Evolution, 4th Edition

All AbouT SEx 267

(Metaphorically, the ruby is thrown out with the rubbish.) The second possibility

is that the beneficial mutation has such a positive effect that its genotype is now

the most fit in the population. It can then spread to fixation. But as it does so, all

the deleterious mutations elsewhere in its genome hitchhike along with it, causing

those loci to degenerate. In contrast, a sexual population avoids both of these fates

(FIGURE 10.24B). Recombination can liberate a beneficial mutation from deleteri-

ous mutations at other loci. That increases the chance that the beneficial mutation

will not be lost immediately, or drag to fixation bad alleles at other loci.

The ruby-in-the-rubbish effect is seen clearly in experimental populations of

yeast that have been manipulated to reproduce either with or without sex [30].

As the yeast adapt, beneficial mutations at many loci begin to sweep through the

populations. In asexual populations, the beneficial mutations drag deleterious

mutations at other loci along with them to fixation (FIGURE 10.25). The picture is

different, however, in sexual populations. As beneficial mutations begin to spread,

deleterious mutations again start to hitchhike along for the ride. But before they

become fixed, recombination uncouples the beneficial mutations from their delete-

rious passengers. The beneficial mutations then spread to 100 percent frequency,

while the deleterious mutations are eliminated. By the end of the experiment

shown in Figure 10.25B, five deleterious mutations became fixed in the asexual

population but none in the sexual population.

One last form of selective interference is Muller’s ratchet (named after the Nobel

Prize–winning geneticist H. J. Muller), which is the irreversible accumulation of

deleterious mutations in an asexual population. In any population, a relatively

small number of individuals have the fewest mutations. There is always a chance

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.25.ai Date 12-15-2016 01-25-17

Total tness increase (%)

Asexual Sexual

10

5

15

0

(A)

Frequency

200 400 600

Generation

Sexual

800

1.0

0.5

0

Frequency

Asexual

1.0

0.5

0

(B)

FIGURE 10.25 Experiments with yeast (Saccharomyces cerevi-

siae) show how recombination speeds adaptation. This species

can be manipulated in the lab to reproduce either sexually or

asexually. (A) After 1000 generations of adaptation to a labora-

tory environment, the fitness of the sexual populations increased

about twice as much as that of the asexual populations. (B) DNA

sequencing at different time points followed the frequencies

of mutations spreading in an asexual and a sexual population.

Separate experiments were used to estimate the fitness effects

of those mutations. The trajectories of beneficial mutations are

shown in green, and of deleterious mutations in purple. In the

asexual populations, eight beneficial mutations had become

fixed by the end of the experiment. As they did so, they dragged

five deleterious mutations to fixation with them. In the sexual

populations, recombination freed the beneficial mutations from

the deleterious ones. As a result, all of the deleterious mutations

were eliminated. (After [30].)

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