Background: To understand speciation and the maintenance of taxa as separate entities, we need information about natural hybridization and gene flow among species.

To understand s p e we need to study the gewtics interspecific h y b r e t i o n still occurs. Two closely z a t e d criterion: Heliconius&mera is endemic to dry forest and Peru, while its sister spzies, H c a t o , is u b i q u G s in wet forest throughout south and central Amzrica. ?;;-three known zones of cozact, the two spzies remain distinct, while h e d s are found at low frequency. Collections in southern EcAador show that the conta=on_e is about 5km wide, half the width of the narrowest clines between c o l o m a t g n races of H. erato. The narrowness of this cline argues that very strong selection (s -1) is maintaining the parsatric distributions of these two species. The zone is closely related with a habitat transition from wet to dry fzest, which suggests that the narrow z o n e of pay2atry is maintain= primarily by ecoJogical adaEtation. Selection on c o l o u x t t c lo& assortative mating and hykrid inviability may also be important. The gene& of hybrids between the two spsies shows that the major geAe control of patsrn elements is similar to that found in previous studies of H. eralo races, and some of the &i are homologous. This suggests that similar g e s t i c processes are involved in thFmorphological divergence of spcies and races. Evidence from related Hcliconius supports a hypzhesis that eaogical a k t a t i o n is the driving force for speciation in the group.-01996 The Lnnran Socirry of London ADDITIONAL KEY WORDS: -parapatry -Pa.~@ora -refugium biogeographyhybridization.