POLYMORPHIC MULLERIAN MIMICRY IN A GROUP OF AFRICAN BUTTERFLIES - A REASSESSMENT OF THE RELATIONSHIP BETWEEN DANAUS-CHRYSIPPUS, ACRAEA-ENCEDON AND ACRAEA-ENCEDANA (LEPIDOPTERA, NYMPHALIDAE)

Mullerian mimicry, in which there is convergence in coloration between unrelated unpalatable species, should lead to uniformity in appearance, not polymorphism, and so the occurrence in tropical Africa of unrelated species of unpalatable butterflies with corresponding polymorphic colour forms suggested a problem of special evolutionary interest. Field work in Uganda and Sierra Leone in 1964-72 demonstrated a statistical association between the occurrence and relative frequencies of polymorphic forms in Danaus chrysippus (Danainae) and Acraea encedon (Acraeinae) which was deemed as confirmation of a Mullerian relationship between them. There were, however, certain anomalies which at the time remained unresolved. Later, in 1976, it was discovered that what had been called A. encedon is in reality two sibling species A. encedon and a new one, named as A. encedana The two differ in the structure of both male and female genitalia and in the coloration and the food-plants of the larvae. The recognition of the additional species has enabled a re-assessment of the polymorphic Mullerian association with D. chrysippus. It emerges that, although there is a close qualitative and quantitative reciprocal mimetic relationship between A. encedana and D. chrysippus, the relationship between A. encedon and D. chrysippus is much weaker, and in places non-existent. The possible origin of the mimetic polymorphism is discussed in terms of hybridization of previously allopatric and monomorphic populations which have met as a consequence of recent expansions of geographical range in all three species resulting from forest clearance and the spread of savanna-like conditions in previously forested areas.