A hybrid zone provides evidence for incipient ecological speciation inHeliconiusbutterflies

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Abstract
InHeliconiusbutterflies, it has been proposed that speciation occurs through a combination of divergence in ecological habitat preferences and mimetic colour patterns. Here we test this hypothesis by investigating a parapatric form of the widespread speciesHeliconius erato. Mendelian (colour patterns) and molecular genetic data permit us to address hypotheses about introgression and genetic differentiation between different populations. Combined analysis of colour pattern, microsatellite loci and mitochondrial DNA showed thatHeliconius erato venusandHeliconius erato chestertoniiform a bimodal hybrid zone implying partial reproductive isolation. In a sample of 121 individuals collected in sympatry, 25% were hybrids representing a significant deficit of heterozygotes compared to the Hardy–Weinberg expectation. Seven microsatellite loci, analysed for a subset of these individuals, showed marked differentiation between the parental taxa, and unambiguously identified two genotypic clusters concordant with our phenotypic classification of individuals. Mitochondrial DNA analysis showedH. erato venusas a monophyletic group well differentiated fromH. erato chestertonii, implying a lack of historical introgression between the populations.Heliconius erato chestertoniiis therefore an incipient species that maintains its integrity despite high levels of hybridization. Moreover,H. erato chestertoniiis found at higher altitudes than other races ofH. eratoand has a distinct colour pattern and mimetic relationship. Hence, there are now two examples of parapatric incipient species related toH. erato,H. himeraandH. erato chestertonii, both of which are associated with higher altitudes, more arid habitats and distinct mimetic relationships. This implies that parapatric habitat adaptation is a likely cause of speciation in this group.