Heterozygosity, fitness and inbreeding depression in natural populations Inbreeding is mating between close relatives and can depress components of reproductive fitness thus having detrimental effects on the populations survival, a phenomenon known as inbreeding depression. There are two principal theories for the mechanism of inbreeding depression. The partial dominance hypothesis (Charlesworth and Charlesworth, 1987) suggests that inbreeding increases the frequency of homozygous combinations of deleterious recessive alleles due to the increased chance of offspring inheriting alleles identical by decent from both heterozygous parents. This is shown in figure 1 and results in a reduction of population fitness.
Figure 1 Adapted from Madsen (1996)
However the overdominance hypothesis suggests that because inbreeding increases homozygotes it reduces the overall frequency of the superior heterozygote's relative to Hardy Weinberg ratios if the population was randomly mating. This results in the loss of the heterozygote advantage and in a decrease in fitness (Charlesworth and Charlesworth 1987). Both hypotheses are likely to be correct and that a combination of the two causes an overall increase in homozygotes and decrease in heterozygotes of the offspring relative to that of the population as a whole if it were mating randomly and in Hardy Weinberg frequencies. As a result there is a loss of genetic diversity, superior heterozygotes and increase in recessive mutations which leads to the loss in the populations' evolutionary adaptability to cope and evolve to environmental change. This is indicated by the following equation for loss of neutral genetic variation in a random mating population: Hg / H0 = [1 1 / (2Ne) ]g = 1 F (Frankham 2002) Where Hg is the heterozygosity at generation g, H0 the initial heterozygosity, Ne the long-term effective population size and F the inbreeding coefficient. As stated by Frankham (2005), the effects of inbreeding and loss of genetic variability are usually inseparable'. In small populations such as those of threatened species, inbreeding becomes inevitable as matings between close relatives become unavoidable resulting in an increase in homozygotes for recessive deleterious mutations which may become fixed at some loci. However, inbreeding is rarer in larger populations typically of non threatened species. This is due to a wider choice in mating partners and wider genetic diversity. The heterozygote advantage can be retained and recessive deleterious alleles are kept rare by selection even though the population may carry many of them. This is because they will rarely be inherited from both parents by offspring and so the population is fitter. There have been many studies done on domestic or captive-bred populations to document the detrimental effects of inbreeding depression (Ralls and Ballou 1983, 1988). Inbreeding reduces reproduction and survival in essentially all well studied species and is virtually unavoidable in captive populations due to their small population sizes. For example, in forty captive populations belonging to thiry-eight species there was an average increase of 33% for mortality in inbred matings (Ralls et al 1988). Fewer studies have been done on natural wild populations due to difficulties in monitoring the populations such as establishing the relatedness between individuals in a breeding pair. Many studies have therefore focused on juvenile traits due to difficulties obtaining complete life history data and also identifying parameters that reflect their lifetime reproductive success. Inbreeding has deleterious effects on reproductive fitness in all well studied species of naturally outbreeding animals and plants and so they exhibit inbreeding depression. Greenwood (1978) found that inbreeding pairs have lower breeding success resulting from higher nesting mortality than normal pairs in the great tit (Parsus...
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