"Clinal variation" is a term used in anthropology. It refers to a scientific model that attempts to describe patterns of human biological variation around the world.
The clinal model of classification of human biological differences was developed in the 1960s. It largely replaced the flawed typological and populational models previously used. The clinal model differentiates itself by measuring the frequency of genetic traits, such as blood type, in regions of the world rather than tallying people with a certain skin color or physical feature. The clinal model does not categorize people into races or other types of groups, though it does demonstrate that the more physically distant population groups are from each other, the fewer genetic traits they tend to have in common.
A central principle of the clinal model is that people are likely to mate with people who live geographically close to them, traveling no more than a few hundred miles from their home to find a committed partner. While the clinal model is more useful than earlier models, it is not entirely accurate because it does not adequately account for long-distance migrations and very isolated communities that have little to no contact with the rest of the world.