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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.
WHY SEX?: See full infographic: JPG | PDFOtto’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. Additional ideas have emerged since Weismann’s hypothesis: sex rids the genome of deleterious mutations; sex rapidly introduces beneficial mutations; sex helps organisms dodge parasitic infections. Yet these evolutionary justifications for sex have remained hypotheses because there is not enough evidence to suggest that any of them provide enough of a benefit to surmount the exquisitely high costs of sex, which include the time and energy it takes to find a mate, the passage of only half of one’s genes to the next generation, and the breaking apart of favorable gene combinations. (See “Why Sex?”)
“Sexual reproduction permeates biology at every level, and yet it’s not intuitively obvious why it should be that way, because the costs are so high,” says Aneil Agrawal, an evolutionary geneticist at the University of Toronto.
More than 99 percent of multicellular eukaryotes ...
