There is a debate on (1) whether genetic diversity is related to population fitness, (2) how to predict fitness using genetic measures and (3) whether population size is important to fitness. In 2003, in an attempt bring clarity to this debate, David Reed and Richard Frankham (Macquarie University, in Sydney) published an article in Conservation Biology. They carried out a meta-analysis of all studies that measured some form of fitness related to molecular data, quantitative genetic data and population size. Although the number of studies that were available for analyses were insufficient to give robust conclusions, they believe there to be enough evidence to support their conclusions; heterozygosity, quantitative genetic variation and population size were significantly correlated with population fitness.
Not only does this article clearly explain the issues surrounding the reasons why genetic diversity is integral in species conservation, Reed and Frankham justify their conclusions with empirical evidence. I recommend that this article should be read by all those interested in conservation biology, especially those that are not directly concerned with biodiversity genetics.
The World Conservation Union (IUCN) recommends that as part of conserving biodiversity, genetic diversity must also be conserved. The IUCN justifies this with the following two reasons: (1) genetic diversity is required for populations to evolve in response to environmental changes and (2) heterozygosity levels are linked directly to reduced population fitness via inbreeding depression. -- A heterozygous population is one with a maximum level of allelic variation at a given locus and inbreeding depression is caused by the breeding of closely related individuals, and leads to increased homozygosity (or reduced heterozygosity).
Thus, if genetic diversity is not managed, or maintained in small or declining populations, (1) the population may not respond to particular environmental changes and (2) increased inbreeding might send the population to extinction, as in Graeme Caughley’s extinction vortex theory.
In their argument, Reed and Frankham explain that although heterozygosity and fitness might not be related --(because (1) molecular markers used to estimate heterozygosity may not affect fitness, (2) of non-additive genetic variation (where phenotypic variation is caused by the interaction of genes at several loci), and (3) increased selection against homozygotes may purge deleterious alleles), they found that heterozygosity, population size, and quantitative genetic variation were positively and significantly correlated with population fitness.
So, how can this theory be applied to conservation management practice? Firstly, methods for simple, non-invasive collection of genetic material from endangered species must be optimised. The following DNA extraction and analysis must also be quick and inexpensive, so that multiple analyses can be carried out. Often, it is difficult to keep pedigrees for wild species, so efficient methods that prove relatedness and define within-generation variation could help conservation managers maximise genetic variation within populations and between generations. Furthermore, it is expected that in the near-term, genome sequencing will become relatively straightforward and cost-effective. This will allow the definitive calculation of genetic variation between individuals and for populations. Unfortunately, it will not be as easy locating specific genes that directly affect fitness.
Reed and Frankham suggest that the migration of individuals to a population greatly increases heterozygosity. This could be managed in captive and wild populations; furthermore, if conservation managers know which genes are fixed in the population, individuals that are known to have a heterozygous copy of that gene could be introduced to that population. However, the take-home message from this article is that population size must be maximised for species to evolve in response to environmental changes and to reduce the negative effects of inbreeding depression.
Photo: The takahe is one such species whos genetic diversity is thought to have been reduced by inbreeding depression.
