Threatened species have small, or declining populations. Once small, they suffer loss of genetic diversity, inbreeding (with consequent reduction in reproductive fitness) and accumulation of deleterious mutations. All these factors increase the risk of extinction. Consequently, Section II considers these factors in detail, as they represent the major genetic issues in conservation biology, and provide the essential background material for the genetic management of threatened species in Section III.

Factors reducing population size

Humans are reducing the size and distribution of wild populations through clearing and fragmentation of habitat, over-exploitation, pollution and the impact of introduced species. Of these, habitat loss is having the greatest impact. Species are becoming extinct before they are described, and unknown numbers of invertebrate and plant species will be exterminated.

Loss of genetic diversity

Loss of genetic diversity in small populations reduces the ability to evolve in response to ever-present environmental change. There are four threats to genetic diversity:

• Extinction of populations or species

• Extinction of alleles due to sampling in small populations

• Inbreeding reducing heterozygosity (redistributing genetic diversity among homozygous individuals and populations)

• Selection reducing genetic diversity by favouring one allele at the expense of others, leading to fixation.

Overwhelmingly the major threat to genetic diversity is extinction of alleles in finite populations by genetic drift. The adverse genetic effects of population size reduction, loss of genetic diversity, inbreeding, and accumulation of deleterious mutations all depend on the effective population size, rather than the census size.