DISCUSSION AND CONCLUSIONS (continued)

3. What are the likely circumstance for cross-species recombination in nature?

While I have never witnessed the process of interspecific hybridization in nature, I have seen areas where hybrids have been collected or observed, I have collected wild hybrids, and I know what is required to establish synthetic hybrid zones in a greenhouse. As long as both species populations are relatively equal in density, it is actually possible to keep cydno and melpomene in the same small 13 ft x 21 ft glass house for years without the occurrence of interspecific courtship or mating (personal observation). My method of obtaining interspecific matings involves releasing a virgin female into an insectary containing males of a second species. The question then becomes when or where does that numerical and sex ratio imbalance occur in natural communities of Heliconius?

In lowland rainforests, Heliconius exist in habitat patches formed by disturbance gaps. These are colonized first by host plants, then by the butterflies. Populations thrive, then disappear as succession converts the gap back to shaded forest. A large gap might allow a second-growth species like melpomene to thrive, but differential dispersal by females (Gilbert 1991) can produce local male bias. Meanwhile, in a cydno population which typically has densities on the order of 1-2 adults per ha. (ibid.) it is not difficult to imagine that a virgin female cydno might wander onto the edge of a forest gap and be surrounded by male melpomene seeking to mate.

Differential colonization and extinction of remote unoccupied habitat patches constitutes a more likely crucible for interspecies crosses. If melpomene arrived in a small mountain valley a month before the first dispersing cydno female parent then no mature cydno males would greet first generation virgin cydno females in that patch even if they emerge with cohort brothers (it typically takes 72 hours before males are competent to mate). Such females very likely will be mated by mature melpomene males. It seems likely that forest fragmentation, either natural or anthropogenic, could provide the conditions for MCS clade species to exchange the contents of their genetic tool boxes. Indeed, areas that show the most active production of novel patterns in Heliconius consist of dissected steep, montane landscapes where isolation, natural disturbance, and funnel points for insects dispersing between valleys provide the ingredients for bringing this patchy hybrid zone scenario to life (e.g., Linares 1997b).