The foregoing pages concern the opposing processes of isolation and migration. In isolation a population maintains itself in (usually) a relatively constant environment, its gene pool remains unadulterated from outside, it evolves under the influences of natural selection, mutation, genetic drift and other random processes, and experiences internal changes in its genetic structure. Migration exposes the migrants to environmental change, so that comparison with those in the same population who did not migrate and are presumed to be of the same genetic constitution demonstrates the effect of change of environment, and so helps to separate out the genetic and environmental components of biological variation. This book aims to illustrate the biological effects of the two processes, to show how the situations to which they give rise may be used to elucidate a variety of biological problems ranging from evolutionary change to disease etiology. This object is achieved by a number of examples, the majority of which have been studied by Asian investigators and are less widely known than the classic Western studies of say the Amish, Yanomama or the Aland islanders (e.g. McKusick, 1978; Neel & Weiss, 1975; Eriksson, 1980). Besides documenting the results, the chapters illustrate the different methods employed in such studies.

Isolates

Isolation comes about or is maintained in many ways, and isolating factors include geographical barriers, distance, religious differences, hostility, mating patterns and other social and cultural variables (Roberts, 1975, 1984). The term isolate is used in many ways, ranging from the conceptual circle of marriages within an otherwise continuous population envisaged by Dahlberg (1929) to the small reproductively sequestered island populations where breeding and geographical boundaries coincide and are virtually absolute.