Sarah "Sally" Otto was sitting in a lab meeting of evolutionary biologist Marcus Feldman's group at Stanford University in 1988 when she overheard a graduate student describe sex as "such a big puzzle." Otto, an undergraduate at the time, didn't agree. Sexual reproduction-genetic recombination between individuals-"obviously" promotes variation, she thought, allowing species to adapt to changing environments. Otto's reaction makes sense, and echoes one of the oldest formal explanations for why sex evolved to be ubiquitous in the animal kingdom. In 1886, German evolutionary biologist August Weismann proposed that sexual reproduction reshuffles genes to create "individual differences" upon which natural selection acts.
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