The effect of a cline as a barrier to gene flow at closely linked, weakly selected loci is investigated using a continuous diffusion model. It is shown that a linked cline induces a step in the frequency of a neutral allele, which is proportional to the gradient in neutral allele frequency and to the inverse of the recombination rate. A barrier to dispersal or a local region of low abundance has a similar effect (Nagylaki, 1976). The cline will block the flow of neutral alleles over a region of chromosome roughly 2√s/t map units long. However, a slightly advantageous allele will be little affected, and must be very tightly linked to be delayed for long.
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Barton, N. Gene flow past a cline. Heredity 43, 333–339 (1979). https://doi.org/10.1038/hdy.1979.86
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