Evolution, 4th Edition

202 CHAPTER 8

Some kinds of chromosomal rearrangements are underdominant (see Chapter 5).

Consider what happens if two populations that are geographically isolated become

fixed for different forms of the rearrangement. If these populations expand their

ranges, come into contact, and begin to interbreed, low-fitness heterozygote off-

spring are produced. Selection against heterozygotes can prevent the introgression

of each rearrangement from the region where it is common into the other region,

where it is rare and selected against. As a result, a stable cline can form. In the foot-

hills of the Alps, the grasshopper Podisma pedestris has populations that differ by the

presence or absence of a chromosome fusion [2]. Steep clines in the frequency of the

fusion, only a few dozen meters wide, are found where these populations meet (FIG-

URE 8.12). These clines result from strong selection against fusion heterozygotes,

which have low fertility. Clines that result from selection against heterozygotes are

called tension zones. They often occur in areas where two species hybridize [1b].

Gene flow and Drift

In Chapter 7 we looked at populations of a terrestrial snail whose allele frequencies

have diverged by drift (see F i g u r e 7. 4). Groups of snails living on the same city block

have similar allele frequencies, but frequencies are quite different between popula-

tions that are separated by a barrier (a street). So drift as well as selection can build up

differences between populations, and its potential to do that is limited by gene flow.

That intuition is confirmed by population genetics theory. Consider two popu-

lations, both with a constant effective size Ne. Each population receives a fraction

m of its individuals as migrants from the other population. For a locus that is free

of selection and that has been evolving long enough to reach an evolutionary equi-

librium, a mathematical model [5] predicts that the genetic divergence between the

populations will be

(8.5)

This equation has an interesting implication. Since Ne is the population size and

m is the fraction of each population that migrates, the product Ne m is simply the

average number of individuals that arrive in each population in each generation.

Equation 8.5 shows that if that number is much smaller than 1, then FST will be

close to 1, meaning that the two populations are expected to have very different

allele frequencies. At the other extreme, if the number of migrants per generation

(Ne m) is 1 or larger, then FST will be close to 0, and the two populations will be



F

N m

1

116

ST

e

Futuyma Kirkpatrick Evolution, 4e

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Podisma pedestris

1 km

Frequency

of the fusion

FIGURE 8.12 ension zones are clines that form when there T

is selection against heterozygotes. The grasshopper Podisma

pedestris is polymorphic for a chromosome fusion. Where

populations with and without the fusion meet in the foothills

of the Alps, clines in the frequency of the inversion form

because heterozygotes have reduced fertility. The filled part

of each pie diagram represents the frequency of the fusion.

The cline in the frequency of the fusion is only a few dozen

meters wide. (After [2].)

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