Evolution, 4th Edition

266 CHAPTER 10

has the A mutation, and that may take a substantial time. In short,

spread of the beneficial A mutation interferes with the establish-

ment of the beneficial B mutation, and adaptation is slowed. The

problem of clonal interference is magnified when more than two

loci are adapting.

The picture is different in a sexually reproducing population.

If mutations A and B are spreading at the same time, recombina-

tion can bring them together in a single individual. The high-fitness AB genotype

can then spread to fixation without the wait for a second B mutation to appear. By

avoiding clonal interference, sexual reproduction accelerates adaptation.

A second type of selective interference is called the ruby-in-the-rubbish effect,

which is the loss of beneficial mutations as the result of their linkage to deleterious

mutations [38]. Recall that deleterious mutations are constantly raining down on

the genome, and as a result every individual carries many of them (see Chapter 5).

When a beneficial mutation appears in an asexual population, one of two things

can happen. If the positive fitness benefit that it gives is small relative to the com-

bined negative fitness effects of all the deleterious mutations in the genome where

it appeared, the beneficial mutation is doomed (FIGURE 10.24A). That is because

the fitness of its genotype will be improved by the beneficial mutation, but not

enough to compete successfully with other, more fit genotypes in the population.

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C

A AB ABC

B

AC

C

AABC

AB

B

AC

(A) Asexual, no recombination

(B) Sexual, with recombination

abc

abc

Frequency

Frequency

Time

FIGURE 10.23 Clonal interference slows adaptation. (A) In an asexual

population that is initially fixed for the genotype abc, beneficial muta-

tions A, B, and C occur at those three loci and begin to spread. Mutation

A is the most fit, and so it drives the clones that carry mutations B and C

to extinction. The population finally gains all three mutations only after B

occurs in a genotype that already carries A, and C occurs in a genotype

that already carries A and B. (B) In a sexual population, recombination

can bring together in a single genotype several mutations that originally

appeared in different individuals. This reduces clonal interference and

accelerates adaptation. The asexual population is therefore at greater

risk of extinction in changing environments. (After [14].)

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(A) No recombination

(B) With recombination

FIGURE 10.24 The ruby-in-the-rubbish effect gives an advan-

tage to sexual populations. (A) In an asexual population with no

recombination, the first beneficial mutation (blue) that appears

occurs in a genotype that is already carrying two deleterious mu-

tations (orange). The fitness of that genotype is less than the one

with no deleterious mutations, and so it is lost. A second beneficial

mutation occurs in a genotype with only one deleterious muta-

tion. It spreads to fixation, but by doing so causes fixation of the

deleterious mutation. (B) In a sexual population with recombina-

tion, the beneficial mutations can recombine away from deleteri-

ous mutations on the chromosomes where they first appeared,

and into genotypes with no deleterious mutations. That increases

the chance they will not be lost and will become fixed without

having deleterious mutations hitchhike to fixation with them.

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