Chapter Summary

• Most species consist of multiple populations. The extent to which these populations are differentiated from one another can be quantified in a number of ways, including Nei's standard genetic distance (D).

• F-Statistics use inbreeding coefficients to partition genetic variation within and between populations: FIT reflects the total amount of inbreeding in a population; FST reflects the amount of inbreeding in a population that is due to the differentiation of subpopulations; and FIS reflects the amount of inbreeding that occurs within subpopulations. The most widely used is FST, which is a measure of how genetically distinct populations are from one another.

• The amount of gene flow between populations is one important determinant of genetic differentiation. Dispersal and gene flow can be quantified using direct methods (such as radio tracking), indirect methods that provide estimates of Nem, and assignment tests.

• Dispersal patterns vary widely across and within taxonomic groups. Dispersal is particularly complex in plants, fungi and invertebrates because many species within these groups have different dispersal mechanisms throughout their life cycle, and also may disperse through time following prolonged diapause.

• Although dispersal does not necessarily lead to gene flow, species with good dispersal abilities generally show correspondingly high levels of gene flow. Species with intermediate dispersal abilities may be most likely to show a pattern of isolation by distance.

• Barriers to dispersal will promote the genetic differentiation of populations. Molecular data can sometimes reveal previously unidentified barriers to dispersal and can be particularly useful in relatively inaccessible places such as the open ocean.

• Reproduction can also influence gene flow and population differentiation. In both plants and animals, outcrossing species have higher levels of gene flow than selfing/clonal species. In plants, this difference is largely attributable to mechanisms of pollen dispersal.

• Species living in ephemeral or fragmented habitats may be either regular or infrequent dispersers. If the former, they may exist within a metapopulation in which local populations are connected by gene flow and are subjected to repeated extinctions and recolonizations.

• The dispersal of species may also depend on the distribution of symbionts. Parasite dispersal often follows the movements of hosts, although hosts will sometimes disperse in response to parasites.

• Genetic drift and natural selection can cause populations to diverge. Gene flow tends to homogenize populations, although divergence between populations can occur despite ongoing gene flow if selection (s) is strong enough. Conversely, if the migration rate (m) exceeds the strength of selection (s), then local adaptation will be hindered by the continued introduction of alleles from other populations.

• Selection can be inferred from the proportion of non-synonymous substitutions, from discrepancies in the levels of differentiation that are revealed at different loci, or from clinal variations in allele frequencies.

• Qst measures the genetic variance of quantitative trait loci (QTLs) within and among populations. If QST is either more or less than FST then this too may be evidence of selection. So far, investigations have found that QST is usually greater than FST.

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