Evolution__3rd_Edition

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Figure 12.2
Non-reproductive sex in
Paramecium. Paramecium
normally contains one
micronucleus and one
macronucleus. When it
prepares for sex, the
macronucleus dissolves and the
micronucleus is duplicated.
Two such cells can then
conjugate, swapping one of
their micronuclei. Meiosis then
occurs within each cell. The
sex act is non-reproductive.
Paramecium cells reproduce by
binary fission. Thus, sex and
reproduction are not associated
in Paramecium. The same is
true in many single-celled life
forms.
Some kinds of sex are cost-free
But a big unsolved problem remains
CHAPTER 12 / Adaptations in Sexual Reproduction 315
conjugate (Figure 12.2). The two cells swap copies of their DNA and then separate.
Meiosis then occurs within each cell. Sex is non-reproductive: there were two cells
before conjugation and there are two after it. Sex has no cost as in Figure 12.1. Sex
acquired its cost as sex became associated with reproduction, perhaps around the
time of the evolution of multicellular life. Sex probably originated in single-celled life,
and had little cost at that time. The origin of sex therefore poses no deep evolutionary
problem. But in many life forms today, sex does have a 50% cost and its existence is a
problem.
Fifty percent is a large cost. The problem of explaining sex is to find a compensating
advantage of sexual reproduction that is large enough to make up for its cost. We are on
the look out for an extraordinarily large selective advantage. Typical evolutionary
events are thought to involve selective advantages of a few percent at most, and more
often 1% or less. Consider this: a female who has survived to adulthood and is about to
reproduce must be fairly well adapted to her environment. If she were to reproduce
asexually, she would just make a copy of herself and produce a daughter as well adapted
to the conditions of the next generation as she would be herself. If she reproduces sexually
instead, she discards half her genes and produces an offspring by mixing the
remaining half with other genes drawn from a stranger. If sex is to outweigh its twofold
cost, the sexual female must by this procedure expect to produce a daughter who will be
twice as fit as a simple copy of herself. The problem, therefore, is not trivial. Indeed,
G. C. Williams has described it as “the outstanding puzzle in evolutionary biology.” The
puzzle is still a puzzle, but we can look at some possible solutions to it. Box 12.1 discusses
how the puzzle has taken on a practical importance, with the rise of cloning technologies.
12.1.2 Sex is unlikely to be explained by genetic constraint
One possibility is that life uses sexual reproduction because it is “stuck with it.” That
is, the mutations to produce asexual reproduction have not occurred. (In terms of