The major forces shaping the genetic structure of populations are genetic drift, selection and migration. Mutation and recombination generate new genetic variation but are not major forces in determining levels of intraspecific differentiation. Most population genetic texts provide explanations and discussions on drift, selection and migration; more detailed accounts on artificial selection in fish can be found in Gall and Chen (1993), Gjedrem (1990) and Kirpichnikov (1981), while the effects of drift and population size on genetic diversity are discussed in Soule (1986, 1987), Soule and Wilcox (1980) and in issues of Conservation Biology and Bioscience and are outlined briefly below.
Selection is the non-random survival of genotypes, brought about through differences between individuals in reproductive output and in survival of offspring. Under natural selection the most fit individuals leave more offspring than the less fit individuals. The degree of fitness is measured by s the selection coefficient, whereby the most fit genotype is given a value of 1 and less fit genotypes 1-s. A selection differential of just a few percent can have a significant impact on a population over several generations.
Three basic types of selection have been recognised in populations: directional selection favouring one genotype or character, disruptive selection favouring two or more different types, and stabilising selection against extreme individuals. Fitness under these three types of selection can be considered in a simplified single gene model as:
Under directional selection allele frequencies would change, but alleles would not be lost unless drift impacted on a small population, because allele a is “protected” in the heterozygous condition. Under disruptive selection the population could diverge into two sub-populations fixed for A and a, the diversity remaining high. Under stabilising selection the heterozygote is advantaged and the proportion of heterozygotes in the population reaches an equilibrium value. In natural populations fitness of a genotype is unlikely to be constant over time and will change with environmental conditions including population density. Frequency-dependent selection is thought to be important in maintaining genetic variation when rare alleles have some selective advantage. The life history characters are likely to be polygenic, controlled by a large number of genes, in which case the response to selection will alter the average allele frequency of the genes coding for the trait (Bentsen 1994).
Genetic drift occurs through the random sampling of gametes at each generation. This produces chance fluctuations in allele frequencies from one generation to the next and in small populations the random changes may result in loss of alleles. Drift is probably the major force determining gene frequencies in very small populations, but is less important in large populations. Inbreeding, the mating between related individuals, also occurs in small populations. Inbreeding is most likely to occur in small isolated populations such as may occur in hatcheries or in endangered species, where there is a greater probability that two related individuals will mate. Mechanisms for avoiding inbreeding need to be considered in aquaculture and in enhancement programmes (Gall 1987).
The rate of loss of selectively neutral variation due to drift is dependent upon population size and the number of generations, and is described as Ht = Ho(1-1/2Ne)t (Wright 1969), where H is heterozygosity, Ne the effective population size and t the time in generations. Ne is usually much smaller than the actual population size as only the breeding adults contribute to the population (Bartley et al. 1992a). Ne is difficult to measure and there are few estimates of Ne for marine species. However, this population statistic is important in managing genetic resources and population geneticists are beginning to develop means to estimate Ne from allelic and genotypic data (e.g. Waples 1990, Bartley et al. 1992a).
The effective population size is decreased by an unequal sex ratio, and by variation in reproductive success which can be large in marine species with high fecundity and overlapping generations (Nelson and Soule 1987). To ensure that 99% of genetic diversity is retained per generation would require a minimum Ne of 50 with equal numbers of males and females. While no single value of Ne is appropriate for all species, due to variation in life histories (Kapuscinski and Lannan 1986), a minimum Ne of the order of 500 has been proposed for the long term preservation of genetic diversity (Franklin 1980, FAO 1981).
Genetic differences between populations evolve when there is little or no gene flow between them and are augmented by selection; migration counteracts the effects of drift and selection by reintroducing alleles to populations. Even a small amount of migration between populations is sufficient to eliminate differentiation due to drift, for example in two ideal populations with sizes of N and which exchange a proportion m through migrants each generation, then no significant divergence will occur between the populations if Nm > 1 (Lewontin 1974, Slatkin 1985). The population size of many commercially important marine species is likely to be large (>1 000) and small absolute numbers of migrants per generation will prevent divergence due to drift.
Drift will act on all loci in the same way whereas selection may act differently on different loci. The relative contributions of drift and selection to the observed allele frequency variation in marine populations is uncertain. There is evidence for strong selection at some enzyme loci for example in blue mussels Mytilus edulis (Koehn et al. 1983) and in mummichog Fundulus heteroclitus (Powers et al. 1983). However, drift would appear to account for the genetic differences between odd- and even- year pink salmon Oncorhynchus gorbuscha which inhabit the same environment (Aspinwall 1974).
Loss of genetic diversity occurs through drift, primarily in small populations, and through the selective removal of specific genotypes. In theory any activities or processes that selectively harvest individual types or severely reduce population size will change the genetic structure of natural populations. In addition diversity can be lost when small populations are swamped by introducing large numbers of individuals from a different gene pool. This situation would be rare in natural populations, but could occur in enhancement programmes and has been recorded with escapes of farmed salmon (Hindar et al. 1991).