194 D. M. Walsh
The phenotype first model needs an account how the mechanisms of develop-
ment generate adaptive phenotypic novelties. Recent research into developmental
biology is beginning to reveal these very mechanisms. The plasticity of organ-
isms is a consequence of the modular architecture of development. Developmental
modules are internally integrated, mutually dissociated, units of developmental
control.
In principle, a modular system allows the generation of diverse phe-
notypes by the rearrangement of its internal module connections and
relatively independent evolution of each module... [Kitano, 2004, 830]The internal structure of a module typically consists of a series of integrated reg-
ulatory feedback relations. This confers on a module both robustness — the capac-
ity to function reliably across a range of perturbations ([von Dassow and Munro,
1999]; [Kitano, 2004]) — and flexibility, the capacity to mount novel adaptive
changes to perturbations ([Greenspan, 2002]; [Bergman and Siegal, 2002]). The
dissociation of modules — the fact that each influences only a few others — has
two distinct kinds of effects. The first is that a module is isolated from poten-
tially deleterious changes in other parts of the developing organism. The second
is that the function of a module can be stably varied, according to its context.
The repertoire of a developmental module, the range of its capacities taken in
isolation, is considerably greater than the range of its realized effects [Greenspan,
2002]. Typically each module is capable of producing any number of a large array
of stable outputs [Von Dassowet al., 2000]. Which of its capacities is manifested
on a particular occasion is determined by the context in which the module finds
itself. Taken together, these properties of modular architecture confer on an or-
ganism the capacity robustly and reliably to produce a viable organism typical of
its kind,andto generate phenotypic novelties as an adaptive response to genetic,
epigenetic or environmental perturbations. These twin capacities are crucial to
adaptive evolution.
Kirschner and Gerhard [2005] propose that the process of adaptive evolution
proceeds through the ‘facilitated variation’. An organism’s development is built
on the foundation of a suite of highly robust ‘conserved core processes’. These
are fixed in their function and are significantly immune to perturbation. These
core conserved processes ‘deconstrain’ other component processes of development.
That is to say, they underwrite the capacity of other component processes of devel-
opment to produce adaptive changes.Hoxgene clusters, for example, are highly
constrained core processes. They orchestrate the development of an enormously
wide range of morphological structures. The sameHoxclusters regulate the devel-
opment of vastly different structures in different organisms. And they initiate the
development of widely varying structures within the same organism. The duplica-
tion ofHoxclusters, their change of timing, or context can bring about remarkable
phenotypic changes. Carrollet al.[2002] conjecture that most of the large scale
changes in metazoan morphology have been caused by changes in the regulatory
roles of core conserved elements likeHoxclusters.