Evolution, 4th Edition

124 CHAPTER 5

Genetic variation that is maintained this way is called multiple niche polymor-

phism. Many herbivorous insects have “host races” that specialize on different

species of host plant. The pea aphid (Acyrthosiphon pisum) has host races adapted

to different crop plants. Each host selects for different alleles at several loci, which

maintains polymorphism in the aphid [22].

A final type of selection that can maintain genetic variation occurs when fit-

nesses vary in space. This situation is similar to multiple niche polymorphism, but

it is usually not considered to be a form of balancing selection because it does not

occur within a single population. FIGURE 5.22 shows a map of the frequency of a

chromosome inversion that occurs in a mosquito that lives in Cameroon, Africa.

The inversion is near fixation in the south, nearly absent in the north, and present

at intermediate frequencies in the center of the country. Analysis of these data

suggests that fitnesses vary along a north-south axis: in the south, chromosomes

with the inversion have higher fitness than those without it, while the reverse is

true in the north [2]. Chapter 8 explores in detail the consequences of selection

that varies in space.

Perhaps surprisingly, selection that fluctuates in time does not generally main-

tain genetic variation. If selection favors one allele in some generations and a dif-

ferent allele in others, in many cases the allele that has the highest fitness on aver-

age will spread to fixation. Likewise, the evolutionary trade-offs discussed earlier

in the chapter do not typically maintain polymorphism. The horn polymorphism

in Soay sheep is unusual. More commonly, despite trade-offs, one allele is on aver-

age more fit than the other, and it will spread to fixation.

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_05.22.ai Date 12-20-2016

(A)

(B)

(C)

Savannah

Highlands

Rainforest

0 20010050 300

km

SS

SI

II

4000 m

Elevation

0 m

Inversion

FIGURE 5.22 A malaria mosquito is

polymorphic for chromosome inversions

in Cameroon, a country in Africa. (A) The

mosquito Anopheles funestus is one of the

species that transmits malaria. (B) A photo

of part of chromosome 3. Inversion 3Ra is

highlighted in green. (C) The pie diagrams

show the frequencies of the three geno-

types in villages along a highway: white are

homozygotes without the chromosome in-

version (SS), black are homozygotes with the

inversion (II), and blue are heterozygotes

(SI). Three groups of populations are visible.

In the north, which is hot and dry savannah,

the inversion is almost entirely absent. In the

south, which is lowland rainforest, the inver-

sion is at almost 100 percent frequency. In

the center of the country, which is a moun-

tainous highland, the inversion is at inter-

mediate frequencies. (B courtesy of Igor V.

Sharakhov, after [37]; C from [2].)

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