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