Studies on the biology of the butterflies anthocharis cardamines (L) and pieis napi (L), in relation to speciation in Pierinae

Abstract

The evolutionary biology of Pierinae is described in three separate studies. In Part One, the population biology of the Orange Tip butterfly (Anthocharis card amines) is described. Colonies of this species are localised in riverbank habitats in Co. Durham. Using mark-recapture studies, the population size and movements of individual males were assessed. Studies of individual pre-adult survival indicated that food-plant related mortality and parasitisation were important causes of death. However a key-factor analysis for one population showed that failure of adult females to lay all their eggs was" the most Important factor influencing population size. The adaptiveness of oviposition behaviour was examined by following individual females and by mapping the distribution of eggs upon foodplants. It was shown that many aspects of oviposition choice were best interpreted as chance outcomes of searching behaviour. The observed differences in larval survival on different foodplants, and the constraints placed by time shortage on oviposition were used to construct a simple optimality model. It was shown that oligophagy, the typical condition of A. cardamines populations, leads to increased individual fitness when compared to monophagy. The effect of this stabilising selection on future speciation in Pierinae is discussed. The concept of effective population size is developed for A. cardainines and it is shown that allelic variation at one locus conforms to the predictions of sampling theory in small populations. Part two describes the mating behaviour of several Pierinae, and demonstrates that reproductive isolation in this group is not a result of male behaviour, but of female discrimination of male characters. The agreement of observed behaviour with the predictions of sexual selection is noted, and the likely effects of sexual selection in past and future speciation is discussed. Part three describes the amount of structural gene change that has occurred during the evolution of Pierinae It is shown that little genetic differentiation at such loci occurs prior to or during speciation. Rather, differentiation of species appears to occur at other loci, including those involved in local adaptation, as was described for montane and lowland populations of the Green-Veined White butterfly (Artogeia napi) in Co. Durham. The importance of these results to the wider field of speciation studies is discussed.