Selection often overrides the effects of random processes in island populations of Philaenus spumarius (Homoptera)

The influence of various evolutionary factors on the populations of the colour polymorphic meadow spittlebug, Philaenus spumarius (Homoptera), was investigated on islands in the Gulf of Finland. Long-term (30-year) investigations showed that the populations can survive through very narrow bottlenecks, but after strong reduction in the size of a population, random genetic drift may remove alleles or change allele frequencies. One of the factors sustaining the presence of almost all the colour polymorphism alleles in almost all the populations is gene flow. Migration seems to be largely by air and by the sea surface (anemohydrochoric). On one rocky island, gene flow was able to carry the top dominant allele, p^T, to a number of scattered plant stands. However, it was not powerful enough to alter the differences in allele frequencies between populations living on the same island in meadows of different types 3 m or 12 m from each other. It is highly probable that the factor operating is meadow-specific selection. Its action appears to be based on the ecological character of the meadow. In the different meadows, the coverages of the food plants of the spittlebug are dissimilar. Differences in plant assemblages are possibly of importance in maintaining the meadow-specific allele frequencies in the spittlebug populations.