Evolution, 4th Edition

348 CHAPTER 14

As a chromosome with a new gene duplicate spreads through

a population, there is polymorphism in the number of copies of

the gene that individuals carry. This situation, called copy num-

ber variation, is found at the AMY1 locus in humans [55]. This

gene codes for amylase, an enzyme in saliva that breaks down

starch. Individuals with more copies of AMY1 have more amy-

lase in their saliva and digest starch more efficiently. The central

African Biaka, who are hunter-gathers with a diet that rich in

protein but low in starch, typically have four or five copies of the

gene. Japanese, whose rice-heavy diet has abundant starch, typi-

cally have six to eight copies of the gene (FIGURE 14.4).

Gene duplication plays a key role in genome evolution. In the

recent evolutionary past of humans, 1 percent of our genes have

been duplicated every million years [15a]. Some 1400 duplicates

have been fixed in humans or chimpanzees since we shared a

common ancestor roughly 7 million years ago (Mya). More of

the base pair differences between humans and chimpanzees

have resulted from gene duplication than from changes at single

nucleotides. Gene duplication has played major roles in the evo-

lution of other species as well (FIGURE 14.5).

Important ecological adaptations can result from gene dupli-

cation. The douc langur (Pygathrix nemaeus) that we met at the

opening of this chapter lives on a diet of leaves. The leaves are

fermented in the gut by symbiotic bacteria, much as in cows,

and the monkeys gain nutrition by digesting the bacteria. One

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Japanese

Biaka

2 3 4 5 6 7 8 9 10 11 12 13 14 15

AMY1 diploid gene copy number

Proportion of individuals

0.30

(A) (B)

0

0.05

0.10

0.15

0.20

0.25 Low starch

High starch

Note: I switched the order in part (B) to have Biaka rst and Japanese second since that is the order of the bars on the left.

The photos in (B) have been adjusted a bit to have the red and green pop out more.

FIGURE 14.4 Copy number variation at the AMY1 locus in

humans. This gene codes for amylase, an enzyme that digests

starch. Individuals with more copies of the gene have a greater

concentration of amylase in their saliva. (A) Distributions of copy

number in populations that have low-starch diets (Biaka and

Mbuti hunter-gatherers) and high-starch diets (Japanese, Euro-

pean Americans, and Hadza). On average, the populations with

a high-starch diet have 6.7 copies of AMY1, while those with a

low-starch diet have 5.4 copies. (B) Photos of regions of the two

chromosome regions that carry duplicates of the AMY1 locus.

Each copy of the gene is labeled with a red and green dye. The

Biaka individual shown here has 6 copies, while the Japanese

individual has 14 copies of AMY1. (A after [55]; B from [55].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

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Note: I thought a standard key would be clear but used
dots rather than boxes to better match the pie charts.

8793 617

Time (Mya)

Human

(+689/–86)

Primate

(+870/–1032)

Rodent

(+1773/–378)

Chimpanzee

(+26/–729)

Mouse

(+1405/–562)

Rat

(+1355/–1120)

Dog

(+607/–2165)

Expanded

Contracted

No change

FIGURE 14.5 Gains and losses of genes in several lineages

of mammals. The numbers beneath each branch show the

numbers of genes gained and lost. The pie charts show the

fraction of gene families that have expanded, contracted, or

not changed. (After [15a].)

14_EVOL4E_CH14.indd 348 3/22/17 2:44 PM