Effect of Population Size on Genetic Variation in a Dioecious Conifer

Habitat fragmentation can result in local population sizes becoming very small, which can reduce genetic variation within populations. Most models concern rates of loss of variation from small populations through genetic drift, in the absence of mutations and immigration that could replace this variation. However, such models are somewhat unrealistic because they predict that eventually every population will lose all variation. Inclusion of mutation and immigration rates leads to the prediction that levels of genetic variation will reach an equilibrium where the amount of variation present at equilibrium will be affected by population size (Lande 1988). In this note, I examine wild populations of a plant with naturally isolated populations in order to determine empirically the relationship between population size and genetic variation. Using allozyme electrophoresis, seventeen discrete populations of the dioecious, anemophilous conifer, Halocarpus bidwillii (Kirk) Quinn (Podocarpaceae), were surveyed for genetic variation at 20 loci. Although not all of these populations will have been at genetic equilibrium, the genetic structure of species with isolated populations may provide a useful insight into the probable genetic consequences of recent fragmentation of more widely distributed species.