Samll-Scle Patterns of Variation at the Sorbitol Dehydrogenase Locus in an Introduced Toad, Bufo marinus (Amphibia:Anura)

Abstract

The spatial distribution of the sorbitol dehydrogenase (Sdh) isozymes in Bufo marinus at 33 localities in south-east Queensland was examined by zone electrophoresis of liver extract. There is significant heterogeneity in isozyme frequencies between localities. The distribution of the frequencies of these localities is non-random, as shown both by the Royaltey-Astrachan-Sokal t-test and by t-tests on spatial autocorrelation coefficients of the sdh*b frequencies. In order to assess the particular pattern type, the autocorrelation coefficients are plotted against increasing inter-locality distance, and a variation of Royaltey et al.'s 'double depression', comprising a 'peak' and a 'depression', is found to best describe the apparent pattern. The colonization history of B. marinus in Queensland is examined for information about movement patterns and the age, in generations, of the population. The fit of the data to different microevolutionary models is evaluated, and a neutralist model of genetic drift is rejected. The data are consistent with an alternative neutralist model of founder events; however, insufficient evidence is available to assess any concurrent effects of natural selection on the observed small-scale pattern of geographic variation.