That force may be pathogens causing repeated cycles of epidemics. To cause repeated epidemics in the same population, a pathogen must switch between different strains to evade detection, like a thief using different disguises to repeatedly rob the same bank. Retaining a large number of pathogen-detection genes among us ensures that there’ll always be a few individuals who can suss out the latest disguise. Each pathogen-detection gene variant thus neither fully dies out nor sweeps into dominance. We carry them around with us, like a treasure chest full of specialized detection tools handed down
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