Ontogeny and Population Biology of a Sex-Limited Colour Polymorphism

Sammanfattning: This study has involved investigation of number of populations of the damselfly Ischnura elegans over several years, looking at frequency changes over time, morphological differences between the sexes and the morphs, differences in growth rate and development time, differences in fecundity between the morphs, and genetic differentiation between populations. I. elegans is a small annual damselfly with three female colour morphs (Androchrome, Infuscans, and Infuscans-obsoleta), one of which (the Androchrome) is considered a male mimic; males are monomorphic. Empirically estimated selection coefficients were used in a population genetic and dynamic model with environmental variation which predicted coexistence of the three morphs. This suggests that negative frequency-dependence on fecundity mediated by male mating harassment is sufficient to explain coexistence of the morphs. Field data also showed that fecundity decreased with increasing frequency in the population for all three morphs. Investigation of sexual dimorphism revealed that the sexes differed in development time, growth rates, and adult size and shape and that sexual size dimorphism is produced in I. elegans by a combination of development time and development rate acting in concert. The female morphs also differed in development time, growth rates, and adult size and shape, although in this case effects of development time and development rate cancelled each other out. In addition, male-like body shape had a negative effect on fecundity in Infuscans-obsoleta females but not in Androchrome or Infuscans females. Quantitative genetic parameters for morphological traits showed that an evolutionary response to selection on body shape is possible. Comparisons of Fst-values and the lack of isolation by distance both suggest that this is a highly dynamic non-equilibrium system, and genetic diversity appears to be influenced by wind direction. The results in this study indicate that the morphs are subject to negative frequency-dependent selection via male mating harassment, and that the differences between the morphs are part of their identity as alternative adaptive strategies.