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The evolution of the genetic load caused by recurrent mutation in small populations : genetic context and demographic history Poon, Arthur F.

Abstract

I have conducted two separate theoretical investigations on the consequences of recurrent mutation for the extinction risk of small populations. The continual generation of deleterious mutations and their subsequent rise in frequency in the population cause genetic load: a reduction in the mean fitness of the population. This evolutionary process is a function of natural selection, mutation, and genetic drift. Predictions made by theoretical studies on the extinction risk depend on what assumptions are made about selection, mutation, and drift. The following work modifies standard assumptions that are made in models that study genetic loads in finite populations such that the sensitivity o f the results can be evaluated. First of all, many models assume that most mutations are unconditionally deleterious — i.e. they have the same effect in any genome. There may be, however, a large fraction of mutations that conceal the expression of another mutation when placed in the same genome. These are compensatory mutations. With compensatory mutation, qualitatively different conclusions are reached about the extinction risk of a small population by genetic causes. Secondly, the models that study the effect of decreasing population size on the genetic load simplify the demographic change to an instantaneous drop from infinite size. Declining populations often gradually decline in size over several generations, however. I have studied the effect of gradual demographic change by numerical methods. The results indicate that transient changes in the mean load caused by recessive alleles can be delayed by a gradual change in population size.

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