394 CHAPTER 15Does phenotypic plasticity contribute to evolution?
In the modern theory of adaptive evolution by natural selection among inherited
phenotypes, reaction norms evolve by selection of mutations (in, perhaps, regu-
latory elements or hormone levels) that differentially affect the expression of the
trait in different environments. Mary Jane West-Eberhard [86] and some other
biologists have proposed a somewhat different process. They suggest that a new
environment can often induce the prevalent genotype to express a novel beneficial
phenotype, by phenotypic plasticity. The expression of this phenotype, they sug-
gest, can subsequently become genetically fixed (that is, genetically assimilated)
by natural selection [34, 48, 68]. The feature may later be further modified by
mutation and natural selection, which West-Eberhard has called “genetic accom-
modation.” According to this hypothesis, phenotypic plasticity paves the way for
standard evolution by selection of advantageous mutations that affect the feature.
West-Eberhard points out many examples of closely related species pairs in which
the reaction norm of one species includes a phenotype that is invariant in the
other species [86]. For example, larvae of the sphinx moth Manduca quinquemacu-
lata develop black pigmentation at low temperatures and green pigmentation at
higher temperatures, whereas larvae in the related species M. sexta develop green
pigmentation at all temperatures [76]. The still unanswered question is whether or
not the green species evolved by genetic assimilation of part of a broader range of
colors—a more plastic ancestral reaction norm.
A possible example of evolution by genetic accommodation has been described
in tadpoles of spadefoot toads in the genus Spea. Like most anuran larvae, these
tadpoles generally feed on detritus, but they often switch to feeding on shrimp and
other animal prey (see Figure 6.24) [56]. This shift involves phenotypic plasticity:
the tadpoles develop a shorter gut and larger jaw muscles. Tadpoles of Scaphiopus,
the sister genus of Spea, normally feed only on the ancestral diet, detritus. But
Scaphiopus tadpoles raised on shrimp were found to develop more slowly and have
shorter guts than those that were fed detritus [35]. This experiment suggests that
the phenotype that is adaptive in one genus (Spea) could have been produced by a
plastic response that was already present, even if not used, in the common ances-
tor of Spea and Scaphiopus.
The hypothesis that plasticity is often the first step toward new adaptations is
controversial. When a population experiences a new environment, an environ-
mentally induced alteration of the phenotype may or may not be in the right direc-
tion (FIGURE 15.28) [18]. In the spadefoot toad experiment, shrimp-fed Scaphiopus
tadpoles developed smaller jaw muscles, not the enlarged muscles seen in Spea tad-
poles that are adapted to eating animal prey. A stressful environment may change
development in ways that are not adaptive. For example, low temperatures reduceFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_15.28.ai Date 02-08-2017Environment 1Mean tnessEnvironment 2A
BCFIGURE 15.28 Phenotypic plasticity may enhance genetic adaptation to a novel environ-
ment, or it may not. Lines A, B, and C are three possible reaction norms of a character,
expressing the pattern of phenotypic plasticity of three genotypes. The population has
occupied Environment 1 for a long time, and has the optimal mean character for that
environment (shown by star). If the environment changes to state 2, and the population
is mostly composed of genotype A, its phenotypic plasticity will produce the pheno-
type that is optimal for the new environment (shown by triangle). But if genotype B is the
prevalent genotype, its plasticity would only slightly enhance its fitness in Environment- If genotype C is prevalent, its phenotypic plasticity would be maladaptive in the new
environment, making its fitness lower than if the genotype’s phenotype were fixed.
15_EVOL4E_CH15.indd 394 3/22/17 1:30 PM