14. Role of Genetics in Conservation Biology
243
mammals) have a higher probability of retaining genetic diversity for any given
period of time (see Hedrick 1992b for review). These species will, however, lose
genetic variation at the same rate as short-lived animals following a bottleneck if
time is measured in generations.
It is crucial to realize that high genetic diversity in long-lived species (e.g.,
One-horned Rhinoceros; Dinerstein and McCracken 1990) does not necessarily
mean that previous population crashes did not affect them, but instead it may
mean that they have not reproduced much since then. Hence, we should realize the
opportunity to maintain the genetic diversity stored through such “walking gene
banks” by maximizing their effective population size for future generations.
To determine effective population size when several factors are of importance,
Chepko-Sade and colleagues (1987) suggested sequential calculation of N e for all
variables by using an iterative process. They used this approach as a first approximation to reach more realistic estimates of the evolutionary potential (N e ) for a
variety of well-studied natural populations (e.g., wild horses, Black Bears, Dwarf
Mongoose), but its theoretical validity has not been tested. Numerous alternative
approaches to estimate N e have been developed to address incomplete data sets
and effects of multiple variables (Frankham 1995a; Rockwell and Barrowclough
1995). In addition to demographic approaches, change in various genetic measures (e.g., allozyme heterozygosity, pedigree inbreeding) has been used to estimate effective population size (Avise et al. 1988; Tomlinson et al. 1991; Briscoe et
al. 1992). Frankham (1995b) argued that genetic versus demographic methods
show comparable results, provided the same variables were used to determine
short-term estimates of N e . In general, he concluded that population fluctuations,
variance in family size, and unequal sex ratio affect the ratio of effective to actual
population size most significantly and that the effective size for most wild population is disconcertingly small (N e /N = 0.10–0.11).
In conclusion, loss of genetic variation in endangered or rare species can be
minimized through breeding programs that aim to maximize effective population
sizes and reduce inbreeding. Outbreeding is generally considered an appropriate
measure to maintain or generate genetic diversity; however, individuals should
not be outbred indiscriminately. If most genetic variation is found in a number of
different geographic regions, regular genetic exchange between those populations
would actually reduce overall genetic diversity and homogenize rather than diversify the species’ gene pool (= genetic cost of dispersal corridors and translocation). Furthermore, populations that experience varying ecological conditions and
have been isolated from each other for an extended period of time might show
important local adaptations. Under such conditions, gene flow would enhance
genetic variation within populations but might also result in reduced fitness of the
less well-adapted outbred offspring (outbreeding depression) (Shields 1982; Templeton et al. 1986; Ballou 1995).
In addition, long-term historical divisions within species may be the sign of
ongoing adaptive radiation, and individual populations might have to be treated as
separate evolutionary entities. Consequently, recommendations for the genetic
management of endangered species should be based on an assessment of the
243
mammals) have a higher probability of retaining genetic diversity for any given
period of time (see Hedrick 1992b for review). These species will, however, lose
genetic variation at the same rate as short-lived animals following a bottleneck if
time is measured in generations.
It is crucial to realize that high genetic diversity in long-lived species (e.g.,
One-horned Rhinoceros; Dinerstein and McCracken 1990) does not necessarily
mean that previous population crashes did not affect them, but instead it may
mean that they have not reproduced much since then. Hence, we should realize the
opportunity to maintain the genetic diversity stored through such “walking gene
banks” by maximizing their effective population size for future generations.
To determine effective population size when several factors are of importance,
Chepko-Sade and colleagues (1987) suggested sequential calculation of N e for all
variables by using an iterative process. They used this approach as a first approximation to reach more realistic estimates of the evolutionary potential (N e ) for a
variety of well-studied natural populations (e.g., wild horses, Black Bears, Dwarf
Mongoose), but its theoretical validity has not been tested. Numerous alternative
approaches to estimate N e have been developed to address incomplete data sets
and effects of multiple variables (Frankham 1995a; Rockwell and Barrowclough
1995). In addition to demographic approaches, change in various genetic measures (e.g., allozyme heterozygosity, pedigree inbreeding) has been used to estimate effective population size (Avise et al. 1988; Tomlinson et al. 1991; Briscoe et
al. 1992). Frankham (1995b) argued that genetic versus demographic methods
show comparable results, provided the same variables were used to determine
short-term estimates of N e . In general, he concluded that population fluctuations,
variance in family size, and unequal sex ratio affect the ratio of effective to actual
population size most significantly and that the effective size for most wild population is disconcertingly small (N e /N = 0.10–0.11).
In conclusion, loss of genetic variation in endangered or rare species can be
minimized through breeding programs that aim to maximize effective population
sizes and reduce inbreeding. Outbreeding is generally considered an appropriate
measure to maintain or generate genetic diversity; however, individuals should
not be outbred indiscriminately. If most genetic variation is found in a number of
different geographic regions, regular genetic exchange between those populations
would actually reduce overall genetic diversity and homogenize rather than diversify the species’ gene pool (= genetic cost of dispersal corridors and translocation). Furthermore, populations that experience varying ecological conditions and
have been isolated from each other for an extended period of time might show
important local adaptations. Under such conditions, gene flow would enhance
genetic variation within populations but might also result in reduced fitness of the
less well-adapted outbred offspring (outbreeding depression) (Shields 1982; Templeton et al. 1986; Ballou 1995).
In addition, long-term historical divisions within species may be the sign of
ongoing adaptive radiation, and individual populations might have to be treated as
separate evolutionary entities. Consequently, recommendations for the genetic
management of endangered species should be based on an assessment of the