Our Expert Voices conversation on why sex exists.

Charles Darwin famously asked why organisms should be produced by combining two sexual elements, instead of by cloning: "The whole subject is as yet hidden in darkness." Evolutionary biologists are still asking this question, but some progress has been made. One idea — with empirical support from studies of New Zealand snails that have sexual and asexual females and lots of virulent parasites — is that sexual reproduction protects populations from disease. That depends on reciprocal evolution between hosts and parasites. The basic idea is that cloning is very good when rare. The clones are all female, and this gives them a massive reproductive advantage. But, once the clone becomes common, it can become the target of rapidly evolving parasites. If these parasites are common and virulent, they could drive the clone back down in frequency, and thus prevent it from replacing the outcrossing females (and males). In contrast, outbred sexual populations can be highly diverse for disease-resistance genes, which reduces the spread of infection among offspring. The bottom line: Sexual reproduction produces genetically variable offspring, which may give sexual females an advantage over asexual females in the face of rapidly evolving parasites. Other voices in the conversation: Sarah Otto, theoretical biologist, University of British Columbia: Sex is evolution's answer to an ever-changing world Joan Roughgarden, evolutionary biologist, University of Hawai'i: Male, female: it goes back to size

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