The Questions of Developmental Biology

Phenotypic Plasticity: Polyphenism and Reaction Norms
The ability of an individual to express one phenotype under one set of circumstances and
another phenotype under another set is called phenotypic plasticity. There are two main types of
phenotypic plasticity: polyphenism and reaction norms. Polyphenism refers to discontinuous
("either/or") phenotypes elicited by the environment. Migratory locusts, for instance, exist in two
mutually exclusive forms: a short-winged, uniformly colored solitary phase and a long-winged,
brightly colored gregarious phase. Cues in the environment (mainly population density)
determine which morphology a young locust will take (Figure 21.6; see Pener 1991). Similarly,
the nymphs of planthoppers can develop in two ways, depending on their environment.
High population densities and the presence of certain plant communities lead to the production of
migratory insects, in which the third thoracic segment produces a large hindwing. Low population
densities and other food plants lead to the development of flightless planthoppers, with the third
thoracic segment developing into a haltere-like vestigial wing (Raatikainen 1967; Denno et al.
1985). The seasonal coat color changes in arctic animals are another example of polyphenism.
In other cases, the genome encodes a range of potential phenotypes, and the environment
selects the phenotype that is usually the most adaptive. For instance, constant and intense labor
can make our muscles grow larger; but there is a genetically defined limit to how much
hypertrophy is possible. Similarly, the microhabitat of a young salamander can cause its color to
change (again, within genetically defined limits). This continuous range of phenotypes expressed
by a single genotype across a range of environmental conditions is called the reaction norm
(Woltereck 1909; Schmalhausen 1949; Stearns et al. 1991). The reaction norm is thus a property
of the genome and can also be selected. Different genotypes are expected to differ in the direction
and amount of plasticity that they are able to express (Gotthard and Nylin 1995; Via et al. 1995).
Seasonal polyphenism in butterflies
Two dramatic examples of polyphenism were shown in Chapter 3. The two phenotypes
of the butterfly Araschnia levana are so different that Linnaeus classified them as two different
species (see Figure 3.3) and the phenotype of the moth Nemoria arizonaria depends on its diet
(Figure 3.4). This type of polyphenism is not uncommon among insects. Throughout much of the
Northern Hemisphere, one can see a polyphenism in the Pieris and Colias butterflies (the cabbage
whites and sulphurs) between those that eclose (emerge from their pupal case) during the long
days of summer and those that eclose at the beginning of the season, in the short, cooler days of
spring.