeated episodes of dauer by producing male offspring instead of hermaphrodites.

Some of this effect was due to males’ better ability to survive dauer state than hermaphrodites. A large portion was because hermaphrodites became more likely to mate with males (with a possibility to produce male offspring) following dauer, though. This kind of facultative sex takes the best of asexual and sexual reproduction — the twofold fitness benefit of asexual reproduction most of the time; and the improved response to natural selection associated with sex in stressful conditions, when it’s needed most.

References

Keightley, P., & Otto, S. (2006). Interference among deleterious mutations favours sex and recombination in finite populations Nature, 443 (7107), 89-92 DOI: 10.1038/nature05049

Morran, L., Cappy, B., Anderson, J., & Phillips, P. (2009). Sexual partners for the stressed: Facultative outcrossing in the self-fertilizing nematode Caenohabditis elegans.
Evolution, 63 (6), 1473-82 DOI: 10.1111/j.1558-5646.2009.00652.x

Getting away from it all: Why are invasive species invasive?

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ResearchBlogging.orgWhen humans move from place to place, we almost always bring other organisms with us. Sometimes it’s intentional — domestic animals carried along with Polynesian colonists, for instance. Just as often, it’s accidental, as with mice stowing away on Viking longships. A lot of these introduced species have done so well in their new habitats that they become invasive, outcompeting natives and disrupting local ecosystem processes. But the species that go crazy-invasive — the cane toads and the purple loosestrife — are probably only the very successful subset of the species that hitch rides in cargo holds and ballast tanks. What sets the dangerously successful invasive species apart from others?


Purple loosestrife, introduced
from Europe, now common in East
Coast wetlands. Photo by Muffet.

A new dataset published in last week’s PNAS suggests that it may be an interaction between available resources in a new habitat and a lack of compatible pathogens [$-a]. This is an amalgam of two hypothesized causes for successful invasion: access to new resources, and escape from antagonistic species. Focusing on European plant species that have successfully invaded North America, the authors, Blumenthal et al., assembled records of viral and fungal infections on each plant species in its native range, and in North America. They classified the plant species based on the habitats each occupies — wet vs. dry, nitrogen-rich vs. -poor — and on whether the plants tended to grow slowly or rapidly. This is because plant species adapted to rich, wet environments are generally thought to evolve fewer defenses against infection and herbivores; they can “afford” to grow new tissue instead of fight to keep it.


Garlic mustard, another invasive
from Europe. Photo by clspeace.

If resource availability interacts with freedom from infectious agents to spur a successful invasion, then invasive plants adapted to rich conditions should tend to host more pathogens in their home ranges than they do in their introduced range; and this difference should be less pronounced in invasive plants adapted to dry, resource-poor conditions. This is exactly what the analysis found — plants adapted to richer habitats saw a larger reduction in the number of pathogen species attacking them in their new ranges than plants adapted to less-productive conditions.

This is a valuable result for its basic application — helping to predict which introduced species are likely to become invasive, and target them for eradication efforts before they become well-established. But it also provides us with an insight into how evolution works. Many authors, particularly G.G. Simpson and Dolph Schluter, have described ecological conditions that set the stage for adaptive radiation — the rapid diversification of a lineage into many species — which sound a lot like the “ecological release” that invasive species seem to experience.

Rapid evolutionary diversification may be triggered by the evolution of a key innovation; by colonization of a new, empty habitat; or the removal of antagonistic species (usually by their extinction). These three classes of conditions are closely related, and they can be mimicked, or even replicated, when humans move species to new habitats [$-a]. Blumenthal et al. suggest, for instance, that species invasions entail both colonization of a new habitat and escape from pathogens. This is a broad observation; a good next step would be to directly compare natural selection acting on invasive plants in their native and introduced ranges. Through day-to-day processes like this, the specific ecology of a species can ultimately shape its evolutionary fate.

Reference

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

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification. Trends Ecol. & Evol., 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017

Seed dispersal by ants: A lousy way to travel, a good way to diversify

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ResearchBlogging.orgNew in the always open-access PLoS One: turns out that a great way to make new species, if you’re a plant, is to have your seeds dispersed by ants. This is because ants aren’t very good at seed dispersal.

Seed dispersal by ants, or myrmecochory, works very much like dispersal by fruit-eating birds and mammals: ant-dispersed seeds typically have a fatty attachment, called an elaiosome, that looks tasty to ants. Ants collect elaiosome-bearing seeds, bring them back to their nest, pry off the tasty bit, and then discard the rest of the seed. This leaves the seed safely underground in an ant-midden, ready to germinate — a great way to dodge seed-eating critters and avoid competition from its parent plant and siblings [$-a].


Bloodroot seeds, with ant-attracting
elaisomes. Photo by cotinis.

I didn’t learn about myrmecochory until after I’d finished undergrad — which is surprising, because it was going on under right my nose every time I went out into the Appalachian woods near campus. Lots of wildflowers [$-a] have ant-dispersed seeds, including bloodroot, touch-me-not, and good old trillium. It’s an extremely popular dispersal mechanism, having evolved independently multiple times on every continent except Antarctica. Really, me not knowing about myrmecochory is kind of like not knowing about fruit!

Ant dispersal is also associated with increased species diversity. In the new article, Lengyel et al. use a classic analysis method called sister group comparison to test the hypothesis that ant-dispersed plant groups contain more species than the most closely-related plant group. And they do, by a long way: on average, myrmecochorous groups contain twice as many species as their non-myrmecochorous sister groups. Why is this? As the authors conclude, it’s probably a side consequence of ant dispersal — ants don’t move seeds very far from where they collect them.

Recent evidence from genetic studies shows that limited seed dispersal in myrmecochory can lead to strong genetic structure within populations even at spatial scales as small as a few meters. The failure of myrmecochores to maintain gene flow across barriers may lead to reproductive isolation of sub-populations, which may facilitate speciation. [In-text references omitted.]

So myrmecochorous plants, like Appalachian salamanders [$-a] and tropical white-eyes [$-a], make lots of new species not because their unique characteristics give them some adaptive advantage (although, to be sure, there are advantages to ant dispersal), but because ants do a lousy job moving seeds between populations, leaving them free to follow their own evolutionary trajectories.

Lengyel et al. argue that myrmecochory is a key innovation, a trait that helps a group of organisms spread and diversify in the process evolutionary biologists call adaptive radiation. Based on their results, I have to agree — ant dispersal is strongly associated with evolutionary diversification. But the speciation that myrmecochory promotes is an accident, a side effect. We often think of key innovations promoting speciation by adaptive means, by allowing one group of species to outcompete others. Clearly, however, a key innovation can also be a trait that makes the accident of speciation a little more likely.

References

Beattie, A.J., & Culver, D.C. (1981). The guild of myrmecochores in the herbaceous flora of West Virginia forests. Ecology, 62, 107-15 DOI: http://www.jstor.org/pss/1936674

Giladi, I. (2006). Choosing benefits or partners: a review of the evidence for the evolution of myrmecochory. Oikos, 112 (3), 481-92 DOI: 10.1111/j.0030-1299.2006.14258.x

Kozak, K., Weisrock, D., & Larson, A. (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon). Proc. R. Soc. B, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326

Lengyel, S., Gove, A., Latimer, A., Majer, J., & Dunn, R. (2009). Ants sow the seeds of global diversification in flowering plants. PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005480

Moyle, R., Filardi, C., Smith, C., & Diamond, J. (2009). Explosive Pleistocene diversification and hemispheric expansion of a “great speciator.” Proc. Nat. Acad. Sci. USA, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105

Why are there so many weevils? Coevolution, maybe.

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ResearchBlogging.orgAsked what attributes of the Creator were manifest in the natural world, the 20th-century biologist J.B.S. Haldane is said to have replied, “an inordinate fondness for beetles.” Beetles are, indeed, the most diverse group of animals on earth, accounting for something less than 40 percent out of five to ten million arthropod species, according to one estimate [PDF]. Naturally, evolutionary biologists would like very much to know how there came to be so many beetles* — and a new paper in this week’s PNAS proposes to answer this question for the largest beetle groups, the weevils.

It seems unlikely to be a coincidence that beetles are widely involved in interactions with the most diverse group of land plants, the angiosperms. In a now-classic 1998 paper, which took Haldane’s apocryphal quip as its title, Brian Farrell presented good circumstantial evidence that living and feeding on flowering plants is associated with beetle diversity. Farrell compared the number of species in groups of angiosperm-feeding beetles with the number of species in closely-related groups of non-angiosperm-feeders, and found that angiosperm-feeding groups were more diverse by orders of magnitude [$-a].





A sample of weevil diversity
Photos by Charles Haynes,
janerc, nutmeg66, and
rizalis Malaysian Macro Team.

Interactions between beetles and their host plants could lead to hyper-diversity in two ways. The evolution of new plant defenses and herbivore counter-defenses could generate alternating cycles of diversification in each interacting group [PDF]. Under this process, diversification doesn’t really happen because of reciprocal natural selection between plant and herbivore — it occurs when plants “escape” their herbivores by virtue of a new defense mechanism, and when herbivores exploit a new food resource made available by innovative counter-defenses. Alternatively, plants and beetles might diversify more simultaneously, with natural selection from plants’ defenses actually driving the speciation of the insect populations that eat them, and vice-versa.

The new paper, on which Farrell is senior author, attempts to distinguish between these two possible scenarios [$-a] using a new phylogeny of the Curculionoidea, the superfamily of beetles more commonly known as weevils. Weevils are distinguished by the rostrum, a noselike appendage they use in feeding — and the estimated 220,000 weevil species feed on an enormous array of plant species. Using DNA sequence data, the paper’s authors reconstructed the evolutionary relationships between 135 weevil genera. They then calibrated the resulting evolutionary tree using the known dates of fossil weevils, so that they could compare the dates of origin of major weevil groups to the history of angiosperm diversification.

Based on this analysis, the oldest weevil groups had their origin millions of years before the first flowering plants. Many of the extant species in these groups still feed on gymnosperms, which predate flowering plants. The most diverse weevil families, which feed on angiosperms, did not emerge until well after the first flowering plants appear in the fossil record, and may not have diversified until angiosperms became the dominant land plants. This lag suggests that, at least on a very broad time scale, weevils diversified because of angiosperm diversity, but probably did not contribute much to creating that diversity:

Thus, the extraordinary taxonomic diversity of weevils appears to have been mediated predominantly by the presence of susceptible, abundant, and diverse host resources, and the ability of weevils to use those resources, rather than by the evolution of host taxa themselves.

In the strictest sense, then, it seems that coevolution isn’t responsible for weevil diversity — yet it is hard to conclude much from results at this broad scale. As weevils took advantage of the “ecological opportunity” created by angiosperm diversity, they would have created myriad opportunities for reciprocal natural selection. Patterns of strict-sense coevolution following the initial colonization of angiosperms may only be apparent over shorter time spans.

References

Ehrlich, P.R., & Raven, P.H. (1964). Butterflies and plants: A study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/stable/2406212

Farrell, B. (1998). “Inordinate Fondness” explained: Why are there so many beetles? Science, 281 (5376), 555-9 DOI: 10.1126/science.281.5376.555

McKenna, D., Sequeira, A., Marvaldi, A., & Farrell, B. (2009). Temporal lags and overlap in the diversification of weevils and flowering plants PNAS, 106 (17), 7083-8 DOI: 10.1073/pnas.0810618106

Ødegaard, F. (2000). How many species of arthropods? Erwin’s estimate revised Biol. J. of the Linn. Soc., 71 (4), 583-97 DOI: 10.1111/j.1095-8312.2000.tb01279.x

———-
* Apart, that is, from the untestable and ultimately unknowable preferences of any putative Creator.

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

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

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

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