Species are not uniformly mixed. All the people in the human population, for example, are completely capable of mating, but we're not totally genetically mixed, hence, the existence of regional heritable differences. The reason for this lack of mixing is remarkably simple: It's a matter of gene flow, which means the movement of genes in space.
The reason the genes within the human population (and the populations of many other species) aren't completely mixed is because we're more likely to mate with others who are nearby. Populations that are close together in space are more likely to exchange genes with each other than populations that are farther away.
Gene flow, the idea that genes can move from one population to another, is a key concept in evolution. You move to Sweden, find a nice, blue-eyed, blond spouse and raise a family. Your children are Swedish, but maybe they've got your eyes — that's gene flow.
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