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Sabine S. Loew
and eventually lost in the population. Consequently, strong selection for particular
traits homogenizes the gene pool, ultimately reducing the adaptive potential of a
population. This relationship is of concern in captive propagation programs that
breed animals for future reintroduction to the wild; any inadvertent selection to
improve management in captivity may reduce survival chances back in the wild.
In general, the interactions between phenotypes, genotypes, and fitness are far too
complex to predict reliably, and conservation biologists generally refrain from
managing captive populations based on specific beneficial traits; instead, they try
to maintain overall genetic diversity on which natural selection can act.
Maintaining Genetic Diversity and Effective Population Size
Ideal populations are defined as infinite populations, consisting of sexually reproducing diploid organisms that mate at random and have nonoverlapping generations and whose allele frequencies are not affected by migration, mutation, or
selection (Wright 1931). The probability of loss of an allele in ideal populations is
equal to its allele frequency.
As real populations almost always violate the assumptions of an ideal population, Sewall Wright (1931) introduced the concept of the effective population size
(N e ) to evaluate the evolutionary potential of populations that deviate from the
ideal. The effective population number is most commonly estimated by relating
the variance in allelic frequency (the “variance effective size”) or the rate of
inbreeding (“inbreeding effective size”) of the real to the ideal population (Wright
1931; Crow and Denniston 1988). Accordingly, the effective population size of a
real population equals the size of an ideal population that has the same amount of
variance in allele frequencies or the same amount of inbreeding as the actual
population (Wright 1931, 1938; Crow and Kimura 1970).
Maximizing the inbreeding effective size maintains heterozygosity within local
populations, whereas high variance effective size decreases loss of genetic diversity across local populations and significantly affects allelic diversity (Crow and
Kimura 1970; Gliddon and Goudet 1994; Ballou and Lacy 1995; Hedrick et al.
1995). In general, N e is smaller than the actual population size, and loss of genetic
variation increases with decreasing effective population size. Numerous factors,
such as unequal sex ratio and fluctuating population size, can significantly affect
effective population size (for detailed discussion, see Falconer and Mackay 1996).
For example, calculations of effective population size are usually based on discrete generation models, which are not applicable to many real populations.
A realistic diploid model with overlapping generations, however, is very complex as it attempts to estimate allele frequency changes per unit time for males and
females that might differ in their age-specific birth and death rates (Felsenstein
1971; Lande and Barrowclough 1987). The effective population size per unit time
is maximized if the generation lengths of males and females are equal, and it
decreases with shorter maturation time. In other words, species that experience
delayed sexual maturation and have long generation times (e.g., large-bodied
Sabine S. Loew
and eventually lost in the population. Consequently, strong selection for particular
traits homogenizes the gene pool, ultimately reducing the adaptive potential of a
population. This relationship is of concern in captive propagation programs that
breed animals for future reintroduction to the wild; any inadvertent selection to
improve management in captivity may reduce survival chances back in the wild.
In general, the interactions between phenotypes, genotypes, and fitness are far too
complex to predict reliably, and conservation biologists generally refrain from
managing captive populations based on specific beneficial traits; instead, they try
to maintain overall genetic diversity on which natural selection can act.
Maintaining Genetic Diversity and Effective Population Size
Ideal populations are defined as infinite populations, consisting of sexually reproducing diploid organisms that mate at random and have nonoverlapping generations and whose allele frequencies are not affected by migration, mutation, or
selection (Wright 1931). The probability of loss of an allele in ideal populations is
equal to its allele frequency.
As real populations almost always violate the assumptions of an ideal population, Sewall Wright (1931) introduced the concept of the effective population size
(N e ) to evaluate the evolutionary potential of populations that deviate from the
ideal. The effective population number is most commonly estimated by relating
the variance in allelic frequency (the “variance effective size”) or the rate of
inbreeding (“inbreeding effective size”) of the real to the ideal population (Wright
1931; Crow and Denniston 1988). Accordingly, the effective population size of a
real population equals the size of an ideal population that has the same amount of
variance in allele frequencies or the same amount of inbreeding as the actual
population (Wright 1931, 1938; Crow and Kimura 1970).
Maximizing the inbreeding effective size maintains heterozygosity within local
populations, whereas high variance effective size decreases loss of genetic diversity across local populations and significantly affects allelic diversity (Crow and
Kimura 1970; Gliddon and Goudet 1994; Ballou and Lacy 1995; Hedrick et al.
1995). In general, N e is smaller than the actual population size, and loss of genetic
variation increases with decreasing effective population size. Numerous factors,
such as unequal sex ratio and fluctuating population size, can significantly affect
effective population size (for detailed discussion, see Falconer and Mackay 1996).
For example, calculations of effective population size are usually based on discrete generation models, which are not applicable to many real populations.
A realistic diploid model with overlapping generations, however, is very complex as it attempts to estimate allele frequency changes per unit time for males and
females that might differ in their age-specific birth and death rates (Felsenstein
1971; Lande and Barrowclough 1987). The effective population size per unit time
is maximized if the generation lengths of males and females are equal, and it
decreases with shorter maturation time. In other words, species that experience
delayed sexual maturation and have long generation times (e.g., large-bodied