The question of whether these two populations are on the road to speciation comes down to sex. When two populations stop exchanging genes-that is, stop mating with each other-then they can be considered distinct species. Uy and his team wanted to see if these flycatchers were heading in that direction. [. . .]Richard Dawkins (in The Ancestor's Tale):
That males from the two populations no longer view the other as a reproductive threat is a good indication that not much mating is taking place between the two groups. Their evolutionary paths are diverging, Uy and his team found-all because of a change in plumage.
The researchers then went a step further. They looked into the birds' genomes to see what genes may have played a role in the different plumage pattern. They found only one: the melanocortin-1 receptor gene (MC1R). The MC1R gene regulates the production of melanin, which gives skin and feathers their color. The all-black and chestnut-bellied birds had different versions of the MC1R gene, which gave rise to the plumage change.
That change appears to have been enough to create a reproductive barrier for flycatchers. Not every species is so picky, so a color change doesn't always drive speciation. Nonetheless, these results suggest that it can take as little as one gene, in the right spot in the genome, to cause a fork in the evolutionary road.
As I said, zoologists define a species as a group whose members breed with each other under natural conditions — in the wild. It doesn't count if they breed only in zoos, or if we have to use artificial insemination, or if we fool female grasshoppers with caged singing males, even if the offspring produced are fertile. We might dispute whether this is the only sensible definition of a species, but it is the definition that most biologists use.
If we wished to apply this definition to humans, however, there is a peculiar difficulty: how do we distinguish natural from artificial conditions for interbreeding? It is not an easy question to answer. Today, all surviving humans are firmly placed in the same species, and they do indeed happily interbreed. But the criterion, remember, is whether they choose to do so under natural conditions. What are natural conditions for humans? Do they even exist any more? If, in ancestral times, as sometimes today, two neighbouring tribes had different religions, different languages, different dietary customs, different cultural traditions and were continually at war with one another; if the members of each tribe were brought up to believe that the other tribe were subhuman 'animals' (as happens even today); if their religions taught that would-be sexual partners from the other tribe were taboo, 'shiksas', or unclean, there could well be no interbreeding between them. Yet anatomically, and genetically, they could be completely the same as each other. And it would take only a change of religious or other customs to break down the barriers to interbreeding. How, then, might somebody try to apply the interbreeding criterion to humans? If Chorthippus brunneus and C. biguttulus are separated as two distinct species of grasshoppers because they prefer not to interbreed although they physically could, might humans, at least in ancient times of tribal exclusivity, once have been separable in the same kind of way? Chorthippus brunneus and C. biguttulus, remember, in all detectable respects except their song, are identical, and when they are (easily) persuaded to hybridise their offspring are fully fertile.