Absence of evidence for isolation by distance in an expanding cane toad (Bufo marinus) population: an individual-based analysis of microsatellite genotypes

The cane toad (Bufo marinus) was introduced in 1935 in Australia, where it spread rapidly. We have tested for isolation by distance by analysing at a local geographical scale a continuous population using seven microsatellite markers and an individual-based method. The matrix of pairwise individual differentiation was not significantly correlated with that of geographical distance. Regression analyses gave a low positive slope of 0.00072 (all ind ividuals) or a negative slope of 0.0017 (individuals with a distance higher than the previously estimated mean dispersal distance). The absence of evi dence for isolation by distance favours the hypothesis that the substantial differentiation and autocorrelation previously observed at enzyme loci, ma inly results from discontinuities in the colonization process with founder effects occurring at the time of the establishment of new populations.