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The Turkana of northwest Kenya are a large population of nomadic pastoralists very thinly distributed over a large geographic area. Such a population structure might be expected to develop local breeding units unless the nature and extent of nomadic movements are sufficient to counteract this tendency. This study demonstrates that nomadic individuals have almost a 50% probability of selecting mates from geographic regions other than their own. For a group of permanently settled Turkana farmers, on the other hand, only about 30% of mates are selected from other geographic regions. While this difference is significant, even the settled group selects enough mates from outside the region to prevent the development of a local breeding population. The Turkana clan exogamy rule has little genetic significance for either settled or nomadic individuals. Not only is this marriage rule not always followed, but the large number of clans results in relatively few individuals being excluded as potential mates even if the exogamy rule were adhered to strictly.