Models of race and cline

Computer models have been constructed to depict in the form of two‐dimensional maps the effect of local selection on gene frequencies, particularly where partial barriers to gene flow are present at some points in an area. It was found that small amounts of uniform gene flow between foci of contrasting selection tend to produce evenly spaced isogenes, giving a “clinal” pattern. The interposition of partial barriers causes isogenes to become closer together in the region of the barriers and to become parallel to the barriers. A corollary result is that in the region of the barriers the isogene systems for different genes tend to become parallel to one another even though the exact foci of active selection for the genes do not coincide. This resembles a “racial” rather than “clinal” trait distribution. It was further observed that if a focus of active selection was peripheral in position (i.e., far from a genetic barrier) a clinal pattern extended across the area it affected between it and the barrier. If such a focus lies close to the barrier, the area it affects becomes rather uniform in gene frequency, with the “hinterland” approximating the gene frequency of the focus of active selection. On the basis of these models interpretation of the distribution of cold‐adapted Mongoloid traits, and of the distribution of skin color across Europe and Africa, are suggested.