Evolution, 4th Edition

114 CHAPTER 5

rapid evolution in a wide range of species [34]. While adaptation to changes to the

environment caused by humans may help some species avoid extinction (see Chap-

ter 6), most are likely to become extinct (see Chapter 17).

Many genes in humans have also evolved recently in response to strong natural

selection. When humans domesticated cattle, cows' milk suddenly offered a rich

source of energy that previously had been available only to infants nursing on their

mothers. Milk contains a sugar called lactose. Children and other young mammals

derive energy from lactose by digesting it with the enzyme lactase. In our ances-

tors, lactase was produced during infancy to digest the mother’s milk, but was

not produced in adults because there was no lactose in their diet. Things changed

with the arrival of domesticated livestock in northern Europe about 10,000 years

ago (FIGURE 5.11). Suddenly, natural selection favored lactase in adults so they

could digest the milk of cows and other domesticated animals [4]. A mutation that

causes lactase persistence, in which lactase is produced throughout life, appeared

in Europe about 4000 years ago and spread very rapidly by natural selection [32].

Genetic evidence suggests that the selection coefficient was very large, perhaps s

= 0.1 or even larger [6]. In northern Europe today, about 98 percent of adults show

lactase persistence. The remaining 2 percent of the population are lactose intoler-

ant: they do not produce the enzyme as adults and so have trouble digesting milk

and cheese. In other populations that independently invented dairy farming, such

as the Maasai in East Africa, other mutations with similar effects spread—a strik-

ing example of parallel evolution [21]. In parts of the world where dairy products

are rarely consumed, lactase persistence is uncommon (for example, less than 10

percent in Southeast Asia).

Light skin color is a second example of strong natural selection causing parallel

adaptation in humans. Modern humans first spread out of Africa roughly 60,000

years ago (see Chapter 21). Those that moved north lived under reduced sunlight,

which generated new selective pressures. The synthesis of vitamin D, which is

Futuyma Kirkpatrick Evolution, 4e

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Atlantic

Ocean

Sweden

Germany

France

1000 km

Spain

FIGURE 5.11 The region in Europe where lactase persistence

is common overlaps with archeological sites where early

Neolithic people had cattle. Dark green areas on the map

show where the lactase persistence allele is at high frequency.

The dashed curve shows the geographic range of the Fun-

nelbeaker culture, which was a pastoral society. Dairy products

were likely consumed by these people roughly 4500 years

ago. Inset: The diet of northern Europeans is still rich with dairy

products. (After [4].)

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