A population isolate is that group of persons within which individuals choose their partners. Such an isolate is also called a Mendelian population. Ideally the population isolate inhabits an island, a mountain valley, a peninsular region, a forested area, or even a large area covering several villages, where the marriage alliance is restricted within that endogamous group.
The general and simple definition of population is the number of people in an area at a given time. It may be used in reference to the number of people possessing a particular character or group of characteristics in an area at a given time. It is difficult to define a particular population strictly, for the actual boundaries around a specific human population are not always easy to find. A human population is usually found in a particular place, and it is a coherent entity largely because of geographical boundaries. Regardless of how they are circumscribed, the significance that populations have for evolutionary genetics lies in the web of genetic relationships within and between them—allele frequencies, consanguinity mating patterns, gene flow , natural selection, etc.
The genetic approach uses the concept of the Mendelian population, which Dobzhonsky has defined as “a reproductive community of sexual and cross fertilizing individuals which share in a common gene pool”. If the isolate or the Mendelian population is not changed by natural selection, nor by mutation, nor by migration, and if the population size is large and if individuals are not mating assortatively (that is, random choice of partners), then the isolate is said to be in equilibrium. These assumptions are fundamental to population analysis and for maintaining an equilibrium of genotypes from generations to generations. Although, all human gene pools are open to varying degree, it is evident that panmixis does not take place within the total species. The more important mechanisms maintaining genetic isolation of populations today are cultural rather than geographical.
Breeding Population
In addition to the problem introduced by the biological openness of human population systems, accurate definition of a human Mendelian population is complicated by the fact that man clusters in social groupings which may or may not serve as biological breeding units. So the first problem of the population geneticist, therefore, is to identify and describe, as accurately as possible, the biological population before he can undertake an analysis of the gene pool and forces acting on it. Because direct analysis of a population’s gene pool is impossible, all conclusions regarding its composition are necessarily inferential, and must be made on the basis of direct examination of the phenotypes of the reproducing individuals. To infer the composition of a gene pool at a single point in time the population geneticist must first enumerate and describe the actual progenitors, that is, the parents in a population. These progenitors constitute the breeding population.