Our Expert Voices conversation on why sex exists.

Why bother with sex when self-cloning would seem more efficient? The advantage: Sexual reproduction in a species, cumulated over all its members, stirs the species' gene pool, rebalancing the genes into different combinations. A species needs a continually rebalanced genetic portfolio to survive over the long term in nature's ever-changing environment. At the cellular level, there are two gamete sizes — big (egg) and tiny (sperm). But why not one intermediate gamete size instead? It's to find one another. Imagine getting separated from your friend at a shopping mall. It's easier if one party stays put while the other does the searching. Likewise, a stationary egg and and mobile sperm can meet up more readily than two gametes of the same size. Hence the origin of the distinction between male and female. The bottom line: The purpose of sex is to mix genes. Species whose members self-clone do not last very long evolutionarily. They are twigs on the tree of life whereas sexually reproducing species form its branches. Other voices in the conversation: Sarah Otto, theoretical biologist, University of British Columbia: Sex is evolution's answer to an ever-changing world Curtis Lively, evolutionary biologist, Indiana University Bloomington: Sex protects populations from parasites

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