The ecological genetics of introduced populations of the giant toad, Bufo marinus. IV. Gene flow estimated from admixture in Australian populations.

Allele frequency variation is described at nine polymorphic enzyme loci in 21 samples from populations of the introduced Giant Toad, Bufo marinus, in the region of Townsville in north Queensland, Australia. Some of these populations appear to have been established through the introgression of other populations that previously had been isolated. Comparisons of allele frequencies at three polymorphic loci between the introgressed populations and the original populations are used to obtain admixture estimates. These are used to estimate a rate of gene flow among the populations of approximately 2 km/year. This is consistent with an estimate based on the rate at which Bufo marinus has colonised new areas in Australia when discontinuities in the pattern of this colonisation are taken into account. The estimate of gene flow is combined with published data on population density to estimate neighbourhood size. The estimate obtained is substantially greater than the effective population size estimate determined previously from data on allele frequency variances in other populations. This discrepancy is most likely due to inaccuracies in the population density estimates, to underestimates of the extent of offspring number variance and perhaps to occasional departures from sex ratio parity. It has important implications for the study of the genetic structure of populations which are discussed.