Development: Three Grades of Ontogenetic Involvement 181view, not of any great theoretical significance [Bowler, 1988]. But here again is a
theory in which the development of traits and their inheritance are consequences
of the same processes of ontogeny. These theories founder, if they do, on empirical
grounds and not because they conflate the processes of inheritance, development
and adaptation.
But why should the modern synthesis require the fractionation of evolution?
The modern synthesis theory, as usually conceived, stands on three conceptual
pillars: Darwinism, Mendelism and Weissmanism. This foundation, I believe, not
only fragments the constituent processes of evolution into three discrete processes,
it also abets the marginalization of development.
2.1 Darwinism
Darwin may not have been the first to realize that biology’s two great explananda
— the fit of organisms to the conditions of their existence and the diversity of
organic form — are the joint consequences of evolution. His great achievement was
the discovery of what sort of process evolution is: it is apopulation-levelprocess.
Darwin taught us to ask not how organisms change in ways that bring about their
adaptedness, but rather to ask howpopulationscome to comprise organisms so
wonderfully adapted. Darwin’s answer, of course, is natural selection. Natural
selection occurs in a population that exhibits heritable variation in fitness. Not
just any variation in fitness will do; the variants must be familial (i.e. passed on
from parent to offspring). Offspring must resemble their parents with respect to
the traits that contribute to differential survival and reproduction if these are to
spread throughout the population in subsequent generations.
While it requires heritable variation in fitness, Darwin’s process makes very
few demands on how variants are generated, or the mechanisms that secure their
reliable recurrence from one generation to the next. True, certain processes of in-
heritance better promote the capacity of selection to drive change in a population.
Selection destroys the variation it requires and a process of inheritance that tends
to preserve variants is more propitious for the continuation of selection. But any
mechanism that underwrites the resemblance of offspring to parents will suffice to
some degree. Nor does selection constrain the nature of development in any par-
ticularly strong way. Any process by which inherited traits develop reliably will
promote selection.^1 So there is nothing about the conceptual structure of natural
selection theory that demands the fractionation of evolution into three discrete
processes of inheritance, development an adaptation. After all, as is well known,
Darwin was happy to endorse the unity of inheritance and development found in
Lamarck. Still, under Darwinism development and inheritance are mostly isolated
from the process ofadaptation, as they make no significant contribution to the
adaptivenessof variants. It is selection that causes adaptation.
(^1) Of course, some mechanisms of development will be more conducive to the capacity of selec-
tion to promoteadaptiveevolution. More on this below.