EFFECTIVE POPULATION SIZE: Ne

One of the most important parameters affecting genetic diversity is effective population size (Ne). Effective population size places an upper limit on the amount of genetic diversity that can be maintained in a population in relation to its pedigree history and potential losses due to genetic dirfit. Some general guidelines for maintaining minimum Ne in distinct, or semi-isolated, populations have been proposed:

Ne > 50 to prevent inbreeding depression and a detectable decrease in viability or reproductive fitness of a population (Franklin 1980)

Ne > 500 to maintain constant genetic variance in a population resulting from a balance between loss of variance due to genetic drift and the increase in variance due to spontaneous mutations (Franklin 1980; FAO/UNEP 1981; Soule 1980; Lande and Barrowclough 1987; Lande 1988).

Ne > 1000 to maintain adaptive genetic variation and survival of small populations (Lynch, M. 1990.)

Ne > 5,000 to maintain a constant variance for quasi-neutral, genetic variation that can serve as a reservoir for future adaptations in response to natural selection and changing environmental conditions (Lande 1995). The rationale here is that Ne needs to be large enough to minimize genetic drift and the potential loss of alleles that may confer a slight, selective advantage under existing and future environmental conditions.

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Source: Washington Hatchery Scientific Review Group, December 2000

References:

Franklin, J.R. 1980. Evolutionary changes in small populations Pp. 135-149, In: M.Soule (ed.) Conservation Biology: and evolutionary-ecological perspective. Sinauer Associates, Sunderland, MA.

Food and Agriculture Organization of the United Nations. 1981. Conservation of the genetic resources of fish: Problems and recommendations. Report of the Expert Consultation on the Genetic Resources of Fish, Rome, 9-13 June 1980, FAO Fisheries Technical Paper 217, 43 pp.

Lande R. and G.F. Barrowclough. 1987. Effective population size, genetic variation, and their use in population management. Pages 87-124. In:M. Soule (ed) Viable populations for Conservation. Cambridge University Press. Cambridge, England.

Lande, R. 1988. Genetics and demography in biological conservation. Science 241:1455-1460.

Lande, R. 1995. Mutation and conservation. Conservation Biology 9:782-791.

Lynch, M. 1990. Mutation load and the survival of small populations. Evolution 44:1725-1737.

Soule, M. 1980. Thresholds for survival: Maintaining fitness and evolutionary potential. In Conservation Biology: An Evolutionary-Ecological Perspective, ed. M.E. Soule and B.A. Wilcox (Sinauer Associates, Sunderland, Mass.), pp. 151-170.