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Sabine S. Loew
Merits of Genetic Diversity
Genetic Diversity and Adaptation
Long-term survival of a species depends on its adaptation to current and future
biotic and abiotic aspects of its environment. Natural selection results in the
survival and propagation of those individuals that are best adapted to prevailing
conditions. In a genetically diverse population, individual fitnesses vary, and
differential survival affects gene frequencies within and between populations. On
changes in the environment, natural selection may favor different genotypes and
hence alter the distribution of gene frequencies. However, a population depauperate of genetic variation may not carry any individuals that are genetically preadapted to the new environment and hence runs an increased risk of extinction. To
maximize the probability of long-term survival of species, especially in changing
environments, conservation geneticists seek to maintain high genetic diversity,
although this variation may be represented by neutral single locus variation or by
variation in fitness-related quantitative traits.
Genetic Diversity and Inbreeding Depression
Inbreeding (nonrandom mating with respect to relatedness) skews genotype frequencies within populations toward increased frequencies of homozygotes. Inbreeding in combination with selection against homozygotes can reduce the reproductive performance of naturally outbreeding populations (Wright 1977;
Falconer and Mackay 1996) and therefore may decrease long-term survival (Ralls
et al. 1988; Thornhill 1993; Frankham 1995c, 1998). In general, inbred offspring
are expected to be less fit than offspring produced by random matings (Ralls et al.
1979, 1988; Thornhill 1993; Falconer and Mackay 1996). This reduction of fitness with inbreeding (inbreeding depression) is manifested by reduced growth
rate, fertility, fecundity, survival, developmental stability, or changed mating
behavior among inbred offspring (Lerner 1954; Wright 1977; Ralls and Ballou
1982; Miller et al. 1993; Keller et al. 1994; Rave et al. 1994; Falconer and Mackay
1996). Likely mechanisms for inbreeding depression have been debated for
decades (Charlesworth and Charlesworth 1987; Shields 1993; Thornhill 1993),
and the two competing explanations (among others such as partial and associative
overdominance), the overdominance and dominance hypotheses, are compatible
with many theoretical and empirical results. Proponents of overdominance argue
that heterozygous genotypes are on average fitter than homozygotes and as inbreeding reduces heterozygote frequencies, it depresses population performance
(Mitton 1993). The dominance hypothesis contends that inbreeding depression is
due to the increased expression of recessive deleterious alleles resulting from an
increase in the number of homozygous loci.
Distinction of these hypotheses is not merely of academic value but could have
implications for conservation management, especially for small inbred populations. If inbreeding depression is a consequence of the reduction of heterozygous
Sabine S. Loew
Merits of Genetic Diversity
Genetic Diversity and Adaptation
Long-term survival of a species depends on its adaptation to current and future
biotic and abiotic aspects of its environment. Natural selection results in the
survival and propagation of those individuals that are best adapted to prevailing
conditions. In a genetically diverse population, individual fitnesses vary, and
differential survival affects gene frequencies within and between populations. On
changes in the environment, natural selection may favor different genotypes and
hence alter the distribution of gene frequencies. However, a population depauperate of genetic variation may not carry any individuals that are genetically preadapted to the new environment and hence runs an increased risk of extinction. To
maximize the probability of long-term survival of species, especially in changing
environments, conservation geneticists seek to maintain high genetic diversity,
although this variation may be represented by neutral single locus variation or by
variation in fitness-related quantitative traits.
Genetic Diversity and Inbreeding Depression
Inbreeding (nonrandom mating with respect to relatedness) skews genotype frequencies within populations toward increased frequencies of homozygotes. Inbreeding in combination with selection against homozygotes can reduce the reproductive performance of naturally outbreeding populations (Wright 1977;
Falconer and Mackay 1996) and therefore may decrease long-term survival (Ralls
et al. 1988; Thornhill 1993; Frankham 1995c, 1998). In general, inbred offspring
are expected to be less fit than offspring produced by random matings (Ralls et al.
1979, 1988; Thornhill 1993; Falconer and Mackay 1996). This reduction of fitness with inbreeding (inbreeding depression) is manifested by reduced growth
rate, fertility, fecundity, survival, developmental stability, or changed mating
behavior among inbred offspring (Lerner 1954; Wright 1977; Ralls and Ballou
1982; Miller et al. 1993; Keller et al. 1994; Rave et al. 1994; Falconer and Mackay
1996). Likely mechanisms for inbreeding depression have been debated for
decades (Charlesworth and Charlesworth 1987; Shields 1993; Thornhill 1993),
and the two competing explanations (among others such as partial and associative
overdominance), the overdominance and dominance hypotheses, are compatible
with many theoretical and empirical results. Proponents of overdominance argue
that heterozygous genotypes are on average fitter than homozygotes and as inbreeding reduces heterozygote frequencies, it depresses population performance
(Mitton 1993). The dominance hypothesis contends that inbreeding depression is
due to the increased expression of recessive deleterious alleles resulting from an
increase in the number of homozygous loci.
Distinction of these hypotheses is not merely of academic value but could have
implications for conservation management, especially for small inbred populations. If inbreeding depression is a consequence of the reduction of heterozygous