Selective versus stochastic processes in the genetic differentiation of populations of the butterfly Erebia embla (Thnbg) (Lepidoptera, Satyridae)

Four populations of the butterfly Erebia embla were studied by allozyme electrophoresis. Three populations were isolated by distance and the fourth by distance and time (flying in different years). Populations differed significantly in allele frequencies at five polymorphic loci, but the difference between the odd year population and the even year populations was not larger than the differences among the latter. Monte Carlo simulations showed that the genetic differentiation was lower than expected if caused by drift and gene flow only, whereas strong heterotic selection could explain the low FST value (0.02). We therefore conclude that the genetic variation of the populations is stabilized by selective forces permitting stochastic processes to operate within narrow limits only.