The Questions of Developmental Biology

By this transfer of induction from an external inducer to an internal inducer, a trait that
had been induced by the environment became part of the genome of the organism and could be
selected. Waddington called this phenomenon "genetic assimilation," while Schmalhausen (1949)
called it "stabilizing selection." Both scientists had used orthodox embryology and orthodox
genetics to explain phenomena that had been considered cases of Lamarckian "inheritance of
acquired characteristics."
A shift from environmental stimulus to genetic stimulus might explain sex determination
in Menidia and caste determination in ants. Similarly, the preexisting developmental plasticity of
arm length in feeding echinoderm larvae may have bridged the transition from pluteus (feeding)
larvae to larvae that lack ciliated arms. The change in the allocation of resources between larval
and juvenile structures parallels that seen where the food reserves are stored in the egg. Thus, the
changes already present as adaptations to external food resources may have become genetically
fixed in those species whose larvae do not need to hunt their food (Strathmann et al. 1992).
If genetic assimilation is the genetic fixation of one of the phenotypes that had been
adaptively expressed, then butterflies would be a good place to look for further examples.
Brakefield and colleagues (1996) showed that they could genetically fix the different morphs of
the adaptive polyphenism of Bicyclus, and Shapiro (1976) has shown that the short-day (coldweather)
adaptive phenotype of several butterflies is the same as the single genetically produced
phenotype of related species or subspecies living at higher altitudes or latitudes.
Genetic assimilation may play an important role in providing a bias for evolutionary
change. If an organism inherits a reaction norm, the developmental pathways leading to a
particular phenotype are already in place, and all that evolution need do is supply a constant
initiator of those pathways. In the next chapter, we will discuss some of the molecular evidence
for genetic assimilation.
*And until this century, writers were recognized by the calluses on their fingers. (Thus, from observing his fingers,
Sherlock Holmes correctly surmised that the red-headed man had been hired as a scrivener.)
Environmental Disruption of Normal Development
From what has been said so far in this chapter, it is clear that the instructions for
development do not reside wholly in the genes or even in the zygote. The developing organism is
often sensitive to cues from the environment. However, this sensitivity makes the organism
vulnerable to environmental changes that can disrupt development.
If you think it is amazing that any
one of us survives to be born, you are
correct.It is estimated that one-half to twothirds
of all human conceptions do not
develop successfully to term (Figure
21.20). Many of these embryos express
their abnormality so early that they fail to
implant in the uterus. Others implant but
fail to establish a successful pregnancy.