EvoluTion And dEvEloPmEnT 393
plants, for instance, the form of a leaf depends on whether it develops below,
above, or on the surface of water (FIGURE 15.27B). Plastic changes in phenotype
result from changes in gene regulation. Environmental stimuli alter levels of
hormones or other signaling molecules that affect levels of transcription factors,
which in turn alter the expression of downstream genes [23].
For many characteristics, the most adaptive norm of reaction is a constant
phenotype, buffered against alteration by the environment (see Figure 15.26C).
It may be advantageous, for example, for an animal to attain a fixed body size
at maturity or metamorphosis, despite variations in nutrition or temperature
that affect the rate of growth. The developmental system underlying the char-
acter may then evolve so that it resists environmental influences on the pheno-
type [66]. This idea was developed independently by the Russian biologist Ivan
Schmalhausen [69] and by British developmental biologist Conrad Waddington,
who referred to it as canalization [16]. Waddington used the concept of canaliza-
tion to interpret some curious experimental results [81]. A crossvein in the wing
of Drosophila sometimes fails to develop if the fly is subjected to heat shock in
the pupal stage. By selecting and propagating flies that developed a crossvein-
less condition in response to heat shock, Waddington bred a population in which
most individuals were crossveinless when treated with heat. But after further
selection, a considerable portion of the population lacked the crossvein even
without heat shock, and this condition was heritable. A character state that ini-
tially developed in response to the environment had become genetically deter-
mined, a phenomenon that Waddington called genetic assimilation.
Although this result is reminiscent of the discredited theory of inheritance of
acquired characteristics, it has a simple genetic interpretation. Genotypes of flies
differ in their susceptibility to the influence of the environment (in this case,
temperature)—that is, they differ in their degree of canalization, so that some
are more easily deflected into an aberrant developmental pattern. Selection for
this developmental pattern favors alleles that channel development into the
newly favored pathway. As such alleles accumulate, less environmental stimulus
is required to produce the new phenotype. The finding that genetic assimilation
does not occur in inbred populations that lack genetic variation supports this
interpretation [66]. We encountered an example of genetic assimilation in natu-
ral populations in Chapter 6 (see Figure 6.25). In high-elevation lakes, exposed
to high levels of ultraviolet radiation (UV), the water flea Daphnia melanica can
develop a range of low to high pigmentation, as a protection against UV. But
dark color makes the Daphnia more visible to fish, and two populations in lakes
with recently introduced trout show constitutive (unchanging) expression of the
low-melanin (light) phenotype. The expression of two genes that affect melanin
production is less affected by UV in these populations than in ancestral popula-
tions. An environmentally inducible expression has become genetically deter-
mined [70].
It is often difficult to tell whether or not constancy of a feature has evolved
by natural selection for canalization, because it can also be an automatic effect
of complex genetic and developmental pathways [72]. Researchers distinguish
environmental canalization, which reduces the effect of environmental variation
on the phenotype, from genetic canalization, which reduces the effect of genetic
mutations. Stabilizing selection on a characteristic is likely to cause environmen-
tal canalization. For example, the floral structures of some animal-pollinated
plants, which are thought to be strongly selected for successful pollination,
are less variable than leaves are [55]. Genetic canalization is much less likely
to evolve, because selection against deleterious mutations is so effective that
few individuals deviate from the optimum, so there is little selection for genetic
modifiers that prevent the mutations from being phenotypically expressed [28].15_EVOL4E_CH15.indd 393 3/22/17 1:30 PM