Evolution, 4th Edition

154 CHAPTER 6

example, smaller individuals tend to have higher metabolic rates per body mass.

A second source of genetic correlations is linkage disequilibrium, the nonrandom

association between alleles at different loci (see Chapter 4).

In some cases, selection favors genetic correlations. We saw earlier that cor-

relational selection on garter snakes favors certain combinations of coloration and

predator escape behaviors. These traits are also genetically correlated, and the high

fitness combinations of traits are more common than they would otherwise be [9].

While we do not know whether the genetic correlation in garter snakes results

from pleiotropy or linkage disequilibrium, in other cases we do. The butterfly Heli-

conius numata has several wing-color morphs that mimic different species of model

butterflies. (See Chapter 13 for more on mimicry.) Selection caused by predation

favors certain combinations of color elements. Genetic analysis showed that the

color morphs are controlled by a small segment of chromosome with 18 genes [29].

The high fitness combinations of alleles that control the colors have been locked

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_06.22.ai Date 11-10-2016 01-10-2017

Relative size of anterior eyespot

–6 –4 –2 0 2 4 6

Relative size of posterior eyespot

–4

–2

0

2

4

6

–6

Color composition of eyespot 4

–3 –2 –1 0 1 2 3

Color composition of eyespot 6

–2

–1

0

1

2

3

–3

(A) (B)

Large-small Large-large

Small-small Small-large

Black-gold Black-black

Gold-gold Gold-black

FIGURE 6.22 Artificial selection experiments showing different

levels of constraint on the evolution of eyespots in wings of the

butterfly Bicyclus anynana. (A) Evolutionary trajectories that result

from selection on the sizes of the two eyespots on the dorsal

side of the wing. The graph shows the results of selection for

both eyespots to be large or small (Large-large and Small-small,

green trajectories) and for one to be large and the other to be

small (Large-small and Small-large, purple trajectories). Each

point represents one generation. Rapid evolution occurred for

all combinations of traits that were selected, showing there is no

evolutionary constraint. (B) Trajectories resulting from selection on

the colors of two other eyespots (shown by arrows) on the ventral

hind wing. Selection to make both eyespots more black or more

gold caused substantial evolutionary change (Black-black and

Gold-gold, blue trajectories). In contrast, selection for different

colors in the eyespots was largely ineffective (Black-gold and

Gold-black, red trajectories). This is an example of an evolution-

ary constraint caused by a genetic correlation between two traits.

(After [1a], courtesy of Cerisse Allen and Paul Brakefield.)

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