POLYMORPHISM AND EVOLUTION IN THE BUTTERFLY DANAUS-CHRYSIPPUS (L) (LEPIDOPTERA, DANAINAE)

Analysis of the genetic structure of a sample of the polymorphic butterfly Danaus chrysippus from Kampala, Uganda shows that the population is undergoing substantial evolutionary change. Comparison with samples from the same area going back to before 1900, indicate that the frequency of form alcippus has increased from 16 per cent to 71 per cent (1909-91) while f. dorippus has decreased from 14 per cent to 2 per cent, f. aegyptius from 66 per cent to 24 per cent and f. albinus from 4 per cent to 3 per cent. Genotype frequency differences between the sexes at two of the three loci examined suggest that a balanced polymorphism is maintained by opposing selective forces acting on males and females. Non-gametic (genotypic) disequilibrium between two pairs of unlinked loci indicates that natural selection is involved, again with sex differences. It is suggested that the polymorphism originated after hybridization of allopatric races which evolved during the Pleistocene but are now maintained sympatrically. The selective agents have not been identified but mimetic relationships, both Batesian and Mullerian, are almost certainly involved.