The Why of Sex

Research is showing that sex confers significant evolutionary advantages on species.

There is a biological cost to sex, as should be evident to us from our own experience. Would things not be so much simpler if each of us could reproduce asexually, saving ourselves the emotional entanglements that take up so much of our being? After all, species which reproduce asexually, and there are many where every individual can give rise to an offspring, should be able to reproduce twice as fast as sexual species, where males cannot give birth.

It has long been speculated that sex confers an advantage by increasing the genetic variability of a species because each offspring (bar identical twins) is a unique mixture of the paternal and maternal genes. In species that reproduce asexually, the offspring is an almost exact copy of its parent, and for one is equally susceptible to disease—thus, say, when an infection sweeps through such a population, it takes a much heavier toll than it does in a more genetically varied population.

The other possibility put forth is that harmful mutations that take place at random tend to accumulate in asexual species while sex breeds them out. This, for example, would be one reason why sex among close relatives is counterproductive.

Now research by University of Oregon biology professor Patrick C. Phillips and his students shows that there is something to both these views. The researchers, working with roundworms, a species that propagates both sexually and asexually, genetically manipulated the worms into two separate populations, one sexual and the other asexual.

Tracking several such collections of the worms over 50 generations, they found that the asexual populations tended to accumulate harmful mutations and were not as well adapted to their surroundings as the sexual populations.

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