Evolution, 4th Edition

THE EVOLUTIONARY STORY OF HOMO SAPIENS 563

of people carry the mutation (see Fig ure 5.11). It must have conferred a very large

fitness advantage indeed.

Three of the other genes that show strong signs of selection are also involved in

adaptation to the new agricultural diet. One, called SLC22A4, carries a mutation

that increases absorption of the amino acid ergothioneine. That amino acid was

abundant in the European diet before agriculture, but it is present only at low levels

in wheat. Ancient farmers with the mutation gained a large fitness advantage. The

remaining eight genes that show rapid recent adaptation affect skin and eye color,

the immune system, and tooth morphology. Other genes that affect height also

show signatures of selection. Body height was selected to increase in some times

and some places, and to decrease in others. The reasons for those selective pres-

sures are not yet known.

The study by Mathieson and colleagues adds to the dozens of examples of recent

adaptation in humans that have been revealed by the analysis of DNA over the

last decade. Other cases discussed earlier in this book include skin color (see Fig-

ure 5.12), malaria resistance (see Fig ure 5.18), EPA S1 (see F i g u r e 7. 2 3), BRCA1 (see

F i g u r e 7. 2 0), and amylase (see Fig u re 14.4). Still more are shown in Figure 6.28. We

have the fantastic fortune to live in a great age of genetic discovery whose revela-

tions are opening whole new vistas on human evolution.

Our genetic loads

Not all the genetic changes in our recent past have been for the best. Some of the

beneficial mutations that spread by positive selection dragged along with them

deleterious mutations by genetic hitchhiking (see Chapter 5). Genetic variants

tightly linked to the beneficial SLC22A4 mutation that we discussed in the last

section are associated with two digestive disorders, celiac disease and irritable

bowel syndrome. Northern Europeans who benefit from more efficient absorbtion

of ergothioneine are also at greater risk from those diseases.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_21.19.ai Date 01-18-2017

9

8

7

6

5

4

3

2

Kya

Lactase

persistance

1 2 3 4 5 6 7 8 9

Chromosome

10 12 14 16 18 22

Relative statistical signicance

LCT

TLR1-6-10

SLC45A2

SLC22A4

MHC

ZKSCAN3

FADS1-2DHCR7

GRM5

ATXN2

Chr13:38.8HERC2

FIGURE 21.19 Twelve genes show strong

signals of adaptive evolution in human

populations in Europe and western Asia

over the last 8500 years. Ancient DNA was

collected at points shown on the map

(inset). Comparison of single nucleotide

polymorphisms in those samples with ones

from modern humans living in the same lo-

cations pinpointed loci that evolved under

positive selection. In the main panel, the

genome is displayed from chromosome 1

to 22. The y-axis shows the strength of evi-

dence for positive selection among more

than 1 million single nucleotide polymor-

phisms. Four genes involved in adaptation

to an agricultural diet are indicated by red

diamonds. The strongest signal is for the

mutation that causes lactase persistence,

which allows adults to digest milk and other

dairy products. It appeared in Europe just

4500 years ago. (After [46].)

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