Evolution, 4th Edition

174 CHAPTER 7

populations now living far from southern Africa therefore experienced many

founder events in their past, while those living closer to Africa experienced fewer.

Remarkably, the signature of those events can still be seen in our genes. Heterozy-

gosity is highest in Africa and declines as we follow the paths that humans took as

they expanded across Earth (FIGURE 7.7).

All species experience fluctuations in population size. The effect of these fluc-

tuations is to make the effective population size much closer to the minimum than

the maximum population size. During an outbreak of the flu, the viral population

grows to an unimaginably large size, but each outbreak starts from a very small

number of viral particles. Estimates based on methods discussed in the next sec-

tion suggest that Ne for the influenza virus is only a few hundred individuals [34].

drift and genetic variation within Species

Genetically speaking, you are very similar to the person next to you. Of the 3.2

billion DNA bases in the human genome, 99.9 percent of them are identical in two

randomly chosen individuals. But while only 1 out of 1000 DNA bases differs, the

genome is so large that you differ from your neighbor at some 3 million bases.

In humans and other species, polymorphism is spread unevenly across the

genome (FIGURE 7.8). Polymorphism is typically high in the regions between

genes, and within introns. Coding regions (exons) are less variable, particularly the

first and second bases of each codon. These patterns are so common across the tree

of life that they must result from very general features of evolution.

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0.75

(A) (B)

0.70

0.65

Mean heterozygosity

0.60

0.55

0.50

0.45

5000 10,000 15,000 20,000 25,000

America

Oceania

East Asia

South/central Asia

Africa

Middle East

Europe

FIGURE 7.7 Genetic variation in humans Distance to Addis Ababa (km)

declines with distance from East Africa,

where the ancestors of modern humans

lived before expanding out of Africa start-

ing about 60,000 years ago. (A) The routes

used by humans to colonize the Earth.

(B) Heterozygosity in modern native popu-

lations plotted against the distance from

Addis Ababa, Ethiopia (shown by the star

in [A]) along the colonization routes. Colors

indicate the geographical origin of the

populations. Heterozygosity is here mea-

sured for haplotypes, which are stretches

of DNA with multiple SNPs. (A after [21]; B

after [20].)

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5 ʹ 3 ʹ

0 200

Exon 1 Exon 2 Exon 3 Exon 4

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 bp

FIGURE 7.8 Nucleotide variation at the Adh locus in Drosophila melanogaster. Four

exons (green) are separated by introns (blue). The vertical lines show the positions of 43

DNA polymorphisms (SNPs) found in a sample of 11 chromosomes. Of all the SNPs, 17

are in exons, but only 1 of these (shown by the triangle) causes an amino acid change

in the alcohol dehydrogenase enzyme. (After [18].)

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