Evolution, 4th Edition

PHEnoTyPiC EvoluTion 155

together by a series of chromosome inversions that prevent recombination

from producing low fitness color patterns (FIGURE 6.23). Selection favored

a genetic correlation between the colors controlled by several loci, and the

inversions spread because they strengthened that correlation.

Phenotypic Plasticity

Most tadpoles (the larval stage of frogs and toads) live on a diet of algae

and detritus. Spadefoot toads (Spea), however, have a remarkable trick (FIG-

URE 6.24). When their eggs hatch in ponds where algae are the main food

source, they develop into typical omnivorous tadpoles. But when they hatch

in ponds with a high density of shrimp and other animal prey, they develop

into carnivorous tadpoles with a greatly enlarged head and sharp horny

beak [39]. The omnivorous tadpoles have large fat reserves that increase

their survival as adults. The carnivorous tadpoles sacrifice these reserves but can

develop more rapidly on their diet of animal protein, which allows them to meta-

morphose at an earlier age. That is adaptive because the conditions that trigger

development of carnivores occur in ponds that dry up quickly, and tadpoles die if

they have not yet metamorphosed when that happens.

This developmental shift is an example of phenotypic plasticity, which occurs

when an individual’s phenotype changes in response to the environment it experi-

ences. In the case of spadefoot tadpoles, the change is developmental and

irreversible. Plasticity can also be physiological and reversible, for example

the tanning that light-skinned people show after exposure to ultraviolet (UV)

light. Plasticity is seen in a wide range of traits that range from gene expres-

sion to morphology and physiology to behavior.

Phenotypic plasticity can be visualized with the reaction norm, which

is a plot showing how environmental conditions affect how a phenotype is

expressed. Reaction norms can differ among genotypes, which means that

reaction norms themselves can evolve. Genetic variation in a reaction norm is

referred to as genotype-environment interaction (or G×E, for short). FIGURE

6.25 shows that reaction norms for increased pigmentation in response to UV

light differ between populations of water fleas (Daphnia). These differences

are adaptive because in some populations survival is increased by plasticity

while in other populations it is not.

Not all phenotypic plasticity is adaptive [20]. When people who live at

sea level ascend to high elevations, physiological changes are triggered by

Futuyma Kirkpatrick Evolution, 4e

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Reference

gene order

Rearrangement

BP1

Rearrangement

BP2

silvana bicoloratus tarapotensis

FIGURE 6.23 A wing-color polymorphism in the butterfly Heliconius numata is

controlled by a small segment of chromosome. Genetic analysis shows that the

segment consists of two overlapping inversions that do not recombine. These

inversions carry loci with alleles that alter the pattern and coloration of the wings.

The different color morphs are favored in different parts of the species’ range

because they mimic other species of toxic butterflies that are common in those

regions. Top: Schematics of the chromosomes showing the changes in gene order

produced by the inversions. Bottom: The wing-color patterns produced by the

different chromosomes. (After [29].)

FIGURE 6.24 A carnivorous tadpole cannibalizes a typical tadpole of the spade-

foot toad Spea bombifrons. This is a dramatic example of phenotypic plastic-

ity: tadpoles of this species develop into either typical or carnivorous morphs

depending on environmental conditions.

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