Evolution, 4th Edition

108 CHAPTER 5

who leaves two offspring to the next generation (a fitness of 2). That individual

enjoys a huge fitness advantage if all others in the population have a fitness of 1,

but it is at a selective disadvantage if all others have a fitness of 4.

We therefore often work with relative fitness, which is the absolute fitness

divided by a fitness reference that we agree on. The choice of the fitness reference

is a matter of convenience, and changes depending on the situation under consid-

eration. Say there are two alleles, A 1 and A 2 , at a locus. We use the symbol W 11 to

represent the absolute fitness of A 1 A 1 homozygotes, W 12 for that of A 1 A 2 heterozy-

gotes, and W 22 for that of A 2 A 2 homozygotes. We write the relative fitnesses of the

three genotypes the same way, but using a lowercase w (for example, the relative

fitness of A 1 A 2 is w 12 ). If we agree to use the A 1 A 1 homozygote as our fitness refer-

ence, then the relative fitness are:

w 11 = W 11 /W 11 = 1

w 12 = W 12 /W 11 (5.2)

w 22 = W 22 /W 11

Relative fitnesses play a critical role in determining the speed and outcome of evo-

lution by natural selection, as you will now see.

Positive Selection:

The Spread of Beneficial Mutations

Whenever one allele has higher fitness than another, natural selection will favor

its spread through the population. This is called positive selection. A hypotheti-

cal example is shown in FIGURE 5.6. A population of mice has two alleles that

affect the coat color. Allele A 2 causes darker fur than allele A 1 , and it is initially

present at a frequency of 0.5. Hawks prey on the mice and kill half of the light-

colored A 1 A 1 and one-fourth of the intermediate-colored A 1 A 2 individuals. The

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A 1 A 1 A 1 A 2 A 2 A 2

Frequency

A 1 A 1 A 1 A 2 A 2 A 2

Frequency

A 1 A 1 A 1 A 2 A 2 A 2

Frequency

Selection Reproduction

Juveniles

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

Surviving adults

X

X

X

X

Offspring

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 2 A 2

A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 1 A 1 A 2 A 1 A 2 A 2 A 2

A 1 A 2 A 2 A 2

FIGURE 5.6 Positive selection causes the allele with higher fit-

ness to increase in frequency. In this population of mice, allele A 2

is at a frequency of p = 0.5 at the start of the generation. Preda-

tion kills half of the A 1 A 1 homozygotes and one-fourth of the

A 1 A 2 heterozygotes, but all of the A 2 A 2 homozygotes survive. The

frequency of A 2 increases to p = 0.58 in the surviving adults. The

frequency of A 2 is also p = 0.58 at the start of the next generation,

and random mating has restored the genotype frequencies to

Hardy-Weinberg proportions.

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