Evolution__3rd_Edition

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Figure 12.3
Evolution in (a) asexual and
(b) sexual populations. The
mutations A, B, and C are all
advantageous. In the asexual
population, an AB individual
can only arise if the B mutation
arises in an individual that
already has an A mutation
(or vice versa). In the sexual
population, an AB individual
can be formed by the breeding
of a B mutation-bearing
individual with an A mutationbearing
individual; the second
mutation of B is not needed.
(c) If favorable mutations are
rare, each will have been fixed
before the next arises, and
sexual populations do not
evolve faster. The relative rates
of evolution in asexual and
sexual populations depends
on the rate at which favorable
mutations arise.
Sexual populations can evolve
faster than asexual populations, ...
. . . if the rate of favorable mutation
is high
(a) Asexual: high rate of favorable mutation
C
A
Time
B AC
Time
CHAPTER 12 / Adaptations in Sexual Reproduction 317
AB ABC
(c) Sexual or asexual: low rate of favorable mutation
A AB
(b) Sexual: high rate of favorable mutation
single individual (Figure 12.3). Suppose, for example, that a sexual and an asexual
population are both fixed for genes A′ and B′ at two loci. In the environment where the
two populations are living, mutations A and B are advantageous. A and B mutations
would be likely to arise initially in different individuals. The asexual population will
then come to consist of A′B and AB′ individuals, because the A mutant cannot spread
into the A′B clone or vice versa. AB individuals cannot appear until an A′ gene mutates
to A within the A′B clone (or B′ to B in the AB′ clone).
In the sexual population, evolution proceeds much faster. After A and B have arisen
in different individuals, they can soon combine in a single individual by sex without
waiting for the mutations to occur twice. Natural selection can therefore take the population
from the state A′B′ to AB faster than under asexual reproduction. This argument
was first put forward by Fisher and by Muller in the 1930s. They concluded that sexual
populations have a more rapid rate of evolution than would an otherwise equivalent
group of asexual organisms.
However, subsequent research has shown that the rate of evolution in sexual populations
is not necessarily faster than in equivalent asexual populations. For instance, the
result depends on the rate of mutation. If favorable mutations are rare, each one will
have been fixed in the population before the next one arises (Figure 12.3c). Sexual and
asexual populations then evolve at the same rate. New favorable mutations will always
arise in individuals that already carry the previous favorable mutation: they must,
because the previous favorable mutation is already in every member of the population.
In terms of the example, the B mutation will arise in an AB′ individual in both sexual
and asexual populations. However, if favorable mutations arise more frequently, Fisher
and Muller’s argument works: the sexual population evolves faster. Each new favorable
mutation will usually arise in an individual that does not already possess other
A
C
B
AC
AB
Time
ABC