Gene flow and effective population size in Lepanthes (Orchidaceae): a casefor genetic drift

Genetic drift can play an important role in population differentiation, par ticularly when effective population sizes are small and gene now is limited . Such conditions are suspected to be common in the species-rich Orchidacea e. We investigated the likelihood of genetic drift in natural populations o f three endemic species of Lepanthes (Orchidaceae) from Puerto Rico. We est imated effective population size, Ne, using three ecologically based method s. Two of the three estimates were based on variance in reproductive potent ial and the third was based on coalescence time. All estimates of Ne were u sually <40% of the standing population size, resulting in values of <20 ind ividuals per population. Based on starch gel electrophoresis of isozymes, N m estimates suggest restricted gene now among populations in the range of o ne or less successful migrant per generation. Genetic differentiation among populations is expected under such conditions from random genetic drift. I ndeed we observed high genetic differentiation among populations (L. rubrip etala, F-ST, G(ST), theta; 0.248, 0.266, 0.293; L. rupestris, 0.148, 0.169, 0.138; L, eltoroensis, 0.251, 0.219, 0.218, respectively). Genetic drift i s likely to be important for population differentiation in Lepanthes as a r esult of small effective population sizes and restricted gene now. (C) 2001 The Linnean Society of London.