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, and what forms of genetic diversity exist.

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 characterize genetic diversity for quantitative characters, especially the concept of heritability. Quantitative characters are centrally involved in the major areas of conservation concern, evolutionary potential, the deleterious effects of inbreeding and the deleterious effects that sometimes occur when different populations are mixed.

In Sections I and II we covered the evolutionary genetics of populations and the deleterious genetic consequences of population size reduction. In Section III we apply this information to the genetic management of threatened populations and species.

Taxonomic uncertainties and management units

A critical first step in conserving a species is to gain a clear understanding of its taxonomy. Is the population of interest a unique species? Does it actually consist of multiple cryptic species? Or is it simply another population of a common species? Without this knowledge endangered species may be denied protection, or resources wasted on populations of common species. The use of genetic techniques to assist in resolving taxonomic uncertainties is described in Chapter 15. To do this we must first define what is meant by a species and consider briefly how speciation occurs. Populations within species may be so distinct that crosses suffer reduced reproductive fitness (outbreeding depression). Consequently, they may require management as separate entities. The chapter concludes by considering means for defining management units within species.

Management of wild populations

The genetic management of wild populations is considered in Chapter 16. Typically wild population management is concerned with increasing population sizes and alleviating the effects of population fragmentation. Species with inadequate gene flow among population fragments will suffer an insidious process of inbreeding depression, loss of genetic diversity and eventually population extinctions, unless gene flow is reestablished. Sadly, there is limited activity in this area.

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.