Evolution, 4th Edition

THE GENETiCAl THEoRy of NATuRAl SElECTioN 113

fitness, selection has little power to increase the frequency of A 2 further, and the

rate of evolution slows dramatically.

The converse situation occurs when the beneficial A 2 allele is recessive. In that

case, when A 2 is rare, its frequency increases very slowly: only the exceedingly rare

A 2 A 2 homozygotes enjoy the fitness advantage. But when the allele is common,

selection is now much more effective than it is for a dominant allele that is com-

mon (see Figure 5.10).

Thus far, we have focused on positive selection favoring the spread of a ben-

eficial mutation. The same logic and same equations (5.3 and 5.4) also apply to

deleterious mutations (those that decrease fitness). In this case, the selection coef-

ficient s is negative, and the change in allele frequency Δp is negative (from Equa-

tion 5.3). Many genetic diseases in humans are caused by mutations that are nearly

or completely recessive. Because they are at low frequency, almost all copies are

in heterozygotes who have fitness close or equal to that of individuals who do

not carry the mutation. Selection is therefore very ineffective at removing these

disease-causing mutations from the population.

We’ve now seen that if we know the fitnesses of the genotypes, we can predict how

many generations it takes for a beneficial mutation to spread. We can turn the tables

around by asking: If we know how many generations it took for an allele to spread,

how strong was selection? This was the strategy used by J. B. S. Haldane to estimate

that the melanic allele of the peppered moth had a selection coefficient of s = 0.5.

The peppered moth is not the only example of rapid evolution in response to

intense selection. Many other animals have evolved melanic forms in polluted urban

environments, so many that the phenomenon has a name: industrial melanism

[30]. Other cases of strong natural selection and rapid evolutionary change include

bacteria evolving resistance to antibiotics, insects evolving resistance to pesticides,

and plants evolving resistance to herbicides. You may have noticed a theme here:

in these examples, selection results from a change to the environment caused by

humans. With antibiotic resistance in bacteria, the change was intentional (when

we administered the drugs), while with industrial melanism it was not. But whether

it intends to or not, human activity is rapidly changing the environment of many,

in fact probably most, organisms on the planet. This is causing strong selection and

Futuyma Kirkpatrick Evolution, 4e

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200 400 600 800 1000 1200

0.2

0.4

0.6

0.8

1.0

0

Generation

Dominant

Frequency of

A

2

A 1 A 1 A 1 A 2

Fitness

No dominance

Recessive

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

Fitness

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

Fitness

A 2 A 2

FIGURE 5.10 Dominance affects the evo-

lutionary trajectories of beneficial alleles.

When the beneficial allele is dominant,

its frequency increases very rapidly while

it is rare, then much more slowly when it

is common. A recessive beneficial muta-

tion spreads very slowly when it is rare, but

rapidly when it is frequent. When there is

no dominance (that is, the heterozygote

has a fitness intermediate between the

homozygotes), the trajectory is intermedi-

ate between the dominant and recessive

cases. In these trajectories, the fitness of the

A 2 A 2 homozygote is 1.01 relative to that of

the A 1 A 1 homozygote, and the beneficial A 2

mutant was introduced at a frequency of 1

percent. The trajectories differ only in the

relative fitness of the heterozygote.

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