Matt Simon, for Wired:

Ever since Darwin and his famous finches, biologists have thought that in order for a species to diverge into two new species, the two populations had to be physically isolated. Those finches, for instance, each live on a different Galapagos island, where their special circumstances have resulted in specialized bill shapes. Yet the two varieties of island scrub jay (they haven’t technically speciated—yet) live on the same tiny island. If they wanted to meet each other for a brunch of acorns and/or pine nuts and perhaps later some mating, they could just fly right over.

This is very, very weird. It’s an affront to a sacred tenet of evolution you probably learned in school: Isolation drives speciation. Well, speciation can also come about in a broadly distributed population, with individuals at one end evolving differently than individuals at the other, but nothing kicks evolution into overdrive quite like separation. Without it, two varieties should regularly breed and homogenize, canceling out something like different bill shapes (though rarely the two types of island scrub jay will in fact interbreed). And the island scrub jay isn’t alone in its evolutionary bizarreness. In the past decade, scientists have found more and more species that have diverged without isolation. Langin’s discovery with island scrub jays, published last week in the journal Evolution, is perhaps the most dramatic illustration of this yet.

So isolation may drive speciation, but strong preferences, over time, may also get you there.