Evolution, 4th Edition

gENETIC dRIfT: EvolUTIoN AT RANdoM 169

homozygous for one allele, roughly half would be fixed for the other allele. Within

each population, allele frequencies would be in Hardy-Weinberg equilibrium, but

the frequency of heterozygotes would be 0 rather than 1/2 as expected if all indi-

viduals were in a single population. Thus drift does not cause large departures from

Hardy-Weinberg equilibrium within a population, but it does cause a deficit of het-

erozygotes when a set of diverging populations is considered as a whole.

We can see the effects of drift in natural populations. Researchers collected 2218

individuals of the garden snail (Cornu aspersum) living on two adjacent city blocks in

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Generation 19

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Generation 15

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Generation 10

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Generation 5

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Generation 1

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107

Initial populations

Number of bw^75 alleles

bw homozygote bw^75 homozygote

Number of populations

FIGURE 7.3 Random genetic drift in experi-

mental populations of Drosophila mela-

nogaster. At a locus that affects eye color,

homozygous bw/bw flies have white eyes,

homozygous bw^75 /bw^75 have brown eyes,

and heterozygotes are intermediate. Each of

107 populations was founded with 16 het-

erozygotes. Populations were then propa-

gated with eight males and eight females

per generation. Differences among popula-

tions accumulated by drift as the experiment

went on. By generation 19, the bw^75 allele

was lost from 30 populations and was fixed

in 28 populations. (Data from [5].)

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