Evolution, 4th Edition

EVoluTioN iN SPACE 197

colonized different parts of Earth. A similar pattern can also result from a balance

between gene flow and random genetic drift, as we’ll discuss shortly.

In other species, the picture of genetic differentiation is quite different. Popula-

tions of the northern dusky salamander (Desmognathus fuscus) living near New

York City have FST values more than four times larger than those in human popu-

lations spread across the entire planet [23]. The contrast between the relatively

low values of FST in humans and the high values in the salamander underlines

the point that simple measures of genetic similarity cannot be used to determine

which populations do and do not belong to the same species.

Geographic differentiation varies across the genome. Marine populations of

the three-spined stickleback (Gasterosteus aculeatus) have invaded thousands of

freshwater streams around the Northern Hemisphere. Adapting to a freshwa-

ter environment involves many genetic changes. These changes cause high FST

between freshwater and marine populations to develop in regions of the genome

with loci that are locally adapted to those very different environments (FIGURE

8.8). Repeated divergence in the same genomic regions in different freshwater

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_08.08.ai Date 11-17-2016 01-24-17

Note: Edits requested by author incorporated. See printout of PDF page for reference.

Genomic position (Mb)

50 100 150 200 250 300 350 400

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

Eda

21 3 4 5 6 7 8 9 10

Chromosomes

11 12 13 14 15 16 17 18 19 20 21 22

0.2

0.4

0.6

0.8

FST

Signicant peak

Bear Paw

Boot Lake

Mud Lake

Marine

(completely plated)

Freshwater

(low-plated)

FIGURE 8.8 Divergence measured by FST between three popu-

lations of three-spined sticklebacks (Gasterosteus aculeatus) that

independently colonized freshwater and the marine populations

from which they evolved. The horizontal axis represents locations

along the 22 chromosomes (indicated by numbers at top), which

have been placed end to end. The vertical axis is FST at points

along the genome. Peaks of significantly greater differentiation

are indicated by red dots. One of the strongest regions of differ-

entiation in all three comparisons is in the region on chromosome

4 that carries the Eda locus. This gene controls a major polymor-

phism in bony plates along the side of the fish, which differs

between freshwater (low-plated) and marine (completely plated)

populations. (After [13]; photos from [1a].)

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