Why is sex so popular among plants and animals, and why isn’t asexual reproduction, or cloning, a more common reproductive strategy? Sex, in fact, has a higher cost than asexual reproduction, as an intriguing experiment published in the peer-reviewed Evolution Letters last week demonstrated.
The investigators adduced their evidence from studying the snail Potamopyrgus antipodarum , which has two kinds of females: one is asexual and the other is sexual and produces both sons and daughters. Asexual females coexist with sexual females in lakes and streams in New Zealand. When they collected snails from a lake where asexual and sexual females coexist and allowed them to reproduce in big outdoor tanks, they found that asexual females increased in frequency from parents to offspring. What was shown in experiment had already been proven, in theory, decades ago. In the 1970s, evolutionary biologist John Maynard Smith mathematically showed that sex is a more costly reproductive strategy than asexual reproduction.
Asexual females, he showed, made more grandchildren than sexual females. For sexual females, approximately half their offspring must be sons, and those sons can’t physically bear grandchildren.
Asexual females don’t make sons, so they make twice as many daughters as sexual females. Maynard Smith called this cost of sex the “two-fold cost of males”.
“Our findings mean that Maynard Smith’s theory does apply to this complex natural system, and sexual females do pay at least a two-fold cost of sex,” said Dr. Amanda Gibson, lead author of the Evolution Letters study in a statement.
The most common explanation for why sex makes sense is that it involves mixing up genes from two different individuals. This would mean that slight changes in the genes, called mutations, when passed over generations, are scattered within a species. Mutations can be harmful or beneficial and too many bad ones make individual life unviable, the argument goes. Sex improves the odds that such harmful mutations are eventually — in aggregate — weeded out of a species.
Another reason is that sex better equips a species to adapt to changing conditions. Experiments confirm that members of a sexual lineage usually adapt faster than asexual members of the same species when conditions change.
