Chapter Summary

• A population is usually defined as a group of potentially interbreeding individuals living within a restricted geographical area. This definition has limited application to asexual species, and even in sexually reproducing species the population boundaries are often difficult to identify.

• Genetic diversity is key to the long-term survival of populations. Estimates of genetic diversity are usually calculated as one or more of the following: allelic diversity (A), proportion of polymorphic loci (P), observed hetero-zygosity (Ho), gene diversity (h) or nucleotide diversity Gene diversity is often equivalent to the heterozygosity that is expected under Hardy-Weinberg equilibrium (He).

• Genetic diversity estimates are usually higher when based on microsatellite data compared with dominant data, which in turn tend to be more variable than allozyme data. Organelle genomes may yield lower estimates of diversity than nuclear genomes.

• Genetic drift can rapidly diminish genetic diversity in small populations through the random fixation of alleles. Genetic drift is such a powerful determinant of genetic diversity and non-adaptive evolution that it forms the basis of what many consider to be the most important theoretical measurement in population genetics, and that is the effective size of a population (Ne).

• The effective size of a population (Ne) is a measure of how rapidly a population loses genetic diversity following genetic drift, and although in an ideal population Ne = Nc, the ratio of Ne to Nc in most populations is substantially less than 1.0 owing to several possible factors, including uneven sex ratios, variation in reproductive success, and fluctuating population size.

• The most reliable way to calculate Ne is from the variance in allele frequencies, although this can be done only if we have allele frequency data from the same population representing at least two different time periods that are separated by at least one generation.

• Population bottlenecks, which include founder effects, often reduce genetic diversity; however, not all measures of diversity will be affected in the same ways, and the long-term impact will depend on both the severity and duration of the bottleneck.

• Natural selection, which includes directional and balancing selection, can either decrease or increase genetic diversity.

• Sexually reproducing populations usually have higher levels of genetic diversity than asexual populations because of the generation of new genotypes through recombination. Inbreeding in sexual populations does not directly alter allele frequencies but it does reduce observed heterozygosity (Ho).

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