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Why does sex -- that is, sexual reproduction
-- exist? In many ways, asexual reproduction is
a better evolutionary strategy: Only one parent
is required, and all of that parent's genes are
passed on to its progeny. In sexual reproduction,
only half of each parent's genes are passed to
the next generation. What's more, a mate must
be found. Yet sex persists.
This essay offers possible explanations of this
evolutionary paradox.
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| Matt
Ridley is the author of The Red Queen: Sex and
the Evolution of Human Nature (1995), The
Origins of Virtue: Human Instincts and the Evolution
of Cooperation (1998), and Genome: The Autobiography
of a Species in 23 Chapters (2000). A former science
editor and Washington correspondent of The Economist,
he now lives in northeast England, where he is chairman
of a science center called The International Centre
for Life. |
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Adapted with permission from New Scientist,
4 December 1993, no. 1902 © 1993 by RBI. (Boldface
added.)
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Sexual
reproduction,
human style |
A variety of theories have been proposed over the years
to explain why sexual reproduction may be more advantageous
than asexual reproduction, and, for that matter,
why sexual reproduction even exists at all. For years
everyone accepted the general proposition that sex is
good for evolution because it creates genetic variety,
which, in turn, is useful in adapting to constantly
changing and challenging environments. But it may give
organisms a very different kind of edge.
By the late 1980s, in the contest to explain sex,
only two hypotheses remained in contention.
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One,
the deleterious mutation hypothesis, was the idea that
sex exists to purge a species of damaging genetic mutations;
Alexey Kondrashov, now at the National Center for Biotechnology
Information, has been its principal champion. He argues
that in an asexual population, every time a creature dies
because of a mutation, that mutation dies with it. In
a sexual population, some of the creatures born have lots
of mutations and some have few. If the ones with lots
of mutations die, then sex purges the species of mutations.
Since most mutations are harmful, this gives sex a great
advantage. |
Salamanders can
reproduce sexually
or asexually. |
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