Scientists want to know: Why is sex so popular?

By Malcolm Ritter
Associated Press

Fruit flies are widely used by scientists for studies on the genetics of development and aging and also for studying the biological advantages of sex. Associated Press

They're looking beyond candy hearts and romantic dinners to the fact that in the cold light of biology, sex is a pretty inefficient way to reproduce. But it's so widespread among plants and animals that there must be some payoff.

After more than a half-century of debate and some 20 published theories, scientists are still trying to pin down just what the payoff is.

"It's clearly one of the most fundamental questions in evolutionary biology," says William Rice of the University of California-Santa Barbara. He has pursued the question by experimenting with fruit-fly reproduction.

It is easy to forget that sex is not the only game in town for the plant and animal kingdoms. In fact, there are thousands of asexual species.

Microscopic creatures called bdelloid rotifers have reproduced asexually by cloning themselves for tens of millions of years. That greatly annoys scientists, who say no creature should be able to keep doing that for so long.

Actually, it is rare for an asexual species to persist for a very long time in evolutionary terms, suggesting again there's something beneficial about sex.

Yet at first blush, it makes some sense to have self-reproducing females and just dispense with males altogether, even before you consider things like singles bars. After all, if your job is to pass on your genes to future generations — and according to evolutionary biology, that is your job — sex just gets in the way.

Consider the notion of two mating sexes. Each female passes only half her genes to each offspring, rather than all of them. What's more, sex breaks up the successful gene combinations found in the parents and gambles on new, untried mixes in the next generation. Does that make any sense?

Maybe so. It's pretty clear, scientists say, that the evolutionary lure of sex has something to do with that gene-mixing.

To understand that, remember that the genetic makeup of an organism is somewhat like a baseball team. Everybody has a full team, with all the positions covered, but who plays at each spot differs. And there are good players and bad players in the same way there are good genes and bad genes.

Clones essentially pass their own rosters on to their offspring, while sexual species create new rosters.

Why tinker with a successful genetic lineup?

Currently, most scientists offer two general theories about why sex is so good: It helps a species get rid of harmful mutated genes, or, alternatively, it helps the population take advantage of beneficial mutations.

The bad-gene idea says sex can make the faulty genes sitting ducks for elimination by natural selection, by separating them from good genes as they pass through generations. It can group bad genes together so they get wiped out in batches when the unfortunate recipients don't reproduce. Sex can also break up harmful combinations of genes, even when each by itself isn't so bad.

The alternative view says sex can help good genes spread through a species or bring favorable combinations together, speeding up evolution and helping species adapt more quickly to changing environments. Rice's experiment with the fruit flies showed that a favored gene spread more quickly if it appeared on a chromosome that participates in gene-shuffling than if it did not. Rice's experiment also found evidence that harmful mutations accumulate faster without gene-shuffling.