The Questions of Developmental Biology

22. Developmental mechanisms of evolutionary change
When Wilhelm Roux announced the creation of experimental embryology in 1894, he
broke many of the ties that linked embryology to evolutionary biology. According to Roux,
embryology was to leave the seashore and forest and go into the laboratory. However, he
promised that embryology would someday return to evolutionary biology, bringing with it new
knowledge of how animals were generated and how evolutionary changes might occur. He stated
that "an ontogenetic and a phylogenetic developmental mechanics are to be perfected." Roux
thought that research into the developmental mechanics of individual embryos (the ontogenetic
branch) would proceed faster than the phylogenetic (evolutionary) branch, but he predicted that
"in consequence of the intimate causal connections between the two, many of the conclusions
drawn from the investigation of individual development [would] throw light on the phylogenetic
processes." A century later, we are now at the point of fulfilling Roux's prophecy and returning
developmental biology to questions of evolution. This return is producing a new model of
evolution that integrates both developmental genetics and population genetics.
The fundamental principle of this new evolutionary synthesis is that evolution is caused
by heritable changes in the development of organisms. This view can be traced back to Darwin,
and it is compatible with and complementary to the view of evolution based on population
genetics that evolution is caused by changes in gene frequency between generations. The merging
of the developmental genetic approach to evolution ("evo-devo") with the population genetic
approach is creating a more complete evolutionary biology that is beginning to explain the origin
of both species and higher taxa (Raff 1996; Gerhart and Kirschner 1999;Hall 1999).
"Unity of Type" and "Conditions of Existence"
Charles Darwin's synthesis
In the 1800s, debates over the origin of species pitted against each other two ways of
looking at nature. One view (championed by Georges Cuvier and Charles Bell) focused on the
differences among species that allowed each species to adapt to its environment. Thus, the fingers
of the human hand, the flipper of the seal, and the wings of birds and bats were seen as marvelous
contrivances, fashioned by the Creator, to allow these animals to adapt to their "conditions of
existence." The other view, championed by Geoffroy St. Hilaire and Richard Owen, was that
these adaptations were secondary, and that the "unity of type" (what Owen called "homologies")
was critical. The human hand, the seal's flipper, and the wings of bats and birds are each
modifications of the same basic plan (see Figure 1.13). In discovering that basic plan, one can
find the form upon which the Creator designed these animals. The adaptations were secondary.
Darwin acknowledged his debt to these earlier debates when he wrote in 1859,
"It is generally acknowledged that all organic beings have been formed on two great laws Unity
of Type, and Conditions of Existence." Darwin went on to explain that his theory would explain
unity of type by descent from a common ancestor. Moreover, the changes creating the marvelous
adaptations to the conditions of existence were explained by natural selection. Darwin called this
concept "descent with modification." As mentioned in Chapter 1, Darwin found that homologies
between the embryonic and larval structures of different phyla provided excellent evidence for
descent with modification. He also argued that adaptations that depart from the "type" and allow
an organism to survive in its particular environment develop late in the embryo. Thus, Darwin
recognized two ways of looking at "descent with modification."