Development: Three Grades of Ontogenetic Involvement 195Kirschner and Gerhart concur:Most evolutionary change in the metazoa since the Cambrian has come
not from changes of the core processes themselves, but from regulatory
changes affecting the deployment of the core processes.... Because of
these regulatory changes, the core processes are used in new combina-
tions and amounts at new times and places. [Kirschner and Gerhart,
2005, 221–222]West-Eberhard summarizes the view succinctly. “A very large body of evidence... shows that phenotypic novelty is largely reorganizational rather than a product
of innovative genes” [2005b, 6547]. Because phenotypic novelties are the result of
an organism’s capacity to make adaptive responses to change, the novelties are not
random. They tend to be adaptive: “Variation... is both less lethal and more
appropriate to selective conditions than would be variation from random change.
Evolutionary change is thereby facilitated” [Kirschner and Gerhard, 2005, 221].
There are two important points to be made about the contribution of pheno-
typic plasticity to adaptive evolution: (i) phenotypic plasticity generates pheno-
typic novelty, and (ii) the phenotypic novelty it produces is not adaptively neutral.
We must recognize, then, that development has a central place amongst the causes
of adaptive evolution. It initiates adaptive novelties; it secures the conditions for
adaptive change in populations; it supplies adaptive novelties within a population;
and it provides a means for making the development of adaptive novelties robust
and routine. On this view, it is the plasticity of development that makes adap-
tive evolution adaptive. “This analysis brings development, largely omitted from
evolutionary biology during the synthesis era..., to the forefront of evolutionary
biology as the source of the variation that fuels natural selection and adaptive
evolution.” [West Eberhard, 2005b, 6544]
This is what I call the ‘Third Grade of Ontogenetic Involvement’. This model of
adaptive evolution confers a role on development that neither Grade I nor Grade
II involvement does. In grades I and II, development plays no role in making
evolution adaptive. The sole adaptation promoting mechanism is natural selection.
Grade III makes no distinction between the organism-level processes of inheritance
and development and the population-level causes of adaptation. The Grade III
maxim:evolution is adaptivebecause ontogeny isadaptive.
In fact, Grade III involvement, emphasizing as it does the importance of pro-
cesses internal to organisms, suggests a particular ‘ontogeny first’ metaphysics of
evolution. Organisms develop and reproduce. The plasticity of development se-
cures the resemblance of offspring to parents — inheritance — despite the (internal
and external) vagaries of the world. So inheritance is the simple consequence of
the plasticity of development plus reproduction. The plasticity of development
also generates novel adaptive phenotypes. Through reproduction, developmen-
tal processes that produce these novel phenotypes, are assorted and recombined.
This lowers the threshold for the occurrence of these adaptive novelties within in-
dividuals and causes their spread throughout the population. So ontogeny causes