Sunday, March 15, 2009

Island biogeography


Nothing could have been more fortuitous for the young Charles Darwin -- and for us -- than that the H.M.S. Beagle stopped among the Galapagos Islands on its voyage around the world. The islands are just large enough, just old enough, just different enough, just far enough apart, and just far enough from the mainland to be an ideal laboratory for the study of evolution by natural selection. Here Darwin could see speciation in action, by geographical isolation and environmental adaptation. A classic example of the right person in the right place at the right time.

As I range my own island -- Exuma in the Bahamas -- I can't help but notice the myriad land snails of the genus Cerion, alive in a variety of environments, and as fossils in the young limestone rock. The Bahamian snails were to the young Stephen Jay Gould and Niles Eldredge what the Galapagos finches were to Darwin -- another case of the right persons in the right place at the right time. The snails unfolded a tale of punctuated equilibrium. As Gould said, it is rare to find such a continuous record of variation -- or the lack of it -- unbroken by time and erosion.

For Darwin, Wallace, Gould, E. O. Wilson and countless others, islands have held the answers to the questions posed by Charles
Lyell in his 1830 Principles of Geology: How do new lands become "clothed and tenanted" with living organisms, and how are the uniqueness of these species to be explained?