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16
Theoretical Properties of
Extinction by Inbreeding Depression
Under Stochastic Environments
Yoshinari Tanaka
Introduction
Reduction of genetic diversity is regarded as one of the important factors for
extinction of small populations (Frankel and Soul´ e 1981; Caughley and Gunn
1996). Loss of genetic variation inhibits adaptive evolution and directly reduces
fitness by inbreeding depression, which results from biased mating with close
relatives or random mating in small populations. Inbreeding depression may
induce rapid extinction of populations. This extinction is the consequence of the
positive feedback between declines in population size and increases in inbreeding.
This phenomenon has been called an extinction vortex (cf. Gilpin and Soul´ e
1986).
The correlations between fitness and heterozygosity and the rates of inbreeding
depression estimated for natural populations provide indirect evidence of inbreeding depression in nature (Mitton and Grant 1984; Zouros 1987; Barrett and
Charlesworth 1991; Mitton 1993; Willis 1993; Ouborg and Treuren 1994; Britten
1996; David et al. 1997; Schierup 1998). Although numerous studies have estimated genetic heterozygosity of endangered species (Loeschcke et al. 1994; Avise
and Hamrick 1996), the proper biological interpretation of these estimates and
their relevance to the extinction process are not completely clear.
There are two shortcomings. First, few examples of population extinction due
to inbreeding depression have been established. A field survey on Glanville
Fritillary Butterflies (Melitaea cinxia) is one of the very few studies that have
reported inbreeding depression as a causal factor of extinction in natural populations (Saccheri et al. 1998). Second, the paucity of investigations on the theoretical properties of the extinction vortex by inbreeding depression makes it impossible to evaluate the genetic risks of extinction based on empirical data.
Previous studies have considered the extinction vortex with genetic and population models, and suggest that rapid extinction due to inbreeding depression is
restricted but possible with observed genetic parameters (Tanaka 1997, 1998,
2000. Gene frequencies, the inbreeding coefficient, and the equilibrium population size achieved by the balance among mutation, selection, and drift are locally
stable, which suggests that an equilibrial population does not spontaneously go
16
Theoretical Properties of
Extinction by Inbreeding Depression
Under Stochastic Environments
Yoshinari Tanaka
Introduction
Reduction of genetic diversity is regarded as one of the important factors for
extinction of small populations (Frankel and Soul´ e 1981; Caughley and Gunn
1996). Loss of genetic variation inhibits adaptive evolution and directly reduces
fitness by inbreeding depression, which results from biased mating with close
relatives or random mating in small populations. Inbreeding depression may
induce rapid extinction of populations. This extinction is the consequence of the
positive feedback between declines in population size and increases in inbreeding.
This phenomenon has been called an extinction vortex (cf. Gilpin and Soul´ e
1986).
The correlations between fitness and heterozygosity and the rates of inbreeding
depression estimated for natural populations provide indirect evidence of inbreeding depression in nature (Mitton and Grant 1984; Zouros 1987; Barrett and
Charlesworth 1991; Mitton 1993; Willis 1993; Ouborg and Treuren 1994; Britten
1996; David et al. 1997; Schierup 1998). Although numerous studies have estimated genetic heterozygosity of endangered species (Loeschcke et al. 1994; Avise
and Hamrick 1996), the proper biological interpretation of these estimates and
their relevance to the extinction process are not completely clear.
There are two shortcomings. First, few examples of population extinction due
to inbreeding depression have been established. A field survey on Glanville
Fritillary Butterflies (Melitaea cinxia) is one of the very few studies that have
reported inbreeding depression as a causal factor of extinction in natural populations (Saccheri et al. 1998). Second, the paucity of investigations on the theoretical properties of the extinction vortex by inbreeding depression makes it impossible to evaluate the genetic risks of extinction based on empirical data.
Previous studies have considered the extinction vortex with genetic and population models, and suggest that rapid extinction due to inbreeding depression is
restricted but possible with observed genetic parameters (Tanaka 1997, 1998,
2000. Gene frequencies, the inbreeding coefficient, and the equilibrium population size achieved by the balance among mutation, selection, and drift are locally
stable, which suggests that an equilibrial population does not spontaneously go