In the English language the term isolate designates ‘a relatively homogeneous population separated from related populations by geographic or biological or social factors or by the intervention of man’ (Webster's Third New International Dictionary, 1971). This broad definition embraces a number of different genetic situations. Since, in any genetic study of an isolate, the expectation with respect to the distribution of allele frequencies and genetic disease differs according to the biological basis of the isolate, it is of some importance in genetics to distinguish between the various types of isolates. I suggest that from the genetic standpoint there are three principal types.

The first, the primary isolate, is typified by tribal populations of presumably very ancient origin which since they emerged as distinct entities have had relatively little biological exchange with other similar groups. In theory these populations should be as close to genetic equilibrium as any contemporary human populations although the stochastic process in these small subdivided groups ensures that equilibrium - if ever realised - is a fleeting phenomenon.

The second and third types of isolate, both of which may be termed secondary, come into being when a group for some reason detaches itself - or is detached - from a larger, usually national population which is a relatively recent amalgamation of smaller groups. The distinction between these two types depends simply on size. Thus, the second type of isolate to be delineated results when a relatively large group detaches itself from a still larger group and migrates into a new setting.