Abstract
We assessed the expected relationship between the level and the cost of inbreeding, measured either in terms of fitness, inbreeding depression or probability of extinction. First, we show that the assumption of frequent, slightly deleterious mutations do agree with observations and experiments, on the contrary to the assumption of few, moderately deleterious mutations. For the same inbreeding coefficient, populations can greatly differ in fitness according to the following: (i) population size; larger populations show higher fitness (ii) the history of population size; in a population that recovers after a bottleneck, higher inbreeding can lead to higher fitness and (iii) population demography; population growth rate and carrying capacity determine the relationship between inbreeding and extinction. With regards to the relationship between inbreeding depression and inbreeding coefficient, the population size that minimizes inbreeding depression depends on the level of inbreeding: inbreeding depression can even decrease when population size increases. It is therefore clear that to infer the costs of inbreeding, one must know both the history of inbreeding (e.g. past bottlenecks) and population demography.
Highlights
Small and isolated populations are associated with high rates of inbreeding and loss of genetic diversity
Understanding the relationship between population viability and inbreeding level is central in conservation genetics
There is still some controversy as to whether higher inbreeding, which is usually associated with small populations, results in a significant decrease in population fitness or on the contrary leads to more efficient purging of genetic load
Summary
Small and isolated populations are associated with high rates of inbreeding and loss of genetic diversity. There is increasing evidence that inbreeding has negative effects both on fitness and probability of extinction due to the accumulation of recurrent deleterious alleles Couvet inbreeding can be deduced from the current level of the inbreeding coefficient. The idea behind this approach is that neutral genetic variation, measured either by means of pedigree analysis and/or neutral genetic markers, can be a reliable predictor of the expected reduction in fitness due to inbreeding [21]. In a comprehensive meta-analysis on 170 threatened taxa, Spielman et al [42], found that threatened species show significantly lower heterozygosity at neutral loci than non-threatened closely-related species concluding that the risk to extinction is correlated to neutral genetic variation
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