Modern human genetic diversity is the result of demographic history, and selective effects that have acted to adapt different populations to their environments. Broad patterns of global diversity are well explained by geography, based on an out-of-Africa model of early human evolution. Genome-wide searches for signals of selection, plus studies of specific candidate loci and candidate phenotypes, have identified genes that show population differences due to adaptation to pathogens, climate, diet and possibly cognitive challenges. Some past adaptations are now maladaptive, and can lead to disease. However, the history of adaptation is complex, and adaptive explanations are often unsupported by hard evidence.

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