Evolution, 4th Edition

118 CHAPTER 5

of hitchhiking is that an allele that does not affect fitness can spread by natural

selection.

Population genetic theory quantifies how much change in an allele’s frequency

will result from hitchhiking. The simplest situation is where two alleles at locus A

are under selection. Allele A 2 has a selection coefficient sA such that the relative fit-

nesses are w 11 = 1, w 12 = (1 + sA), and w 22 = (1 + 2sA). The two alleles at locus B have

no fitness effects. The change in the frequency of allele B 2 in one generation is then

ΔpB ≈ sA D (5.6)

where D is the linkage disequilibrium between allele A 2 and allele B 2. (Recall from

Chapter 4 that D = PAB – pApB, where PAB is the frequency of gametes that carry both

alleles A 2 and B 2 , pA is the frequency of allele A 2 , and pB is the frequency of allele B 2 .)

If Equation 5.6 looks vaguely familiar, it is because it resembles Equation 5.3. The

rate of evolution at locus B depends on two quantities. The first is the strength of

selection acting on locus A, which is measured by the selection coefficient sA. The

second quantity is the linkage disequilibrium, D, which plays a role analogous to the

term p(1 – p) that represents genetic variation in Equation 5.3. Equation 5.6 tells us

that hitchhiking will happen at locus B only if it is in disequilibrium with locus A (that

is, D does not equal 0). Equation 5.6, like Equation 5.3, is an approximation that is very

accurate for selection coefficients less than 0.1.

Hitchhiking is responsible for the evolution of genes that themselves do not impact

survival or fecundity, but that do have other effects. In some environments, mutations

spread in bacterial populations that drastically increase mutation rates throughout the

genome. They do so because they generate mutations at other loci that are beneficial,

and as those mutations spread, the mutator allele hitchhikes along with them.

Evolutionary biologists exploit hitchhiking to find genes that have recently

evolved by positive selection. When a beneficial mutation first appears, it is in per-

fect linkage disequilibrium with all the other alleles on its chromosome (FIGURE

5.15). As the mutation spreads, recombination breaks down the disequilibrium.

The breakdown is most rapid between the selected locus and distant sites on the

chromosome (simply because there is more recombination between distant sites

than neighboring sites). Sites close to the selected locus do not have a chance to

recombine before the mutation becomes fixed. As a result, those sites carry the same

alleles that were on the original chromosome where the mutation appeared. All

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Neutral variant Benecial mutation Polymorphic site

Site with no

variation

FIGURE 5.15 When a beneficial mutation spreads to fixation,

the selective sweep eliminates polymorphism at nearby regions

of the chromosome. The beneficial mutation (in yellow) first ap-

pears on a chromosome that has selectively neutral variants in

its DNA sequence at two sites nearby (in blue). As the mutation

spreads to higher frequency, the neutral variants hitchhike with

it to higher frequency. When the mutation becomes fixed, ge-

netic variation is eliminated in the region nearby. Regions of the

chromosome further from the beneficial mutation retain variation

because recombination joins together chromosomes that carry

the beneficial mutation with chromosomes that carry different

neutral variants.

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