Adaptive Novelty through Introgression in Heliconius Wing Patterns: Evidence for Shared Genetic “tool Box” from Synthetic Hybrid Zones and a Theory of Diversification

The neotropical genus Heliconius has long fascinated evolutionary biologists because of puzzles presented by variation of wing pattern within and among species. The strikingly diverse wing patterns which characterize different races of a species apparently evolve rapidly (Brower, 1994a) and at different rates from overall genomic divergence (Turner et al. 1979). Studies of gene flow in hybrid zones between races show that genes controlling conspicuous pattern elements exhibit much steeper clines of frequency than genes with little or no effects on pattern (Mallet 1986, Mallet et al. 1990, Jiggins et al. 1996, Linares 1989) and manipulative experiments (Benson 1972, Mallet and Barton 1989) support the idea that visual selection by predators exerts a strong conservative effect to preserve ruling phenotypes within the boundaries of a race. Perhaps the biggest puzzle then, is how such qualitatively distinct patterns arise, persist, and come to characterize an area of a species’ range in the first place. Many aspects of this general problem have been thoroughly explored and convincingly explained by Turner and Mallet (1996) but the source of the novelty that fuels this adaptive radiation of mimetic forms remains an unexplored question.