Why Sex Is Necessary: Evolutionary Biologists Explain

        By NICHOLAS WADE

          It is hard to see why economics is called the dismal science when the title is so much better deserved
           by evolutionary biology.

        For biologists, the human race is being loaded down with bad mutations, the more so as medical
        advances thwart the scythe of natural selection from culling the weak.

        Another thing that bugs biologists is sex. Like James Thurber, they have
        long asked why it is necessary. Yeah, babies, but you could make them
        much more easily by parthenogenesis -- virgin birth -- the mode of
        procreation favored by stick insects and other sensible creatures.

        When it comes to sexual intercourse, the biologists stand foursquare
        with Lord Chesterfield, who advised his son that "the pleasure is
        momentary, the position ridiculous and the expense damnable."

        The biologists' two interests, doom and sex, have intersected in a new
        report that shows there is indeed a purpose to sex: to sweep away the
        bad mutations that would otherwise drag us toward terminal
        degeneracy.

        Two British biologists, Adam Eyre-Walker and Peter Keightley, came
        up with an ingenious way of estimating the number of bad mutations that
        have flowed in and out of the human genome during the 6 million years since we and chimpanzees
        stopped calling each other cousins. They found that at least 1.6 and probably 3 mutations per person
        must have been eliminated in each generation.

        This is an amazing number when you consider that a mutation can only disappear through a "genetic
        death" -- when its owner, or child or grandchild, dies without progeny.

        "That implies three genetic deaths per person!" said population geneticist James Crow of the University
        of Wisconsin in a commentary on the new study. "Why aren't we extinct?"

        Evidently we manage to beat the odds somehow or other. Last week Service Corporation International,
        a major operator of funeral homes and cemeteries, forecast meager earnings because of reduced
        mortality rates, a grim outlook that resulted in a 44 percent tumble in its stock price.

        What magic draught cleanses us of bad mutations without demanding one life for each?

        Now you begin to see how sex works. If we bred like stick insects, our children would have the exact
        same faults as their parents, knowledge of which would destroy all respect between generations.

        In sex, however, life's biggest non-regulated lottery, one's genes first get shuffled among themselves and
        a random assortment is then cut into someone else's deck. Bad mutations get left on the cutting room
        floor, or if they combine in a fetus, their synergistic effect will probably cause a speedy death,
        eliminating many mutations at once.

        The fact is, the gamble of sex beats death. By not acting like stick insects, we escape extinction. Maybe
        those evolutionary biologists aren't such dismal killjoys after all.