Evolution__3rd_Edition

320 PART 3 / Adaptation and Natural Selection
The balance argument is not
decisive
Group selection may or may not
explain sex
Sex may help clear deleterious
mutations, ...
reproduce in either way. Therefore, when an aphid reproduces sexually, it must be
advantageous to the individual, because if it was not the aphid could have reproduced
asexually. Both sexual and asexual reproduction must have “balanced” advantages to
maintain them in the species’ life cycle, otherwise the inferior one would be lost.
The group selectionist proposes that sex is disadvantageous to the individual, and
only advantageous to the group. But in aphids and other heterogonic species in which
individuals have a “choice,” sex almost has to have an individual advantage. The argument
can be extended. If sex is advantageous in aphids, it is probably also advantageous
to the individual in non-heterogonic species too. We have no good reason to think that
sex is exceptional in aphids, or that special factors favor sex in heterogonic species. If we
must find an individual advantage for sex in aphids, that same advantage will probably
also exist in other species. If group selection can be ruled out for aphids, it can probably
also be ruled out for other species.
Williams’ argument is powerful, but not decisive. In most heterogonic species, the
asexual and sexual propagules differ in other respects besides being asexual and sexual.
For example, the cladoceran sexual offspring form special winter eggs that are adapted
for winter survival. Any cladoceran that gave up sex would also lose its overwintering
stage: in practice, the loss of sex while retaining the winter egg would need two mutations,
one for the loss of sex and the other for transferring the winter egg phenotype to
asexual eggs. So the balance argument is not perfectly clear-cut.
In summary, group selection will tend to favor sexual over asexual reproduction
because sexual populations will have a lower rate of extinction. The taxonomic distribution
of asexuality suggests that asexual populations tend to go extinct relatively
quickly in evolution. However, biologists doubt whether group selection is the reason
why sex exists, for two main reasons. One is a general disbelief in group selection; the
other is Williams’ balance argument. Neither of these objections is completely convincing,
and group selection cannot finally be ruled out. However, the objections are strong
enough to have inspired biologists to look for a short-term, individual advantage to sex.
12.2 There are two main theories in which sex may have a
short-term advantage
12.2.1 Sexual reproduction can enable females to reduce the number of
deleterious mutations in their offspring
A certain number of deleterious mutations arise every generation, and every individual
contains some defective genes. Selection acts to remove these deleterious mutations.
Here we shall consider how effectively selection removes them, depending on whether
reproduction is sexual or asexual. The theory that sex exists because it enhances the
power of selection against deleterious mutation, was proposed by Kondrashov (1988).
It is sometimes called the mutational theory of sex. Maynard Smith has given an analogy
to explain Kondrashov’s theory. Imagine you have two cars, with two different defects.
One is broken down because it has faulty brakes, and the other because it has a faulty
ignition. What should you do? One thing you could do is swap the components
..