Chapter Summary

• Although in most taxonomic groups only a small proportion of species have been assessed, the numbers of threatened species have led many people to believe that we are on the brink of a sixth mass extinction.

• Conservation strategies tend to assume that species can be classified accurately, although none of the >20 species concepts currently in the literature are universally accepted. The most recent approach to species identification is DNA barcoding. Conservation may also be based on management units and evolutionary significant units, whereas the protection of hybrids is more controversial.

• Threatened populations are usually small and therefore lose genetic diversity at a relatively rapid rate following genetic drift. Because drift is more important than selection in determining the fate of alleles in small populations, deleterious alleles are more likely to reach fixation and increase the genetic load.

• Small populations are susceptible to inbreeding, and if this leads to a reduction in fitness as a result of either dominance or overdominance then the population is experiencing inbreeding depression.

• Estimates of inbreeding depression are sometimes based on correlations between one or more fitness components and individual heterozygosity values (heterozygosity fitness correlations). In some cases, purging of deleterious alleles may reduce inbreeding depression.

• Sex-biased dispersal and mate choice are two important mechanisms of inbreeding avoidance, although inbreeding may be unavoidable in populations with Ne < 500.

• Translocations can slow or reverse the decline of small populations through genetic rescue, although source and destination populations should be genetically compatible, and founder effects should be avoided.

• Outbreeding depression can result from the mating of genetically dissimilar individuals if locally adapted alleles are lost or if positive epistatic interactions break down. The former will be evident in the F1 generation, whereas the latter will not become apparent until the F2 generation.

• Small population sizes mean that captive breeding programmes must be managed carefully so as to minimize inbreeding depression and loss of genetic diversity. Pedigree information can be used to reduce inbreeding, and Ne can be maximized if bottlenecks are avoided and variation in reproductive success (VRS) is kept as low as possible.

• A last resort for preserving genetic diversity is through banks that may comprise DNA, tissue, sperm, seeds and other sources of genetic material. Some of these are destined for long-term preservation, whereas others have shorter-term goals such as the avoidance of inbreeding in small populations.

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