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types, complex patterns of gene expression, and mechanisms of cell signaling), a long-term synergism may exist between nonadaptive evolution at the DNA level and adaptive evolution at the phenotypic level. There is no need to abandon the idea that many of the external morphological and/or behavioral manifestations of multicellularity in today’s organisms are adaptive. However, if the view promoted above is correct, the relatively simple phenotypes of the Earth’s smallest organisms is not an inevitable outcome of the prokaryotic body plan nor a reflection of selection for metabolic efficiency, but an indirect consequence of the barrier to the passive emergence of genomic complexity imposed by high Ng.

In summary, the near-complete absence of the concept of nonadaptive processes from the lexicon of those concerned with cellular and developmental evolution does not reflect any formal demonstration of the negligible contribution of such mechanisms, and indeed, there is no fundamental reason why development should be uniquely immune to non-adaptive evolutionary forces. One could even argue that the stringency of natural selection is reduced in complex organisms with behavioral and/or growth-form flexibilities that allow individuals to match their phenotypic capabilities to the local environment. Some of these shortcomings have recently attracted attention, and a scaffold for connecting evolutionary genetics, genomics, and developmental biology is slowly beginning to emerge (Johnson and Porter, 2000, 2001; Stern, 2000; True and Haag, 2001; Delattre and Felix, 2002; Rockman and Wray, 2002; Wray et al., 2003; Force et al., 2005).

THE PASSIVE EMERGENCE OF MODULAR GENE INTERACTIONS

King and Wilson (1975) inspired the view that modularity and repatterning of regulatory-element utilization are the central determinants of the evolution of organismal complexity (Carroll, 2005a,b; Davidson, 2006). Although this view is not universally accepted even among developmental biologists (Alonso and Wilkins, 2005), because development always involves cross-talk between gene products, one must start with a consideration of the origins of the mechanisms that allow such transactions to take place. There is no evidence that gene-regulatory modules associated with complex functions arise as de novo integrated units, although some biologists seem to feel otherwise (Davidson and Erwin, 2006). Rather, like all aspects of evolution, the origins of changes in genetic pathways must be a function of descent with modification. Mutant alleles arise independently at individual loci, with features being defined by prior historical contingencies. Thus, although the idea that regulatory modules with functional significance in today’s organisms can only have arisen via natural



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