An important consideration for population studies is the effect of inbreeding on reproductive fitness (the combination of fecundity, fertility, and survival). There is strong evidence to demonstrate that inbreeding reduces reproduction and survival in naturally outbreeding species and to a lesser extent in self-fertilization species, whereas increasing the incidence of outbreeding can reverse these deleterious effects. Not only does inbreeding reduce individual reproductive fitness, a population of inbred individuals often results in a declining population and an increased rate of inbreeding. As such, inbreeding is also expected to increase the risk of population extinction. This process begs the question: how much inbreeding can be tolerated without inbreeding depression.? All finite closed populations will eventually become inbred without outbreeding, and even a low frequency of inbreeding is expected to result in some inbreeding depression. Effective population sizes of 50 or greater have been suggested as sufficient to avoid inbreeding depression, although it is generally unknown how large populations must be to avoid inbreeding depression in the long term. This number will also be highly variable among populations as a result of local dynamical processes and extrinsic forces.
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