WHAT CAN HYBRID ZONES TELL US ABOUT SPECIATION - THE CASE OF HELICONIUS-ERATO AND HELICONIUS-HIMERA (LEPIDOPTERA, NYMPHALIDAE)

To understand speciation we need to study the genetics and ecology of intermediate cases where interspecific hybridization still occurs. Two closely related species of Heliconius butterflies meet this criterion : Heliconius himera is endemic to dry forest and thorn scrub in southe rn Ecuador and northern Peru, while its sister species, H. erato, is u biquitous in wet forest throughout south and central America. In three known zones of contact, the two species remain distinct, while hybrid s are found at low frequency. Collections in southern Ecuador show tha t the contact zone is about 5 km wide, half the width of the narrowest dines between colour pattern races of H. erato. The narrowness of thi s dine argues that very strong selection (s approximate to 1) is maint aining the parapatric distributions of these two species. The zone is closely related with a habitat transition from wet to dry forest, whic h suggests that the narrow zone of parapatry is maintained primarily b y ecological adaptation. Selection on colour pattern loci, assortative mating and hybrid inviability may also be important. The genetics of hybrids between the two species shows that the major gene control of p attern elements is similar to that found in previous studies of H. era to races, and some of the loci are homologous. This suggests that simi lar generic processes are involved in the morphological divergence of species and races. Evidence from related Heliconius supports a hypothe sis that ecological adaptation is the driving force for speciation in the group. (C) 1996 The Linnean Society of London