Rapid Evolution and the Conditions Which Make It Possible

A number of distinct situations contribute to rapid evolutionary change. The majority, but not all, depend upon the sub-division of a population into small isolated or semi-isolated units. These will be considered first. The effect of such isolation, when least compli-cated by other issues, may conveniently be dis-cussed in the light of an example drawn from stud-ies on the butterfly Manlola]urtina (Satyridae) in the Isles of Scilly. In this insect the number of spots on the under-side of the hind-wings can vary from 0 to 5, and is genetically determined on a multifactorial basis. This variation is sex controlled. In the male, the frequency-distribution is every-where the same, with a single mode at two spots,