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Generation of consanguineous mating

Figure 2.16 A graphical depiction of the predictions of the dominance and overdominance hypotheses for the genetic basis of inbreeding depression. The line for dominance shows purging and recovery of the population mean under continued consanguineous mating expected if deleterious recessive alleles cause inbreeding depression. However, the line for overdominance as the basis of inbreeding depression shows no purging effect since heterozygotes continue to decrease in frequency. The results of an inbreeding depression experiment with mice show that litter size recovers under continued brother-sister mating as expected under the dominance hypothesis (Lynch 1977). Only two of the original 14 pairs of wild-caught mice were left at the sixth generation. Not all of the mouse phenotypes showed patterns consistent with the dominance hypothesis.

such as mice, rats, and Drosophila, intentionally inbred by schemes such as full-sib mating for 10s or 100s of generations to create highly homozygous, so-called pure-breeding lines, are also not immune to inbreeding depression. Such inbred lines are often founded from multiple families and many of these family lines go extinct from low viability or reproductive failure with habitual inbreeding. This is another type of purging effect due to natural selection that leaves only those lines that exhibit less inbreeding depression, which could be due to dominance, overdominance, or epistasis. Purging is not universally observed in all species and it is likely that inbreeding depression has several genetic causes within species as well as different predominant causes among different species.

The social and economic correlates of inbreeding depression in humans mentioned above are a specific example of environmental effects on phenotypes. Inbreeding depression can be more pronounced when environmental conditions are more severe or limiting. For example, in the plant rose pink (Sabatia angularis), progeny from self-fertilizations showed decreasing relative performance when grown in the greenhouse, a garden, and their native habitat, consistent with environmental contributions to the expression of inbreeding depression (Dudash 1990). In another study, the number of surviving progeny for inbred and random-bred male wild mice (Mus domesticus) were similar under laboratory conditions, but inbred males sired only 20% of the surviving progeny that random bred males did when under semi-natural conditions due to male-male competition (Meagher et al. 2000). However, not all studies show environmental differences in the expression of inbreeding depression. As an example, uniform levels of inbreeding depression were shown by mosquitoes grown in the laboratory and in natural tree holes where they develop as larvae and pupae in the wild (Armbruster et al. 2000).

The degree of inbreeding depression also depends on the phenotype being considered. In plants, traits early in the life cycle such as germination less often show inbreeding depression than traits later in the life cycle such as growth and reproduction (Husband & Schemske 1996). A similar pattern is apparent in animals, with inbreeding depression most often observed for traits related to survival and reproduction.

Inbreeding depression is a critical concept when thinking about the evolution of mating patterns in plants and animals. Suppose that a single locus determines whether an individual will self or out-cross and the only allele present in a population is the outcrossing allele. Then imagine that mutation produces an allele at that locus, which, when homo-zygous, causes an individual to self-fertilize. Such a selfing allele would have a transmission advantage over outcrossing alleles in the population. To see this, consider the number of allele copies at the mating locus transmitted from parents to progeny. Parents with outcrossing alleles mate with another individual and transmit one allele to their progeny. Self-fertilizing parents, however, are both mom and dad to their offspring and transmit two alleles to their progeny. In a population of constant size where each individual contributes an average of one progeny to the next generation, the selfing allele is reproduced twice as fast as an outcrossing allele and would rapidly become fixed in the population (Fisher 1999; see Lande & Schemske 1985). Based on this two-fold higher rate of increase of the selfing allele, complete self-fertilization would eventually evolve unless some disadvantage counteracted the increase of selfed progeny in the population. Inbreeding depression where the average fitness of outcrossed progeny exceeds the average fitness of selfed progeny by a factor of two could play this role. If outcrossed progeny are at a two-fold advantage due to inbreeding depression, then complete outcrossing would evolve. Explaining the existence of populations that engage in intermediate levels of selfing and outcrossing, a mating system common in plants, remains a challenge under these predictions (Byers & Waller 1999).

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