Quaternary Refugia in Tropical America: Evidence from Race Formation in Heliconius Butterflies

The hypothesis of Haffer, Turner, and others, that patterns of race and species formation in the tropical forests of South America are the result of the isolation of populations in forest refugia during widespread climatic changes in the geologically recent past, is supported by the distribution of races in the butterfly genus Heliconius: the location of the refuges for these butterflies shows an excellent accord with the refuges deduced by Haffer in his studies of forest birds. The strict parallel variation through most of South America of the various races of H. melpomene, H. erato and of ten similarly-patterned species shows the result of selection for Mullerian mimicry; as the patterns must be subject to strong stabilizing selection, and as the low vagility of the butterflies normally produces isolation by distance even in a continuous population, it is suggested that the extreme divergence of pattern that some (but not all) Heliconius underwent in the forest refugia results from selection pressure in favour of mimicking the most abundant or distasteful local species, which would vary from refuge to refuge, rather than from geographical isolation per se.