MACRO- AND MICROGEOGRAPHIC STRUCTURE OF A SPATIALLY SUBDIVIDED BEETLE SPECIES IN NATURE.

Abstract

Spatial subdivision of species can affect their population structure by allowing processes such as limited dispersal, spatial heterogeneity in selective pressures, small population sizes, and random events to operate. By studying species restricted to islands or "island" habitats, one can attempt to determine which of these factors have affected the current structure of the population. Collops georgianus (Coleoptera: Melyridae), a beetle species endemic to the "island" habitat of granitic rock outcrops, was chosen to see how its spatially subdivided distribution has affected its genetic structure. Its genetic structure was examined on both a macrogeographic and a microgeographic level using protein electrophoresis. Macrogeographically, 12 populations throughout its range were sampled. The discontinuous distribution of outcrops, and thus populations, throughout its range, has determined the connectivity of the populations. Significant variation in allele frequencies and substructuring (FST = 0.192) was found throughout the range, but there was no spatial autocorrelation. Microgeographically, in the central part of the range, where outcrops are denser and more continuously distributed in space, there was evidence of isolation by distance. Very little variation in allele frequencies was found, but a low but significant level of substructuring occurred among the populations. Comparison of disjunct and continuous populations microgeographically revealed no effect of disjunct distributions, although a significant effect of distance was detected. Effective population size variation among populations and between years, compounded with the effects of local extinctions, suggest that random processes such as drift and founder effects are important determinants of the population's genetic structure.