The Questions of Developmental Biology

The population genetics model contained some major assumptions that have now been
called into question.
1. Gradualism. The supposition that all evolutionary changes occur gradually was debated by
Darwin and his friends. Thomas Huxley, for instance, accepted evolution, but he felt that Darwin
had burdened his theory with an unnecessary assumption of gradualism. A century later, Eldredge
and Gould (1972), Stanley (1979), and others postulated punctuated equilibrium as an
alternative to the gradualism that characterized the Modern Synthesis. According to this theory,
species were characterized by their morphological stability. Evolutionary changes tended to be
rapid, not gradual. At the same time, molecular studies (King and Wilson 1975) showed that 99%
of the DNA of humans and chimpanzees was identical, demonstrating that a small change in
DNA could cause large and important morphological changes. New findings in paleontology and
molecular biology prompted scientists to consider seriously the view that mutations in regulatory
genes can create large changes in morphology in a relatively short time.
2. Extrapolation of microevolution to macroevolution. The idea that accumulations of small
mutations result in changes leading to new species has also been criticized. Richard Goldschmidt
(1940) began his book The Material Basis of Evolution by asking the population genetic
evolutionary biologists to try to explain the evolution of the following features by accumulation
and selection of small mutations: hair in mammals; feathers in birds; segmentation in arthropods
and vertebrates; the transformation of the gill arches into structures including aortic arches,
muscles, and nerves; teeth; shells of molluscs; compound eyes; and the poison apparatus of
snakes. Interestingly, both Goldschmidt and Waddington saw homeotic mutations as the kind of
genetic change that could alter one structure into another and possibly create new structures or
new combinations of structures. These mutations would not be in the structural genes, but in the
regulatory genes. Few scientists paid attention to Goldschmidt or Waddington, however, because
they were not working under the population genetics paradigm of the Modern Synthesis and
because their scientific programs were suspect. (Goldschmidt did not believe in Morgan's notion
of the gene as a particulate entity, and Waddington's work was misinterpreted as supporting the
inheritance of acquired traits: see Gilbert 1988; 1991; Dietrich 1995.)
3. Specificity of phenotype from genotype. Developmental biologists have found that life is
more complicated than a 1:1 relationship between genotype and phenotype. Chapter 21
documents numerous cases wherein the genotype can permit any of several phenotypes to form.
These cases include polyphenisms induced by predators, diet, day length, or antigenic or visual
experience. Moreover, development always mediates between genotype and phenotype.
The same gene can produce different phenotypes depending on the other genes that are present
(Wolf 1995). A mutant gene that produces limblessness in one generation can produce only a
mild thumb abnormality in the next (Freire-Maia 1975). That evolution is the result of heritable
changes in development (Goldschmidt 1940) is as true for whether a fly has two or three bristles
on its back as for whether an appendage is to become a fin or a limb. One way of visualizing this
is to use a mathematical analogy (Gilbert et al. 1996):
Functional biology = anatomy, physiology, cell biology, gene expression
Developmental biology = [functional biology]/t
Evalutionary biology = /[developmental biology]/t