Development: Three Grades of Ontogenetic Involvement 197of development for the process of adaptive evolution and (ii) the reunification of
evolution’s constituent processes. In Grade I involvement development plays no
role in either inheritance or in the production of the fit and diversity of organ-
isms. Adaptive evolution and the divergence of lineages are the results of random
mutation and transmission of inherited variants and their selection. This requires
the recognition of three discrete processes; inheritance of replicators, development
of organisms and adaptive change of populations, each with its proprietary causal
process. Grade II involvement draws development more centrally into the process
of evolution; the functioning of developmental systems — and not the replication of
genes — explains inheritance; inheritance is simply the reliable transgenerational
re-occurrence of developmental processes (systems). There is, then, no distinction
between the processes that underwrite inheritance and those that underwrite de-
velopment; they’re both ontogeny. In Grade II, development makes no positive
contribution to the adaptiveness and diversification of lineages. This is exclusively
the domain of natural selection. Grade III involvement preserves the unity of the
processes of inheritance and development from Grade II, but it casts ontogeny in
an even more central role. According to Grade III, the plasticity of organismal
development is the principal cause of adaptive evolution. Grade III involvement is
consistent with natural selection, of course. But selection — adaptive evolution —
is just a consequence of the development of individual organisms. The processes
of development, inheritance and adaptation are all fundamentally just ontogeny.
As far as I can tell, these are all coherent positions. They are seldom, if ever,
articulated in quite this way, but I think it’s fair to say that, implicitly or explicitly,
each has its adherents. Nor do these positions seem to be empirically equivalent;
each has observational consequence that the others don’t. Which of these positions
is the most plausible, then, looks like an empirical issue. It is one that I believe
developmental biology only now has the resources to address.
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