Here is another article that I think offers insights to how groups get separated from each other …
The Galapagos finches have been intensely studied by biologists Peter and Rosemary Grant since 1973. At that time, the Galapagos island Daphne Major was occupied by two finch species: the medium ground finch and the cactus finch. Then, in 1981, a hybrid finch arrived on Daphne Major from a neighboring island. It was part ground finch, part cactus finch, and quite large compared to the locals. It also happened to have an extra-wide beak and an unusual song — a mash-up of the songs sung by ground finches in its birthplace and on Daphne Major. The immigrant paired up with a local female ground finch (who also happened to carry some cactus finch genes), and the Grants followed these birds’ descendents for the next 28 years.
The new immigrant finch (left), a cactus finch (middle), and a ground finch (right)
After four generations, the island experienced a severe drought, which killed many of the finches. The two surviving descendents of the immigrant finch mated with each other, and this appears to have set the stage for speciation. In December of 2009, the Grants announced that, since the drought, the new lineage has been isolated from the local finches: the children and grandchildren of the survivors have only produced offspring with one another.
Several factors probably contributed to the isolation of the new lineage. Since males mainly learn their songs as juveniles in the nest, the immigrant’s male descendents also sang his strange, mixed song. This likely affected which females were willing to mate with them. In addition, female finches tend to choose mates with beak sizes similar to their own, so the extra-wide beaks of the new lineage probably also biased it towards within-group mating.
Both the blackcaps and the finches demonstrate the important role that behavioral shifts may play in the early stages of speciation, as well as the many ways these shifts can arise. For example, the blackcaps’ split was triggered by a persistent, human-caused change in the environment, while the finches’ split was kicked off by a fluke series of natural events. The behavioral shifts that result in reproductive isolation also differ between the two cases. The change in the blackcaps’ migration pattern is genetically controlled, while the finches’ unusual song, which contributed to their divergence, is a learned trait.
These two examples make it clear that the division between species is not a black-and-white issue. Rather, speciation occurs as many different sorts of traits (physical, behavioral, and genetic) diverge from one another along a continuum. Because of this, biologists sometimes disagree about where to draw the line between incipient species — about when a division has become deep enough to warrant a new species name. Whatever we choose to call them, these two cases clearly illustrate how a lineage can split and begin to make its way down two separate evolutionary paths.
Of course, there’s no way to know if these paths will converge at some point in the future or are even completely distinct now. Another chance event on Daphne Major could cause the new finch lineage to begin interbreeding with the local population again. And blackcaps may never evolve differences beyond a slight change in wing and beak shape. While we can’t know the fates of these lineages, directly observing such divergences in real time highlights the fact that we don’t always need to look into the distant past or far off future to find examples of speciation in action. Evolution is occurring all around us. We just need to learn where and how to look for it.