Friday, November 25, 2005

Islands and Shifting Balance?   posted by Razib @ 11/25/2005 05:41:00 PM

Living the Scientific Life has a great summary of this recent paper which suggested that islands can serve as sources for biological diversity via back-migration to continental sources. Am I the the only one to think of the Shifting Balance Theory? To be succinct, Sewall Wright and R.A. Fisher spent several decades disputing (PDF link) the importance of population substructure and random genetic drift as salient forces in evolutionary genetic dynamics (Wright tended to emphasize their importance to a far greater extent than Fisher). In this scenario, where there was diversification of the original colonizers on islands, it seems plausible that the rate of migration1 might have been low enough for substructure to become a significant parameter. Of course islands are special cases which are not ubiquitous, so the biggest take home message might be that evolutionary genetic dynamics needs to considered in context.

1 - Fst, which is a reflection of interpopulation vs. intrapopulation genetic variation, can be related to the # of migrations per generation via 1/(4Nm + 1), where N ~ number and m ~ proportion of migrants per generation, so that Nm is the # of number migrants per generation. 1 migrant per generation is usually sufficient to serve as a break on interpopulational differences. Random genetic drift's power is inversely proportional to N, so when the population is large there isn't much sampling variance which the 1 migrant has to work against, while in a small population 1 individual is a much larger proportion of the population. More here.