Why sex exists

We take sex for granted — it got us all here, after all. But, it isn't the way of life for everything. Sex, as humans and other animals do it, is bizarre.

Most forms of life on Earth don't have sexes and go about reproducing by cloning complete versions of themselves. Other species have multiple sexes and, like us, combine and shuffle their genes every generation. Sexual reproduction seems an inferior strategy — only half of a parent's genes (the good and the bad) are passed on and it requires a mate. Yet, sex exists and persists.

Male. Female. Why not more? Why not one sex? We asked three researchers: why is there sex at all?

• 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
• Curtis Lively, evolutionary biologist, Indiana University Bloomington: Sex protects populations from parasites