"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.
Photo by Jean Peccoud
The authors collected 1090 pea aphids feeding on 19 different species of wild and cultivated legume plants across Europe. When they developed a family tree for these aphids, based on their DNA, they found that they fell into 11 distinct groups. The most closely related aphids were those feeding on the same host species, rather than those from the same geographic area.
There were 11 distinct "biotypes", but are these all separate species? In other words, how complete was reproductive isolation? About 9% of aphids were found on the wrong host for their genotype. These "migrants" could, in theory, have mated with individuals there. But was there enough interbreeding to prevent speciation?
To find out, they looked for hybrids, whose DNA showed they resulted from mating between different biotypes. Three of the biotypes are apparently separate species, because no hybrids between them and other biotypes were found. The other 8 biotypes did produce hybrid offspring with other biotypes, at least occasionally. Genetic differences between biotypes were greater, but not much greater, for those identified as separate species.
Given the rarity of hybridization among some of the other biotypes, will some of these also evolve into separate species? This could happen if differences among host plants result in tradeoffs, such that aphids that grow well on one host grow poorly on others. In that case, hybrids may do poorly on either host and there would be strong selection to feed and mate only on the preferred host.
Here's my answer to the creationist question:
"Have you ever had sex with a chimp? No? Neither has anyone else, for a long time. That's why there are still chimps."