All AbouT SEx 265
change, we expect that a set of cards that were successful the last time
will also be successful the next time. For the same reason, an individ-
ual that has survived selection is likely to carry a good combination
of alleles, and mixing them with the alleles of a sexual partner can
produce offspring that are genetically less fit. Suddenly the genetic
mixing of sexual reproduction does not seem like such a good idea.
One way in which evolution can favor sexual reproduction is the
Red Queen hypothesis, named in honor of the colorful character in
Lewis Carroll’s Through the Looking-Glass who must run constantly
to stay in the same place. According to this hypothesis, all species
are in an evolutionary arms race with other species, such as patho-
gens that are evolving rapidly to defeat their host’s defensive systems
(see Chapter 13). This creates a shifting adaptive landscape for the
host. Recombination can increase the frequency of rare combinations
of alleles that are good at defending the host against attacks from
its pathogens. Returning to the analogy of the card game, evolution-
ary changes in the pathogens change the rules about which cards are
best. In that case, changing cards can be the winning strategy.
Support for the Red Queen hypothesis comes from the New Zea-
land mud snail (Potamopyrgus antipodarum), which has both sexual
and asexual genotypes. Populations that are exposed to higher densi-
ties of parasites have higher frequencies of sexually reproducing indi-
viduals [29]. Furthermore, because sexual females have lower infec-
tion rates, the fitness of sexual females is sometimes more than twice
that of asexual females (FIGURE 10.22) [48]. This result suggests that
the evolutionary benefit of recombination can sometimes compensate
for the twofold cost of males.
Parasites and pathogens can also receive an evolutionary benefit
from the genetic mixing caused by recombination. People develop
immunity to the genotypes of influenza virus they have been exposed
to. New outbreaks result when a novel viral genotype is produced that
can efficiently infect humans. The genome of the influenza virus is
not a single molecule, as in most viruses. Instead, its genome is broken into eight
segments of RNA. This arrangement is an adaptation that enhances recombina-
tion. You can think of the next outbreak of the flu as a dramatic demonstration of
the evolutionary power of recombination.Selective interference favors sex and recombination
Alleles are not selected independently. The chance that a particular copy of an
allele is passed to the next generation depends in part on the rest of the genome
in which it is carried. Adaptation can be hampered as a result, particularly if there
is not much genetic mixing. This phenomenon is called selective interference
(also known as the Hill-Robertson effect). Many of the advantages of sex revolve
around the fact that sex reduces selective interference because it separates alleles
from their genomic backgrounds and allows selection to act more efficiently [17].
One form of selective interference is called clonal interference, which hap-
pens when two or more beneficial mutations spread through a population at the
same time (FIGURE 10.23). Consider the outcome in an asexual population if ben-
eficial mutations A and B appear at two loci in different individuals at about the
same time. Selection causes the two clones with those alleles to spread. When
they become common, they compete. If clone A has higher fitness than clone B,
it will drive clone B to extinction. The genotype AB, with the highest fitness, can
be established only after a second B mutation appears in a genotype that already
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_10.22.ai Date 12-15-2016 01-25-17wsexual2001 2002 2003
Year2004 20051.00.51.502.52.03.53.0FIGURE 10.22 Attacks by parasites give an evolu-
tionary advantage to sexual reproduction in the New
Zealand mud snail (Potamopyrgus antipodarum), as
predicted by the Red Queen hypothesis. The curves
show the relative fitness of sexual females, using
asexual females as the fitness reference, in three
populations across 5 years. Sexual females usually have
higher fitness than asexual females (points above the
solid horizontal line). In some years, sexual females
are more than twice as fit (points above the dashed
horizontal line), showing that they have overcome the
twofold cost of producing males. The higher fitness of
sexual females results because they have lower rates of
infection by parasites. (After [48].)10_EVOL4E_CH10.indd 265 3/22/17 2:25 PM