The effective study of the influence of isolation on an animal population necessitates the choice of a species which is not only abundant and easy to recognize, but which is readily contained by barriers of reasonably small dimensions. The satyrine butterfly,
Maniola jurtina
, has proved to be ideal material for this kind of work in the Isles of Scilly and elsewhere. Its large numbers, coupled with a reluctance to cross certain types of obstacles, sometimes little more than a hundred yards in extent, have led to the formation of many distinct populations. These provide ideal material for investigating adaptation and the action of natural selection. During our studies of population dynamics in
Maniola jurtina
on the small uninhabited island of Tean, Isles of Scilly, in 1946 (see figure 1), E. B. Ford and I noticed that the number of spots on the underside of the hind wings of the butterfly appeared to vary in both sexes, ranging from 0 to 6 on each wing. The capture of representative samples confirmed our opinion that the spot distribution was characteristic for each sex, being unimodal in the male and bimodal in the female. Moreover, this condition appeared to be a constant one, the distribution of spots, whatever its selective value may be, having become stabilized in the population by the action of natural selection (Dowdeswell, Fisher & Ford 1949; Dowdeswell & Ford 1952, 1953, 1955; Dowdeswell, Ford & McWhirter 1956).
Main Trends in the Vertebrate Animal Population Dynamics in a Clayey Semidesert of the Trans-Volga Region Since the Mid-18th Century
