Isolation by distance

Isolation by distance (IBD) (note: the acronym IBD is also used for another important concept in population genetics, Identity by descent) is a term used to refer to the accrual of local genetic variation under geographically limited dispersal The IBD model is useful for determining the distribution of gene frequencies over a geographic region. Both dispersal variance and migration probabilities are variables in this model and both contribute to local genetic differentiation. Isolation by distance is usually the simplest model for the cause of genetic isolation between populations. Evolutionary biologists and population geneticists have been exploring varying theories and models for explaining population structure. Yoichi Ishida compares two important theories of isolation by distance and clarifies the relationship between the two. According to Ishida, Sewall Wright’s isolation by distance theory is termed ecological isolation by distance while Gustave Malécot’s theory is called genetic isolation by distance. Isolation by distance is distantly related to speciation. Multiple types of isolating barriers, namely prezygotic isolating barriers which includes isolation by distance, are considered to be the key factor in keeping populations apart limiting gene flow.

Wright introduced two different models of population structure, one not taking short-distance dispersal into account and one model incorporating short-distance dispersal. The "island model" is quite artificial and proposes the idea that a population is divided into two geographically, unique subpopulations (islands) with random mating occurring with exchange of individuals occurring when a migrant is drawn randomly from the total population. In a more realistic model, where short-distance dispersal is taken into account, a population is compiled of continuously distributed individuals over a region of space. Populations in remote locations may become differentiated simply by isolation by distance, restricting the probability of individuals mating with one another. Local populations are small in comparison to the total population and reproduction occurs solely within the local population. This ecological isolation by distance, according to Wright, can create genetic differentiation among subpopulations, leading to evolutionary change. Individuals within the subpopulation are neighbors in the sense that their gametes may come together and inbreeding within the subpopulation increases homozygosity. Wright’s statistical theory for isolation by distance looks at population genetic consequences measured by F-statistics where the correlation of randomly uniting gametes within a subpopulation relative to those of the total population is the FST value.

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