Chance events, habitat age, and the genetic structure of pond populations

By MARC G. BOILEAU and BARBARA E. TAYLOR

Abstract

We surveyed the genetic structure of zooplankton populations in small natural ponds to measure geographic patterns of differentiation and to evaluate the rates of genetic exchange. Populations of eight taxa (the calanoid copepods Aglaodiaptornus clavipoides, A. conipedatus, A. stagnalis, Aglaodiaptornus sp., Onycbodiaptornus sanguineus, and Osphranticum labronectum, the cladoceran Dapbnia laevis, and the conchostracan Lynceus gracilicornis) were sampled from 26 ponds in the uplands and alluvial terraces of the Savannah River Site near Aiken, South Carolina, USA. Genetic divergence 0 was high, with values of 0.053 - 0.297 for three calanoid copepods and one cladoceran. Dispersal, estimated from 0 by assuming equilibrium with genetic drift, yielded dispersal rates of 0.6-4.5 animals per generation. However, because the populations are large, founder effects in divergence should decay very slowly. Dispersal may thus be underestimated, but would still represent only miniscule proportions of the populations. For the two species sampled in both regions, insignificant differences in divergence between populations on the upland and populations on the alluvial terrace, which is younger, suggested persistent founder effects. Weak or insignificant correlations between genetic and geographic distances indicated that dispersal was poor over even very short distances or that local selection was strong. Comparisons of spatial patterns lead us to speculate that founder effects may persist longer in genetic composition than in taxonomic composition.