THE ECOLOGICAL GENETICS OF INTRODUCED POPULATIONS OF THE GIANT TOAD,BUFO MARINUS. III. GEOGRAPHICAL PATTERNS OF VARIATION

The allele frequencies at ten polymorphic loci are described from 31 Bufo marinus populations in the Moreton Bay region in southeastern Queensland, Australia and the variation of these is found to be non-random in all cases. The pattern of non-randomness varies among loci, being clinal in two instances. The allele frequencies at the same ten loci are also described for 12 populations sampled from throughout B. marinus' Australian range. The frequency variation on this larger geographical scale is non-random at all but two loci (Mpi and Hbdh) and also varies among loci, in this case being clinal in four instances. In both cases, the patterns of variation are most reasonably explained as having resulted from genetic drift occurring during the recent range expansion which B. marinus is known to have experienced in Australia. It seems that natural selection has played little, if any, role in generating the observed gene frequency patterns. These results emphasize the need for caution in interpreting geographical patterns of variation. They show that even when clinal patterns exist at some loci but not at others, one cannot conclude that the patterns result from natural selection, unless the demographic histories of the studied populations are known and are inconsistent with the alternative hypothesis that the patterns result from genetic drift.