by
Clyde Freeman Herreid
Department of Biological Sciences
University at Buffalo, State University of New York
Birds do it. Bees do it.
Even educated fleas do it.
Let’s do it. Let’s fall in love.
—Cole Porter
Why do so many organisms go through sexual reproduction? It seems like every organism we think about does it: clams, jellyfish, trees, and elephants. And while we’re thinking about it: why only two sexes? It doesn’t have to be that way. Some fungi have dozens of sexes, enough to keep a romance novelist and a scriptwriter of soap operas ecstatic for years.
Sex really isn’t necessary for reproduction. Bacteria and many one-celled organisms like amoebae reproduce quite nicely by simply dividing in half (binary fission). They produce identical copies of themselves, quite an efficient way of sending one’s genes on to the next generation. They do it alone. For them, it doesn’t take two to tango.
Complex organisms can do it too. Some lizard species have only one sex—females. They reproduce parthenogenetically—that is, females produce eggs that spontaneously start development without sperm being involved at all. They are completely asexual.
Some species have it both ways: they reproduce both sexually and asexually. Queen bees when they produce females (workers) release sperm out of a storage sac and fertilize the egg in the normal way, but when they want to produce males (drones) they hold the sperm back and the eggs develop by parthenogenesis.
Water flea (Daphnia) populations seem to switch from asexual to sexual depending on environmental conditions. And some species of fish actually switch from being one sex to the other depending on which gender is in short supply. Science fiction writers should love these gender benders.
So, this brings us to a fundamental question that biologists have not solved: If organisms can survive well without sex—in fact, may do better without it—why has sexual reproduction evolved?
Date Posted: 09/05/05 nas
Image Credit: © Alwyn Cooper.
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