Evolution, 4th Edition

392 CHAPTER 15

phenotypic plasticity (see Chapter 6). In other cases, the phenotype may be quite

constant, irrespective of environment (as in Figure 15.26C): it has a flat reaction

norm, and is sometimes said to be canalized.

When the effect of environmental differences on the phenotype differs from

one genotype to another in a population, the reaction norms of the genotypes are

not parallel, and the phenotypic variance includes a variance component (referred

to as VG×E) that is due to genotype × environment (G×E) interaction (see Figure

15.26B). If all the genotypes have parallel reaction norms (see Figure 15.26A),

there is no G×E interaction (VG×E = 0). If there is G×E interaction (that is, if reaction

norms differ in slope among genotypes in a population), selection could change

the average degree of phenotypic plasticity. For example, the effect of temperature

on the number of bristles differed among ten genetic strains of Drosophila pseu-

doobscura (FIGURE 15.26D). If different bristle numbers were optimal for flies in

colder versus warmer environments, it is likely that the ability to develop the right

number, depending on temperature, could evolve.

Some environmentally determined phenotypic effects are not adaptive and may

be unavoidable: for example, most organisms grow more slowly at lower tempera-

tures, and most of us will weigh more if we eat too much. In many species, how-

ever, natural selection has resulted in norms of reaction that most nearly yield the

optimal phenotype for the various environments the organism commonly encoun-

ters [67, 86]. Phenotypic plasticity includes rapidly reversible changes in morphol-

ogy, physiology, and behavior as well as “developmental switches” that cannot

be reversed during the organism’s lifetime (FIGURE 15.27A). In some semiaquatic

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_15.27.ai Date 12-20-2016

(A)

(B)

Catkins Caterpillars

Submerged Air-water interface Aerial

FIGURE 15.27 Examples of phenotypic

plasticity. (A) Larvae of the geometrid

moth Nemoria arizonaria that hatch in

the spring (left) resemble the oak flowers

(catkins) on which they feed. Larvae that

hatch in the summer (right) feed on oak

leaves and resemble twigs. (B) The form

of a leaf of the water-crowfoot Ranun-

culus aquatilis depends on whether it

is submerged, aerial, or situated at the

air-water interface during development.

(A, photos courtesy of Erick Greene; B

from [12].)

15_EVOL4E_CH15.indd 392 3/22/17 1:30 PM