Population Diversity

Population diversity refers to variation in the quantitative and spatial characteristics of populations, such as the numbers of individuals present and the geographic range of the population. An estimate of the overall population size provides a measure of the potential genetic diversity within the population; large populations usually represent larger gene pools and hence greater potential diversity.

The geographic range and distribution of populations (that is, their spatial structure) are key factors in analyzing their diversity, since they give an indication of the likelihood of the movement of organisms between populations and subsequent genetic interchange.

Isolated populations, with very low levels of interchange, show high levels of genetic divergence (Hunter, 2002, p. 145), and often show unique adaptations to the biotic and abiotic characteristics of their local environment—for example, competition with other organisms, local topography, and climate. Less isolated populations may show greater genetic exchange, and those populations are likely to be more homogenous.

Populations can be categorized according to the level of divergence between them. Isolated

Cottontail rabbit. Some rabbit species undergo population cycles of impressive abundance alternating with extreme scarcity, often influencing the population densities of their predators. (D. Robert and Lorri Franz/Corbis)

and genetically distinct populations of a single species may be referred to as subspecies. Presumably, in time, these populations, or subspecies, will become sufficiently genetically distinct that they can no longer interbreed and hence will represent different species. Populations that show less genetic divergence might be recognized as "variants" or "races." However, the distinctions between subspecies and other infra-subspecific categories can be somewhat arbitrary.

Studies of population diversity also include analyses of seasonal changes in population dynamics and distribution. These studies identify cyclical changes in population size, and whether certain populations migrate to different regions and habitats to reproduce or in search of food.

Individual organisms periodically disperse from one population to another. Groups of contiguous populations thereby form a larger so-called metapopulation. For instance, let us look at the distribution of five populations of field mice, randomly distributed over an area of 2,500 square meters.

Figure 2 represents a patchy distribution of populations of field mice in a landscape. Populations 1 and 5 are isolated from all other populations, are genetically quite distinct, and may be considered a subspecies. Populations 2 and 3 are adjacent to each other and closely related, with quite extensive genetic transfer between them. Populations 3 and 4 are also adjacent to each other, but a seasonal stream temporarily separates the two populations. Genetic exchange occurs only occasionally between 3 and 4, when the stream is dry; thus they represent partially isolated populations. In

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