Evolution, 4th Edition

THE GENETiCAl THEoRy of NATuRAl SElECTioN 115

critical to calcium metabolism, requires ultraviolet (UV) light. If not enough UV

light penetrates the skin, children develop a disease called rickets. Mutations that

decrease skin pigmentation likely had a selective advantage in the north because

they enhanced vitamin D synthesis [39]. Mutations at several loci that lighten the

color of the skin appeared and spread to high frequency under strong selection

(FIGURE 5.12). Different alleles at different loci were selected in different popula-

tions: the alleles that cause light skin color in Asia are different than those that

cause it in Europe. It is no accident that human populations farthest from the

equator have very light skin: it is an adaptation. But like many adaptations, light

skin comes with a cost. The increased pigmentation in African skin provides an

increased sun protection factor (SPF) of 10 compared with white skin. People with

light skin suffer higher rates of skin cancer.

Chance and adaptation: The probability that a

beneficial mutation spreads

Our discussion so far might lead you to think that if a beneficial mutation appears

in a population, it is certain to spread by natural selection and become fixed. But

that is not true: chance also plays a role. Consider an adult who is heterozygous

for a mutation that increases the probability of survival. If he or she has two off-

spring, there is a 50 percent chance that the mutation will not be passed to the first

offspring, and a 50 percent chance that it won’t be passed to the second offspring.

No matter how large a survival benefit the mutation gives, there is a 25 percent

chance that it will leave no descendants in the next generation. This example

shows how an allele can be lost through the random segregation of alleles dur-

ing meiosis. Other factors also contribute to the chance that the allele will be lost:

even if it increases survival on average, any particular individual who carries the

allele might not survive.

This example shows just one aspect of an important evolutionary process called

random genetic drift. As you will see in Chapter 7, drift is particularly important

in small populations, but it plays a role in very large ones as well. When an allele

first appears in a population by mutation, it is represented by only a single copy. It

may be lost by chance then, or in a later generation while it is still rare. Population

genetic theory shows that the probability that a single copy of a new beneficial

mutation will survive and become fixed in the population is

Pr(fixation) = 2s (5.5)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.12.ai Date 12-29-2016

The red portion of

each pie diagram

shows the frequency

of the SLC24A5

allele for light skin color.

FIGURE 5.12 The “golden” gene (technically,

a locus called SLC24A5) contributes to light skin

color in European populations. Lighter skin results

from a nonsynonymous mutation that changes an

alanine to a threonine in the gene’s protein prod-

uct. Patterns of DNA polymorphisms around this

locus show that strong selection caused this allele

to spread when humans spread from Africa into

Europe. The cause of the selection was likely that

reduced sunlight in the north led to a deficiency

in vitamin D. Light skin color in other populations

(for example, Asians) results from mutations at other

loci. Inset: The arm of this book’s second author

after removal of a basal cell carcinoma. Increased

risk of skin cancer is a negative side effect of light

skin. (After [13].)

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