The following brief news item seems to lend support to a hunch I have had for some years:
New Scientist: Bird song goes out of fashion too
… Behavioural ecologists have long known that some songbirds develop local dialects, and that individual birds respond more strongly to their own dialect than to a foreign one. Less is known about how, or how quickly, such differences arise.
To study how a dialect changes over time, Elizabeth Derryberry, a behavioural ecologist at Duke University in Durham, North Carolina, compared recordings of male white-crowned sparrows' song from 1979 […] and 2003. The modern song, she found, was slower and lower in pitch.
This difference mattered to the birds… [Derryberry] found that females solicit more copulations and males showed more aggressive territorial behaviour to the contemporary song than to the older ones… The result shows that meaningful differences in song styles can arise within just a few years, and thus that mating barriers can be erected quickly, says Derryberry.
I have long suspected that sexual selection might be more important in species-creation than it is generally given credit for. Sexual selection tends to play second fiddle to natural selection in discussions about speciation. In fact, I would argue that sexual selection is simply a sub-category of natural selection—albeit a very important sub-category.
It seems to me that differing sexual preferences amongst potential mates creates evolutionary niches without the need for haphazard geographical isolation. This must greatly increase the opportunities for speciation. In fact, my hunch is that sexual selection could prove to be more important than traditional natural selection in terms of speciation.