We heard a BBC discussion today about urban birds in Britain. An interviewee, a bird expert of some sort (we did not catch his name or profession, but he seemed to be a biologist), was describing the development of locally different dialects in urban areas of the country. The local birds recognize each others' 'language', but these are different among localities. When played the call of a bird of the same species, but with a different 'accent', birds didn't respond with nearly as much interest as when the call was of a bird from the same locale as the listener.
This is interesting, because there is so much stereotyping in popular culture, guidebooks, and the like, that describes what 'the' so-and-so bird does. But over time, for all sorts of animals, domestic and wild, local speech dialects have been detected, so the story here is not a great surprise. The idea here is that in each city, birds become more distinct over time, so that they no longer recognize each other's songs.
The discussion then took a rather predictable evolutionary turn. If these birds continued to have locally differing dialects, then birds from different areas could no longer communicate to mate, and this would lead to the evolution of new species. Partly, this is simply a matter of our own--the scientists'--definition of what a species is. There, there's not really a difference (that one can test) between 'don't mate' and 'can't mate', and in either case we declare the two groups to be different species.
This is classic, but rather superficial Darwinism, one of the issues we write and think a lot about. There is nothing wrong with the logic itself. Long-term isolation is likely eventually to lead to the accumulation of so much genetic difference that members of each area could no longer mate successfully.
But how long does this take? Probably hundreds, or thousands of generations (or more). Think about that in this context. How stable are urban areas in a place like Britain, relative to such long times? For birds, that would mean centuries or millennia (or more). Given the rapid change of urban landscapes, transportation, and environmental changes, the odds that simple local dialects, which have been observed to develop in a short period of time, would persist or remain sufficiently isolated for such lengths of time seem remote, even if certainly not impossible in principle.
Species can remain mating-compatible even after hundreds of thousands or even millions of years of separation. Humans (who have dialects if any species do!) inhabit the entire terrestrial globe, and at the end points (the tips of South America and Africa) have been isolated for around 100,000 years (5,000 generations at least), and are still fully mating-compatible.
Oversimplified evolutionary explanations are pat, often irrefutable because untestable, and the problem is that they cover over some of the more interesting questions about how things actually happen (one of the issues we raise in The Mermaid's Tale). Speciation may be due to the accumulation of large amounts of small genetic differences due to local adaptation (behavioral, such as by mating calls, or otherwise); that was certainly Darwin's idea of what happened. But this need not be so. Small genetic changes in chromosomal compatibility can also lead to mating isolation (some of these are called hybrid sterility mutations), without any of the usual kinds of adaptations due to natural selection. And what happens over eons of time is unlikely in most or at least many cases to be easily extrapolated from what is observed in just a couple of generations.
It's for this reason that we caution against such simplifications. They give a semblance of understanding according to a widely, if often uncritically, accepted theory. But they are a reflection of scientific impatience that can obscure the facts that may be important for a deeper understanding.