genetic changes needed for phenotypic change, increase the number of targets for regulatory change, reduce lethality, and increase genetic variation retained in the population. Although the core processes are constrained in their own change of function, they deconstrain regulatory change.
Is this a testable hypothesis or merely a post hoc rationalization? To begin with, we should say that the theory emphasizes the targets of change and their consequences for phenotype, not the paths of change, although we especially like the plasticity-based paths because of what they say about targets and reuse of components. Basically, we accept any kind of regulatory change, arising by any path of genetic change, as long as it affects the combinations, amounts, states, times, and places of conserved core processes. Included would be the neo-Darwinian possibility of a rare, favorable, nonlethal, penetrant mutation that is selected to fixation of a new phenotype, and also included would be the Baldwin possibility of physiological adaptation at first without genetic change (in response to environmental change), followed by regulatory changes (via new allele combinations) enhancing and fixing a new phenotype. In both cases, genetic change results in regulatory change, which modifies the use of the conserved core processes.
The theory predicts that developmental biologists will continue to find (i) more examples of core processes used in diverse developmental and physiological traits in different combinations, amounts, and states, and (ii) in each new case a few small regulatory changes sufficing to redeploy core processes, which are themselves robust and adaptable. When introduced experimentally, such regulatory changes should significantly alter the phenotype, and other processes should accommodate to the directly altered ones, giving viable outcomes. Furthermore, it predicts that, as comparative experimental studies uncover the history of evolutionary innovation in animals, regulatory types of changes will predominate. Indeed, as is already clear, altered cis-regulation of gene expression and altered production of secreted signals lie behind specific phenotypic changes in stickleback fish and Drosophila (Sucena et al., 2003; Crickmore and Mann, 2006; Shapiro et al., 2006).
A recent example of bone morphogenetic protein (Bmp) and calmodulin signaling supports facilitated variation via robust adaptable processes. As described in the Introduction, Darwin noted the rapid divergence of beak morphologies by Galapagos finches. If we think mostly about selection and not phenotypic variation, we might imagine that selection acted repeatedly on many small changes occurring independently in the upper and lower beaks, adjacent skull, and head muscles to coordinate and order them into viable intermediate beaks throughout divergence. Many regulatory changes and many selections would be needed for this detailed coevolution of parts. Recent results, however, make a different