Founder effects and silvereyes

Patterns of variation in nature have played a large role in the development of explanations for biological richness at the species level. One such influential pattern has been the morphological distinctiveness of small populations on islands at the periphery of a large continental land mass (1). If the island differs from the mainland in habitat or composition of the biological community, the morphological differences often can be explained in terms of adaptation by natural selection (2). But, if the environments are more or less the same, then an explanation has to be sought elsewhere. Fifty years ago, Ernst Mayr (3) suggested that the key to island evolution lies in the circumstances and immediate consequences of colonization. The model he proposed, called the “founder effects model,” postulated major changes in the genetic constitution of a newly established population that were set in motion by the reduced genetic variation carried by the few colonizing individuals in the founding event (4). The model was a bold explanation, because nothing was known of the genetic basis of the morphological traits. It has been challenged repeatedly on theoretical grounds (refs. 5–7; but see refs. 8 and 9) and has fared little better empirically (10). The preferred method of testing has involved creating in the laboratory new populations with a few individuals of small organisms with short generation times (11–13). These experimental studies have yielded some (11, 13) but generally little or no support (10, 12). Adopting a different tactic, Sonya Clegg et al. (14) have returned to the source of the problem, bird populations on islands, and in this issue of PNAS, they report the results of some novel tests. Silvereye is not the name of a Drosophila mutant but the name of a small, warbler-like …