Population Size Genetic Diversity and Inbreeding

Endangered species have, by definition, small or declining population sizes and are therefore sensitive to environmental perturbations simply because small populations lack a 'buffer' that helps them to survive periods of high mortality, for example following a disease outbreak or a temporary reduction in food supplies. Of equal or greater relevance to the long-term survival of small populations are their levels of genetic diversity. We know from Chapters 3 and 4 that the amount of genetic diversity within a population depends on the balance between mutation, gene flow, drift and selection (summarized in Figure 3.9). Although the effects of natural selection are variable, genetic diversity will be eroded by genetic drift and, in most cases, enhanced by gene flow. Because genetic drift acts more rapidly in small populations, we would expect overall genetic diversity to be roughly proportional to the size of a population, and this indeed appears to be the case. A review published by Frankham (1996) examined the relationship between population size and genetic diversity in 23 studies of plants and animals. Twenty-two of these species revealed a significant positive relationship between population size and genetic diversity when the latter was measured as He, Ho, allelic diversity (A) or proportion of polymorphic loci (P). Because endangered species typically have smaller population sizes than non-endangered species, they should also have relatively low levels of genetic diversity, and this too is generally the case (Table 7.3).

So what exactly are the dangers associated with genetically depauperate populations? For one thing, reduced levels of genetic diversity mean that populations may be unable to adapt to a changing environment. In addition, the fate of alleles in small populations is more likely to be determined by genetic drift than by

Table 7.3 Mean heterozygosity values in endangered avian populations, calculated from allozyme data. Adapted from Haig and Avise (1996) and references therein

Number of populations

Mean

Species

surveyed

heterozygosity

Wood stork (Mycteria Americana)

15

0.093

Trumpeter swan (Cygnus buccinator)

3

0.010

Hawaiian duck (Anas wyvilliana)

2

0.035

Laysan duck (Anas laysanensis)

1

0.014

Blue duck (Hymenolaimus malacorhynchos)

5

0.002

Lesser prairie chicken (Tympanuchus pallidicinctus)

1

0.000

Guam rail (Rallus owstoni)

1

0.030

Piping plover (Charadrius melodus)

5

0.016

Micronesian kingfisher (Halycyon cinnamomina)

1

0.000

Red-cockaded woodpecker (Picoides borealis)

26

0.078

selection. All populations harbour deleterious alleles at low frequencies, and if drift is a stronger force than selection then these deleterious alleles are much more likely to reach fixation.

The accumulation of harmful mutations contributes to a population's genetic load, which is defined as the reduction in a population's mean fitness compared with the mean fitness that would be found in a theoretical population that has not accumulated deleterious alleles. Genetic load can be measured as the number of lethal equivalents, which is the number of deleterious genes whose cumulative effect is the equivalent of one lethal gene. Because a relatively high proportion of deleterious alleles will become fixed in a small population, genetic load tends to be inversely proportional to population size. If populations are small enough then the accumulation of lethal equivalents can lead to a substantial reduction in reproductive fitness, at which point the population will experience mutational meltdown, which means that it will continue to decline until it goes extinct. It is not clear just how often mutational meltdown occurs in the wild, although it will be accelerated by inbreeding, which poses the biggest short-term threat to small populations.

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