Evolution__3rd_Edition

318 PART 3 / Adaptation and Natural Selection
Figure 12.4
The taxonomic distribution of
asexual reproduction is spindly
and is found in odd isolated
taxa.
Sex may exist despite an individual
disadvantage
Asexual
favorable mutations; the greater speed with which the different favorable mutations
combine together causes the sexual population to evolve faster. The higher the rate
at which favorable mutations are arising, the greater the evolutionary rate of a sexual
relative to an asexual population.
Other factors can also influence the relative rate of evolution in sexual and asexual
populations. However, the basic Fisher–Muller result remains valid in many, if not all,
circumstances. Rice & Chippindale (2001) experimentally demonstrated that the
Fisher–Muller theory can be realistic. They found that the rate of evolution was faster in
the presence, than in the absence, of sexual recombination.
12.1.4 Is sex maintained by group selection?
Asexual
Perhaps the commonest answer to the question of why sex exists is that it speeds up the
rate of evolution. This is the “group selection” theory of sex. It accepts that sex is disadvantageous
for the individual, because of its 50% cost, but claims that the cost is more
than made up for by the reduced extinction rate of populations, or groups, of sexually
reproducing organisms. Sexual populations, or groups, can accumulate superior adaptations
more rapidly than asexual populations, or groups. The asexual population will
then be out-competed and go extinct faster. Each sexual female is in a sense sacrificing
herself (she could produce more offspring by reproducing asexually) in order to save
the group from extinction.
The main argument for group selection, as an explanation for sex, comes from
the taxonomic distribution of asexual reproduction. In multicellular life, exclusively
asexual reproduction is mainly confined to small twigs of the phytogenetic tree (Figure
12.4). 1 A few exceptions have been suggested, but it is difficult to be sure that an
1 The spindly taxonomic distribution of asexuality only applies to multicellular life. In single-celled and viral
life, there probably are large chunks of the phylogenetic tree in which asexual reproduction prevails. Many
(and arguably all) bacteria, for instance, may do without sex, though this is far from confirmed. But we are not
concerned with those single-celled forms here.
Asexual
Asexual
..