The Stepping Stone Model of Population Structure and the Decrease of Genetic Correlation with Distance
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Genetics 49 (4): 561-576 (April 1964)

H E N a species occupies a very large territory, local differentiation is usually noticeable in the form of geographical races. Each race may in turn consist of numerous colonies which are differentiated to a less noticeable extent. The underlying differentiation in genetic constitution may reflect the local differences of selective pattern or may be the results of chance occurrence of different mutant genes, but these factors cannot act effectively unless some sort of isolation ensures the accumulation of genetic differences. It is well known that existence of geographical barriers greatly favors the for- mation of races and new species. However, even if such barriers do not exist, the large size of the whole area as compared with the migration distance of an indi- vidual may prevent the species from forming a single panmictic unit, and this will produce a sort of isolation which WRIGHT called “isolation by distance” (WRIGHT 1943). He proposed a model of population structure in which a popula- tion is distributed uniformly over a large territory, but the parents of any given individual are drawn from a small surrounding region. He studied, by his method of path coefficients, the pattern of change in the inbreeding coefficient of sub- groups relative to a larger population in which they are contained (WRIGHT 1940, 1943, 1946, 195 1 ) . The problem of local differentiation may also be studied (1948, in terms of change in correlation with distance as considered by MAL~COT 1955, 1959) ; individuals living nearby tend to be more alike than those living far apart. In the mathematical theory of population genetics, the problem of local differentiation of gene frequencies in a structured population is one of the most intricate, and so far the main results are due to these two authors.
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