Founder effects and selection

Many people that have some minimal understanding of evolution think that all differences we find among populations must be attributed to the effect of natural selection.  That's not true at all.  While selection is certainly the driving force behind much of evolutionary change, it's not the only process at work.  A major factor, particularly in small populations, is a random process that we call genetic drift.  A special case of genetic drift known as the founder effect can take place when a new habitat, like an island, is colonized by a small subset of organisms from a larger population.  Many times, that small random sample of founders differs, sometimes dramatically, from the source population from which it was drawn.  This difference may have nothing whatsoever to do with selection.

In a fascinating new paper appearing in Science and discussed here at Science Daily, Jason Kolbe and his colleagues investigate the interaction of founder effects and natural selection on populations of brown anoles that they established on small islands in the Bahamas.  After the native populations of anoles were wiped out by a storm, the researchers replaced them with pairs of anoles collected from nearby Great Abaco.  The introduced anoles, adapted to the forests of their home island,  had longer legs than those typically found on lizards inhabiting the scrub habitats of the small islands.  However, as the result of random nature of the simulated founding event, the seed populations of the islands differed in limb length. 

After four years of monitoring, the biologists found that the (now well-established) anole populations on the small islands had indeed evolved toward shorter leg length.  However, the initial differences in populations were still in evidence, as the ranks of limb lengths of the founding populations had been retained.  In other words, the limb length of the island anole populations were a result of both the founder effect, and the subsequent effects of selection.