All AbouT SEx 263
Why Sex?
The simplest but most profound question about sex is why it exists at all. Sex is not
the only option for reproduction. About 1 percent of plant species and 0.1 percent
of animal species reproduce by making genetic clones of themselves, a reproduc-
tive mode called parthenogenesis (FIGURE 10.20) [34]. These asexual species are
typically found on the tips of the tree of life: their closest relatives reproduce sexu-
ally. This implies that most asexual species have a short life expectancy on an evo-
lutionary time scale: if they did survive a long time, we would expect to see groups
of related asexual species that are connected to sexual relatives back in the distant
past. The most famous exception to this generalization is the bdelloid rotifers. This
group of small aquatic animals has lived in sexual abstinence for more than 100
million years [18].
The rarity of parthenogenesis is one of the deep puzzles in evolutionary biology.
Parthenogenesis has evolutionary advantages that should make it more common
than sexual reproduction. By far the most important of these is the twofold cost of
males: if all else is equal, the production of males in a sexual population reduces its
reproductive potential by a factor of two. Consider the following thought experi-
ment. Females of a sexual species each produce two offspring (FIGURE 10.21A).
Half of the individuals are males that do not give birth, so the population size is
constant. An asexual female then appears, for example by mutation, and she also
has two offspring. But since all of her offspring are asexual females, the number
of asexual individuals doubles in each generation. In short, a mutation for asexual
reproduction enjoys a 100 percent fitness advantage over an allele for sexual repro-
duction, and it will spread to fixation in only a handful of generations (FIGURE
10.21B). The mystery is why that scenario is not constantly playing itself out, very
quickly causing all sexual species to evolve parthenogenesis.
Parthenogenesis has other evolutionary advantages as well. Sexual reproduc-
tion requires finding a partner, which is sometimes difficult. Weeds such as the
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_1020.ai Date 11-02-2016(A)(C)(B)FIGURE 10.20 Asexual reproduction is very rare among ani-
mals and plants. Examples include (A) bdelloid rotifers (Philo-
dina roseola is shown here), (B) some whiptail lizards (Aspidos-
celis uniparens is shown here), and (C) the common dandelion
(Taraxacum officinale).10_EVOL4E_CH10.indd 263 3/22/17 2:25 PM