Never in the history of biology has a single set of scientists so thoroughly studied a group of organisms in their natural environment, following them through the span of multiple decades. The work, outlined in a book we highly recommend, The Beak of the Finch by Jonathan Weiner, has been a masterpiece—science at its very finest. So thorough is their analysis that in this PNAS article they are able to actually demonstrate the key events in the formation of a new species as it occurred before their very eyes. A single species has diverged to become, in essence, two species—they are no longer inter-mating. One would have to follow the organisms for a longer time to see if they become firmly established as two distinct species, but the important point is that as we approach the 150th anniversary of the publication of Darwin's Origin of Species next Tuesday, careful analysis has showed that it is actually possible to observe the key events of speciation. And the events associated with it happened in less than one human lifespan!
The findings describe the mini-evolutionary saga that took place on their island of Daphne Major in the Galapagos chain. The Darwin finches of the Galapagos islands (technically from a family called "tanagers") first provided Darwin with a clear picture of evolutionary divergence, as birds from each island have adapted unique traits to meet the unique conditions of their respective islands. The new species observed by the Grants can be traced back to a medium ground finch (Geospiza fortis) that arrived from a neighboring island in 1981. The bird was "unusually large, especially in beak width and sang an unusual song" (The Grants tape recorded the immigrants song!)
Despite being a newcomer, this ground finch was able to successfully mate with a female from the local island. Together, they produced five sons. However, because young Darwin finches learn their songs from their father, the offspring learned a new variation of the local bird call, a product of the song being mimicked through the voice of their foreign father, similar to a human singing a song in a language they do not know.
Despite this slightly different tune and their unusual size (which they also inherited from their father), this new generation was still able to find mates. However, the Grants theorized that within a few generations, this new lineage would become reproductively isolated from the local population. Sure enough, after four generations when a drought killed off all but a brother and sister of the new lineage, the remaining siblings mated. Their children began to do the same. After three generations of reproductive isolation, the Grants have declared that the birds are now in the "secondary stages" of species formation.
The ultimate future of the newly reproductively isolated group remains to be seen. Will they be out-competed for resources or will genetic problems from the variations in the patriarch and matriarch of the lineage become magnified over time? Perhaps, even, members of this new group will return to the island of their forefather and interbreed with them, leading to even more genetic diversity.
At the very least, the saga of these finches shows that while rarely observed, speciation can occur quickly given the proper circumstances.