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?