Since our objective in conservation genetics is to preserve species as dynamic entities capable of evolving to adapt with environmental change, it is essential to understand the natural forces determining evolutionary change. Such information is indispensable if we are to understand how to genetically manage threatened and endangered populations. Since evolution at its most basic level is a change in the genetic composition of a population, it only occurs when there is genetic diversity. Consequently, we need to appreciate how genetic diversity arises, how it is lost, how much genetic diversity exists in populations, what forms of genetic diversity exist and how it is measured.

Extent of genetic diversity

Chapter 3 introduces methods for measuring genetic diversity for DNA, proteins, deleterious alleles and quantitative characters, and documents levels of genetic diversity for them. Most large populations of animals and plants contain extensive genetic diversity. However, levels of genetic diversity are often reduced in small populations, island populations and endangered species.

Genetic constitution of populations

To evaluate changes in genetic diversity, we must have means for quantifying it. Chapter 4 covers the estimation of allele (gene) frequencies and heterozygosity that are used to describe diversity at single loci. Chapter 5 describes the measures used to characterise genetic diversity for quantitative characters, especially the concept of heritability. Quantitative characters are centrally involved in the major areas of conservation concern, the ability to genetically adapt to environmental change (evolutionary potential), the deleterious effects of inbreeding (inbreeding depression) and the deleterious effects that sometimes occur when different populations are mixed (outbreeding depression).

Terms

Binomial distribution, bottleneck, effective population size, evolutionary potential, fixation, idealized population, inbreeding, Poisson distribution, random genetic drift, stochastic

Importance of small populations in conservation biology

Small or declining populations of threatened species are more prone to extinction than large stable populations. Species whose adult population sizes are less than 50, 250 or 1000 are designated as critically endangered, endangered and vulnerable, respectively, by IUCN (Chapter 1). Only ∼ 100 critically endangered northern hairynosed wombats survive in Australia (adults plus juveniles), while the Mauna Kea silversword in Hawaii declined to about two dozen plants. Some species have reached such low numbers that they exist, or have existed, only in captivity. These include Arabian oryx, black-footed ferret, European bison, Pére David's deer, Przewalski's horse, scimitarhorned oryx, California condor, Guam rail, 11 species of Partula snail, Cook's kok'io plant, Franklin tree and Malheur wirelettuce (WCMC 1992; Falk et al. 1996).

Some species have experienced population size reductions (bottlenecks), but have since recovered. The Mauritius kestrel was reduced to a single pair but has now recovered to 400–500 birds (Box 8.1).