Heterozygosity, Fitness and Inbreeding Depression in Natural Populations

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

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