Quantifying Gene Flow

Before looking at ways to quantify gene flow, we have to understand exactly what constitutes gene flow and how it differs from dispersal and migration. Dispersal generally refers to the movement of individuals or propagules between discrete locations or populations, whereas migration refers to periodic movement to and from a specific geographical area, often occurring seasonally and along a consistent route. Although either dispersal or migration must precede gene flow, neither results in gene flow unless individuals successfully reproduce once they have arrived at their new location. Nevertheless, dispersal is sometimes used as a surrogate for gene flow and we need to be aware of this potential limitation when evaluating different methods of quantification. Understanding the movements of individuals, gametes (e.g. pollen) and genes between populations is fundamental to studies of ecology and population genetics because, as we shall see later in the chapter, gene flow can profoundly influence a host of relevant variables, including population size, genetic diversity, local adaptation and ultimately speciation. Unfortunately, quantifying the movements of organisms and their genes is seldom straightforward. In this section we shall look at several methods that are commonly used to estimate ongoing dispersal and gene flow. These methods can be divided into three main categories: direct, indirect and assignment tests.

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