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March 25, 2011

The beaks of the finches

This week's paper is "Divergence with gene flow as facilitated by ecological differences: within-island variation in Darwin's finches", published in Philosophical Transactions of the Royal Society.

"If humans evolved from chimps", ask the creationists, "why are there still chimps?" Good question! Humans didn't evolve from modern chimps, any more than I am descended from my cousins. We just share ancestors.

But still... why are there now chimps and humans, rather than one species? More generally, how does one species split in two? If part of a species becomes isolated enough to be inaccessible for mating, or at least "geographically undesirable," then the two populations can diverge through natural selection or random genetic drift, eventually losing the ability to reproduce with each other.

But genetic divergence doesn't always require geographical isolation. For example, if birds with medium-size beaks get less food than those with large or small beaks (on an island with many large and small seeds but few medium seeds), will the resulting "disruptive selection" tend to split the population into two subpopulations, with genes for small and large beaks? Maybe, but not if small- and large-beaked birds interbreed, combining genes for large and small beaks. And why wouldn't they?

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May 8, 2009

"If evolution is true, why are there still chimps?"

I once heard PZ reply to this popular creationist question by pointing out that, although many Minnesotans are descended from Norwegians, there are still Norwegians. This isn't really a good analogy, however, because Minnesotans and Norwegians aren't separate species. We know this because they can interbreed, producing healthy children. At the end of this post I suggest a better answer, indirectly inspired by this week's paper.

Two of evolutionary biology's central questions are: how do species change over generations? and how does one species split into two? We have many detailed examples of small evolutionary changes occurring over days (in bacteria) or years (in animals and plants), so one would have to be very close-minded to deny major evolutionary change over millions of years. But major evolutionary change is not enough, by itself, to split one species into two. One subpopulation within a species must change, while the rest of the species either stays the same or changes in different ways. This divergence cannot happen if the two subpopulations continue to interbreed at high rates. In other words, speciation requires some reproductive isolation.

Often, reproductive isolation is a byproduct of geography. After a few individuals (or a pregnant female) cross a mountain range or are blown from the mainland to an island, they no longer interbreed with their ancestral population. Over many generations, random genetic drift or nonrandom natural selection can change the isolated population enough that they can no longer produce healthy offspring with the original population, even if they come back into contact.

Sometimes speciation can occur without a major geographic barrier, but reproductive isolation is still required. This week's paper shows that this has happened and is still happening in Europe.

"A continuum of genetic divergence from sympatric host races to species in the pea aphid complex", by Jean Peccoud and others, was just published online in the Proceedings of the National Academy of Science.
aphid.jpg
Photo by Jean Peccoud

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October 5, 2007

Faster speciation in the tropics?

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Pygmy salamander, Desmognathus wrighti, from the southern Appalachian Mountains (photo by Matt Chatfield)

This week's paper "Climatic zonation drives latitudinal variation in speciation mechanisms" is by Ken Kozak, a new member of my department, and John Wiens of Stony Brook University, published in Proceedings of the Royal Society. They used data on salamanders to test an old hypothesis to explain why there is so much species diversity in the tropics.

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March 21, 2007

Splitting species: sneak attacks from strategic hamlets

This week's paper is "Colour pattern as a single trait driving speciation in Hypoplectrus coral reef fishes?" by Oscar Puebla (Smithsonian Tropical Research Institute, Panama) and colleagues in Canada and the UK, published on-line (no volume or page number yet) in Proceedings of the Royal Society. (I was planning to review a paper on the evolutionary history of genetic differences between chimps and humans, suggested by a reader, but decided I didn't understand it well enough myself to explain it clearly. Is there a volunteer guest blogger out there?)

Actually, there's a bit of a connection between the two papers. At some point, the ancestors of humans must have stopped having babies with the ancestors of chimps. Otherwise, we'd still be one species. We might have evolved a lot from our common ancestor, but we'd be evolving together, not separately. Interbreeding is a problem for the production of new species in general; the resulting "gene flow" can prevent differentiation into separate species.

One easy solution is geographic separation. Finches on different islands in the Galapagos group rarely interbreed with each other, and never with their ancestral species on the mainland. So natural selection, working in different directions on the different islands, isn't swamped by interbreeding. This eventually produced enough change that the finches would at least hesitate to mate if brought back together.

But can species separate without being physically separated? There are already a few known examples of this, but the authors of this week's paper may have caught "sympatric" speciation in reef fish known as hamlets red-handed. Uh, finned.

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