Why Do We Have Sex?

One of the big questions in evolutionary biology is why some organsims utilize sexual reproduction. Although the mechanics are pretty straight forward, how different species respond to subquently becoming a parent can range from years of personal care (such as in humans) to outright abandonment right after birth (seen in many,many species). Either way requires a bigger investment in time and energy which can draw away from successfully reproducing in the first place.

With this in mind, it would seem that asexual species (e.g. bacteria, archaea) have a tremendous advantage. Parthenogenetic reproduction produces exponentially more offspring with none of the investment of finding a mate or rearing offspring, thus reducing their own fitness. Before you females begin to question the utility of the male gender, let me suggest a couple of reasons why sex persists (besides the obvious ones).

One idea holds that sexual reproduction (and the genetic recombination associated with it) helps protect species from parasites and other potentially dangerous hanger-ons. With our genome constantly changing, it is more difficult for a bacteria, virus, or other microbe to establish themselves.

A study out in the journal Nature (last week) provides some direct evidence for another explanation. It seems that asexual reproduction leads to a greater accumulation of deleterious mutations. The authors demonstrated this by studying the humble water flea (Daphnia pulex). Daphnia is a great model because the species includes both sexual and asexual populations, which can be compared. The authors compared 14 populations of each and found that the asexual populations had many more mutations with some sort of functional effect. Of these mutations, 90% were under selective pressure. This suggests that selection is not as efficient at getting rid of deleterious mutations compared to reproductive populations. This isn't the final nail in the coffin, but it is an important study in examining the role selection has in maintaining sexually reproducing populations. So, we males have some use after all!

Nielsen, R. 2006. Why Sex? Nature 311: 960-961.

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