The Origin of Species

Traditionally, evolutionary biologists assumed speciation occurred when a population was split geographically (e.g. a canyon forms dividing a population in two, also known as allopatric speciation). The subdivided populations then undergo differential adaptation and eventually become reproductively isolated. Ta da! New species!

More controversially, species can also form in a non-geographically subdivided population (i.e. sympatric speciation. Huber et al. report on a possible example of sympatric speciation in progress in the archetypal symbol of evolution: Darwin's finches.

Here a population of the medium ground finch, Geospiza fortis, features large and small
beak morphs with relatively few intermediates.On the right is the large beaked morph, and on the left is the small beaked morph of the medium ground finch. In one of the best characterized examples of evolution in action Peter and Rosemary Grant showed that the Galapagos finches probably diverged due to diverged to variation in local food availability and inter- or intraspecific competition. Presumably these factors are also driving the differentiation of these finches.

However, the most important part of Huber et al.'s part is there microsatellite DNA analyses showing that the large and small beak morphs represent two partially distinct gene pools caused by assortive mating. That is, small beaked females prefer to mate with small beaked males and vice versa. One factor driving these preferences might be selection against birds with intermediate sized beaks, which have been shown to suffer increased mortality relative to the other morphs. Come back in a few years, perhaps by then these two morphs will be reproductively isolated, and officially, different species.

Top Image: Species of Darwin's finch, from a colour plate by John Gould in Zoology of the Voyage of H.M.S. Beagle (1839–43), based on specimens collected by Charles Darwin on the Galapagos Islands.