Evolution, 4th Edition

200 CHAPTER 8

A very different outcome occurs when migration onto an island is much stron-

ger than selection (s << m). The frequency of the locally adapted allele evolves to

0, and the allele is lost entirely. This outcome, in which gene flow overwhelms

local adaptation, is called gene swamping. Like many insects, populations of the

mosquito Culex pipiens have evolved resistance to insecticides. On the island of

Corsica, however, resistance has not evolved, despite the presence of resistance

alleles at low frequency. The best explanation is gene swamping. Patches of habitat

on Corsica where insecticide is applied are small. Mosquitoes disperse into those

patches from neighboring populations that are not resistant, swamping the resis-

tance alleles that are favored inside the patches [19, 29].

The evolutionary tension between selection and gene flow plays out in continu-

ous habitats as well as on islands. Consider a grass growing in a prairie that has

two kinds of soil that meet at a sharp boundary. The soil to the west of the bound-

ary favors allele A 1 , while the soil to the east favors allele A 2. To be specific, say that

allele A 1 has a relative fitness advantage of s in the west, while A 2 has the same

advantage in the east, and there is no dominance. What happens? Without gene

f low, A 1 will become fixed everywhere to the west of the boundary, and A 2 will be

fixed everywhere to the east. But with gene flow, alleles move across the boundary.

This introduces into each habitat the allele that is not favored there, and a cline

develops (FIGURE 8.11). The cline can be short or long, depending on the relative

strengths of gene flow and selection. The width of the cline is

(8.4)

This is the width of region over which allele A 2 increases from a low frequency

(p = 0.1) to a high frequency (p = 0.9).

wc=2.5 σ^2 ms

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_08.10 .ai Date 11-17-2016 01-24-17

Lava

O’Neill

Xmas Tule West Mid East O’Neill

Arizona,

USA

Sonora, Mexico

50

km

Frequency of melanic allele^0

0.5

1

FIGURE 8.10 The rock pocket mouse (Chaeto- Light rock Dark lava Light rock

dipus intermedius) typically has light-colored fur

where it lives on light-colored granite, but is dark

colored where it lives on dark lava flows in Arizona.

The dark coloration, which results from a melanic

allele at the Mc1r locus, camouflages mice on the

lava from predators. Selection favoring the melanic

allele on the lava is intense, with a selection coef-

ficient estimated to be as high as s = 0.4. (After [12];

photos from [24].)

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