Evolution, 4th Edition

196 CHAPTER 8

results in an excess of homozygotes and a deficit of heterozygotes (see

Chapter 7). A second effect of migration is to generate linkage dis-

equilibrium (see Chapter 4). In the United States, males who are bald

are more likely to have white skin than black skin. That is because

the U.S. population includes people with ancestry from northern

Europe and from Africa. In northern Europe, alleles for white skin

and baldness are common, while in Africa they are rare. The mixing

of people from those two populations has caused linkage disequilib-

rium between alleles that affect baldness and those that affect skin

color. Departures from Hardy-Weinberg ratios and excess linkage dis-

equilibrium can be used to estimate gene flow between populations.

Other indirect methods for estimating migration rates from genetic

data are discussed later in this chapter.

Genetic Divergence between Populations

Genetic differences between populations can be described in several

ways. One of the most widely used approaches is based on a statistic

called FST, which measures the fraction of the total genetic variance

found across two or more populations that results from genetic dif-

ferences between them. A value of FST = 0 means that the populations

are genetically identical, while a value of FST = 1 means that each

population is fixed for a different allele (FIGURE 8.6). For a locus with

two alleles, it is calculated as

(8.2)

where Var(p) is the variance of the allele frequency among popula-

tions, and p– is the mean allele frequency across all the populations.

FST is often used to measure genetic differences among human populations. The

International HapMap Project analyzed differentiation in single nucleotide poly-

morphisms (SNPs) across the genomes of East Asians, Europeans, and Yoruba from

Nigeria [35]. Across all the autosomes, FST is 0.12. This tells us that only 12 percent of

all the genetic variation in these populations is caused by differences among them.

A full 88 percent of all the variation in our species can be found within a typical

population. The striking phenotypic differences we see among human populations

are therefore not representative of the genome as a whole. FIGURE 8.7 shows how

FST increases as the distance between pairs of populations increases [28]. This pat-

tern is called isolation-by-distance. In humans, it reflects the history of how we

( )

=

−

F

p

pp

Var( )

1

ST – –

Q: This gure might benet from some balloon

text at start and end points?

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Sinauer Associates

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Evolution4e_08.06.ai Date 11-17-2016

FST = 0

A 1

A 1

A 1 A 1

A 1

A 2

A 2

A 2 A 2

A 2

A (^1) A
1 A 1
A 1
A 1
A 2
A 2
A 2
A 2
A 2
FST = 0.36
A 1
A 1
A 1 A 1
A 1
A 1
A 1
A 1
A (^2) A
A 2 2
A 2 A 2
A 1
A 1
A 2
A 2
A 2
A 2
A 2
FST = 1
A 1
A 1
A 1 A 1
A 1
A 1
A 1
A 1
A (^2) A
A (^22)
A 2
A 1 A 2
A 1
A 2
A 2
A 2
A 2
A 2
Population 1 Population 2
FIGURE 8.6 FST is a statistic used to measure genetic
differences between two or more populations. In this
schematic, two alleles at a locus are represented by red
and blue circles. Top: FST = 0 when allele frequencies are
equal in the populations. Middle: FST = 0.36 when allele
frequencies are 0.2 and 0.8 in the two populations. Bot-
tom: FST = 1 when the populations are fixed for different
alleles.
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Troutt Visual Services
Evolution4e_08.07.ai Date 03-16-17
FST
0 5000 10,000 15,000 20,000 25,000
Distance between populations (km)
0.10
0.20
0
Within a
single region
Between the Americas
and Oceania
Between Africa and Eurasia
FIGURE 8.7 solation-by-distance in human popula-I
tions. The horizontal axis has been corrected for large
bodies of water that could not be crossed when hu-
mans first spread across Earth. The vertical axis gives FST
estimated from 783 loci. Each dot represents a compari-
son between a pair of populations from the indicated
region(s). (After [28].)
08_EVOL4E_CH08.indd 196 3/23/17 9:12 AM