Theory tells conservation biologists to beware genetic problems in small populations that may arise through loss of genetic variation. Genetic variation is determined primarily by the joint action of natural selection and genetic drift (where the frequency of genes in a population is determined by chance rather than evolutionary advantage). The relative importance of genetic drift is higher in small, isolated populations, which as a consequence are expected possible genetic problems in small populations to lose genetic variation. The rate at which this happens depends on the effective population size (Ne). This is the size of the 'genetically idealized' population to which the actual population (N) is equivalent in genetic terms. As a first approximation, Ne is equal to or less than the number of breeding individuals. Ne is usually less, often much less, than N, for a number of reasons (detailed formulae can be found in Lande & Barrowclough, 1987):
1 If the sex ratio is not 1 : 1; for example with 100 breeding males and 400 breeding females N = 500, but Ne = 320.
2 If the distribution of progeny from individual to individual is not random; for instance if 500 individuals each produce one individual for the next generation on average N = 500, but if the variance in progeny production is 5 (with random variation this would be 1), then Ne = 100.
3 If population size varies from generation to generation, then Ne is disproportionately influenced by the smaller sizes; for example for the sequence 500, 100, 200, 900 and 800, mean N = 500, but Ne = 258.
The preservation of genetic diversity is important because of the long-term evolutionary potential it provides. Rare forms of a gene (alleles), or combinations of alleles, may confer no immediate advantage but could turn out to be well suited to changed environmental conditions in the future. Small populations that have lost rare alleles, through genetic drift, have less potential to adapt.
A more immediate potential problem is inbreeding depression. When populations are small there is a tendency for individuals breeding with one another to be related. Inbreeding reduces the heterozygosity of the offspring far below that of the population as a whole. More important, though, is that all populations carry recessive alleles which are deleterious, or even lethal, when homozygous. Individuals that are forced to breed with close relatives are more likely to produce offspring where the harmful alleles are derived from both parents so the deleterious effect is expressed. There are many examples of inbreeding depression - breeders have long been aware of reductions in fertility, survivorship, growth rates and resistance to disease - although high levels of inbreeding may be normal and nondeleterious in some animal species (Wallis, 1994) and many plants.
How many individuals are needed to maintain genetic variability? Franklin (1980) suggested that an effective population size of about 50 would be unlikely to suffer from inbreeding depression, whilst 500-1000 might be needed to maintain longer term evolutionary potential (Franklin & Frankham, 1998). Such rules of thumb should be applied cautiously and, bearing in mind the relationship between Ne and N, the minimum population size N should probably be set an order of magnitude higher than Ne (5000-12,500 individuals) (Franklin & Frankham, 1998).
It is interesting that no example of extinction due to genetic problems is found in Table 7.5. Perhaps inbreeding depression has loss of evolutionary potential risk of inbreeding depression magic genetic numbers?
Percentage due to each cause *
Habitat loss Overexploitationf
Predators Other Unknown
2B 11 B2 4
2O 22 42 BO
Table 7.5 Review of the factors responsible for recorded extinctions of vertebrates and an assessment of risks currently facing species categorized globally as endangered, vulnerable or rare by the International Union for the Conservation of Nature (IUCN). (After Reid & Miller, 1989.)
Threatened extinctions Mammals 68
S4 BO 6B 29 12
* The values indicated represent the percentage of species that are influenced by the given factor. Some species may be influenced by more than one factor, thus, some rows may exceed 100%. f Overexploitation includes commercial, sport, and subsistence hunting and live animal capture for any purpose.
Figure 7.18 Extinction vortices may progressively lower population sizes leading inexorably to extinction. (After Primack, 1993.)
occurred, although undetected, as part of the 'death rattle' of some dying populations (Caughley, 1994). Thus, a population may have been reduced to a very small size by one or more of the processes described above and this may have led to an increased frequency of matings amongst relatives and the expression of deleterious recessive alleles in offspring, leading to reduced survivorship and fecundity, causing the population to become smaller still - the so-called extinction vortex (Figure 7.18).
Evidence of a role of genetic effects in population persistence was reported in a study of 23 local populations of the rare plant Gentianella germanica in grasslands in the Jura mountains (Swiss-German border). Fischer and Matthies (1998) found a negative correlation between reproductive performance and population size (Figure 7.19a-c). Furthermore, population size decreased between 1993 and 1995 in most of the studied populations, but population size decreased more rapidly in the smaller populations (Figure 7.19d). These results are consistent with the hypothesis that genetic effects resulted in a reduction in fitness in the small populations. However, they may equally have been caused by differences in local habitat conditions (small populations may be small because they have low fecundity resulting from low-quality habitat) or because of the disruption of plant-pollinator interactions (small populations may have low fecundity because of low frequencies of visitation by pollinators). To determine whether genetic differences were, indeed, responsible, seeds from each population were grown under standard conditions in a common garden experiment. After 17 months, there were significantly more flowering plants and more flowers (per planted seed) from seeds from large populations than those from small populations. We can conclude that genetic effects are of importance for population persistence in this rare species and need to be considered when developing a conservation management strategy.
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