262
Richard Frankham
eukaryotes. I am aware of no case in which Drosophila experiments have yielded
qualitative results at variance with equivalent experiments with other such species
(Frankham 1982).
Nevertheless, conclusions from laboratory studies are greatly strengthened
when complemented by meta-analyses of wildlife data (of the kind done by Ralls
and Ballou 1983). Both a laboratory animal study and a meta-analysis of wildlife
data have verified the predicted relationship between population size and allozyme genetic variation (Frankham 1996; Woodworth 1996). Similarly, low
ratios of effective-to-adult population size found in large Drosophila populations
(Briscoe et al. 1992) were found to apply to wildlife following a meta-analysis
(Frankham 1995a). If a meta-analysis is not possible, it is desirable to carry out
studies in a second laboratory species, as has frequently been done in animal
breeding (see Ralls and Meadows 1993).
Modeling Problems in Conservation Genetics
Using Drosophila
Inbreeding and Extinction
The fundamental assumption underlying the application of genetics within conservation biology is that inbreeding and loss of genetic variation increases the risk
of extinction. Although there is ample evidence that inbreeding causes depression
in components of reproductive fitness in humans, domestic animals, outbred
plants, and wildlife in zoos and in nature (see Wright 1977; Ralls et al. 1988;
Frankham 1995b; Falconer and Mackay 1996), there is little direct evidence of its
effects on extinction. Further, the only paper on this issue (Soul´ e 1980) failed to
distinguish genetic and environmental causes of extinction. Methods were
devised to separate these and applied to data sets from Drosophila melanogaster,
D. virilis, and Mus musculus (Frankham 1995c). Inbreeding clearly increased
extinction rates. Notably, there was a threshold relationship in all species, with
little extinction until intermediate levels of inbreeding but increasing rates of
extinction thereafter. Consequently, populations that are not being closely monitored may give little warning of impending inbreeding crises. Subsequently, Saccheri and colleagues (1998) have found that inbreeding increases extinction risk
for butterfly populations in nature.
Effective Population Size
Genetic effects of small population size are predicted to depend on the effective
population size (N e ) rather than the census size (Wright 1969; Crow and Kimura
1970; Falconer and Mackay 1996). Thus we have concentrated on factors predicted to affect N e , for instance, variance in family size, unequal sex ratios, and
fluctuations in numbers between generations. Much of the relevant theory is more
than 60 years old (Wright 1931) but was untested.
Richard Frankham
eukaryotes. I am aware of no case in which Drosophila experiments have yielded
qualitative results at variance with equivalent experiments with other such species
(Frankham 1982).
Nevertheless, conclusions from laboratory studies are greatly strengthened
when complemented by meta-analyses of wildlife data (of the kind done by Ralls
and Ballou 1983). Both a laboratory animal study and a meta-analysis of wildlife
data have verified the predicted relationship between population size and allozyme genetic variation (Frankham 1996; Woodworth 1996). Similarly, low
ratios of effective-to-adult population size found in large Drosophila populations
(Briscoe et al. 1992) were found to apply to wildlife following a meta-analysis
(Frankham 1995a). If a meta-analysis is not possible, it is desirable to carry out
studies in a second laboratory species, as has frequently been done in animal
breeding (see Ralls and Meadows 1993).
Modeling Problems in Conservation Genetics
Using Drosophila
Inbreeding and Extinction
The fundamental assumption underlying the application of genetics within conservation biology is that inbreeding and loss of genetic variation increases the risk
of extinction. Although there is ample evidence that inbreeding causes depression
in components of reproductive fitness in humans, domestic animals, outbred
plants, and wildlife in zoos and in nature (see Wright 1977; Ralls et al. 1988;
Frankham 1995b; Falconer and Mackay 1996), there is little direct evidence of its
effects on extinction. Further, the only paper on this issue (Soul´ e 1980) failed to
distinguish genetic and environmental causes of extinction. Methods were
devised to separate these and applied to data sets from Drosophila melanogaster,
D. virilis, and Mus musculus (Frankham 1995c). Inbreeding clearly increased
extinction rates. Notably, there was a threshold relationship in all species, with
little extinction until intermediate levels of inbreeding but increasing rates of
extinction thereafter. Consequently, populations that are not being closely monitored may give little warning of impending inbreeding crises. Subsequently, Saccheri and colleagues (1998) have found that inbreeding increases extinction risk
for butterfly populations in nature.
Effective Population Size
Genetic effects of small population size are predicted to depend on the effective
population size (N e ) rather than the census size (Wright 1969; Crow and Kimura
1970; Falconer and Mackay 1996). Thus we have concentrated on factors predicted to affect N e , for instance, variance in family size, unequal sex ratios, and
fluctuations in numbers between generations. Much of the relevant theory is more
than 60 years old (Wright 1931) but was untested.