Thursday, May 24, 2007

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.


  1. I haven't kept up with this literature in a long while, so thanks for this summary.

    What a nice extension of the Grants' work. (Of course, this will be meaningless to creationists who will argue that it's just microevolution - you can't turn a finch into an eagle or a bat. Sigh.)

  2. Very interesting, I've seen many illustrations of the different finches but never a side-by-side photo.
    By the way, thanks so much for adding me to your blogroll! Just one thing, I did some editing recently and changed my title from "Biology, life..and what else is there" to "Pondering pikaia", if you want to update your blogroll, sorry about the change but just thought I'd mention it. I think you're the first fellow blogger to acknowledge me, thanks again!

  3. Thanks for updating that, again I really appreciate the add, for while I was afraid I was just talking to myself on my blog and the recognition is definitely a morale-booster!

  4. The plate at the top of the page is of _Tanagra darwinii_ [Gould]. Darwin collected only one specimen from Maldonado in mainland Uruguay.

    Its a tanager rather than a finch.

    Other than that (and my colossal pedantry) great blog!