Population Genetics and Endangered Species

In applying the principles of population genetics to the field of endangered species conservation, biologists are interested primarily in genetic variation, in particular its distribution and maintenance. In general genetic diversity is considered to be a good thing, and the more the better. In terms of population structure, multiple large populations of a species which are in some way in contact with each other provide a good situation for maintaining variation. However, in the case of endangered species, we are generally faced with the opposite situation: a small number of populations which are isolated from one another, each containing a small number of individuals. Applying population genetic theory to such situations may point the way to management strategies which will maximize the maintenance of existing variation.

Why is genetic variation good?

One fundamental property of natural environments is unpredictability. Think about how difficult it is to predict the weather. We humans are experts at manipulating our artificial surroundings so that day-to-day fluctuations in climate affect us relatively little. However, for an insect, every day may present a challenge: to find water, to find food, to get out of, or into, the sun. There is no one strategy which will always guarantee success. However, if sufficient genetic variation, and thus life strategy variation, exists, the population can be assured of persistence through the survival and reproduction of at least some of its members. As an example, only 1-2% of the offspring of a Bay Checkerspot Butterfly survive to reproductive age. The other 98-99% are weeded out by the environmental challenges they face. It is partly due to genetic variation that the offspring are not all destroyed. Yet when populations are small the tendency for genetic variation to diminish makes survival of the population even less likely.

Population Structure

One of the most important applications of population genetic principles is the understanding of connectedness among populations of a species (or subspecies). Species usually exist as some number of populations occurring in different areas of appropriate habitat. Individual populations may have varying degrees of contact, from frequent genetic interchange to nonexistent. Connectedness is generally measured by examining the frequencies of different alleles, or forms of a specific gene, at several different genes. If the frequencies differ significantly between two areas, it is likely that there is some restriction in gene flow between them. If it appears that there is no difference in frequencies from one area to another, it may be supposed that there is some genetic connection preventing differentiation (other interpretations are, of course, possible). The interpretation of genetic connectedness is difficult as its significance is situation dependent. Strong interpopulational connectedness (presumably through frequent migration) will be good for promoting the maintenance of overall genetic diversity; rare alleles are less likely to disappear in a larger population. However, a disadvantage to strong interpopulational connectedness is that deleterious alleles and diseases may more easily spread through a species whose populations are in frequent contact with each other. It is important to understand the threats to a species in order to properly interpret information on interpopulational structure.

Metapopulations

Metapopulations are frequently mentioned in endangered species conservation literature. Metapopulations represent a specific type of population structure in which several small relatively isolated populations collectively function like a single large population. On a landscape scale, this situation often arises when patches of appropriate habitat are small and dispersed. Each individual patch may be susceptible to year-to-year fluctuations in quality for the species of interest, and subpopulations may often go extinct. However, connectedness among areas is generally so high that suitable habitat will be quickly reinvaded. The importance is in understanding. It is crucial to understand when such a situation is operating within a species and that apparently isolated subpopulations cannot persist indefinitely. Numerous subpopulations must be protected and allowed to maintain contact in order for the metapopulation to survive. Habitat fragments are often preserved in isolation. While all such efforts should be applauded, it also must be recognized that it will represent merely the first of several necessary steps in the protection of some species.

Degree of Genetic Difference

Understanding degrees of genetic differentiation is also pertinent to endangered species conservation. A measure of how distinct a particular species or subspecies of concern is from its nearest relative may allow us to prioritize our efforts to some extent. A strong application of this idea seeks repeated patterns of distinctness in order to identify entire systems which require our attentions. This whole idea of prioritization is very controversial, however. Genetic distinctness and ecological importance are not necessarily correlated and which is more important is unanswerable. Genetic distinctness should be viewed as only one of many criteria in assessing conservation merit.

For further reading:
Avise, J. C., 1994. Molecular Markers, Natural History, and Evolution. New York, Chapman and Hall.

Moritz, C., 1994. Applications of mitochondrial DNA analysis in conservation: a critical review. Molecular Ecology 1994(3): 401- 411.