positive, negative, or both) and the chromatin structure in which such factors operate (Hershberg and Margalit, 2006).
Hence, the tinkering/borrowing process at the molecular level is somewhat more channeled, thus restricted, than the metaphor of bricolage might suggest. In addition, however, it has become equally clear over the past decade that the recruitment process is often not a gene at a time but a functional grouping of genes, a network “module” (Davidson, 2006). This is most obviously relevant in the cases where a preexisting signal transduction pathway becomes recruited, via an enabling mutation, to a new use. But it almost certainly also involves certain functional groupings of TFs. The Six-Dach-eya functional ensemble of TFs was first identified as a key component in the fruit fly eye development network (reviewed in Kumar, 2001) but later found in muscle development (Heanue et al., 1999) and then in still other developmental processes (Li 2003). In principle, the initial mutational event may elicit only a single activity, but that single recruited gene then induces or activates other members of the network modules. The distinction is that between “primary” and “secondary” recruitment (Wilkins, 2002). The induction of eye development in nonstandard sites by ectopic expression of Pax6 in Drosophila (Halder et al., 1995) is currently one of the best pieces of evidence that network module recruitment can take place in this way. The crucial point is that the recruitment of modules is made possible by the prior evolutionary-selective history that constructed and optimized performance of the network module.
The relevance of these points to organismal evolution is that it is the particular combination of network modules used that determines the composition of the entire genetic network governing a trait (Fig. 4.1) (Davidson, 2006; Wilkins, 2007b). Each module, however, is not, in itself, a rigidly delimited gene set, in either evolution or development, particularly in its downstream (“output”) target genes. The set of target genes is almost certainly evolutionarily labile (Davidson, 2006) while it seems inevitable that the overall molecular machinery, with its plethora of regulatory devices (from alternative splicing to a host of posttranslational modifications), will affect the particular degree of activity of various members of the output target gene set differentially in different cellular contexts (within and between organisms).
The network module, however, is only one kind of modular unit in the genetic and regulatory machinery that influences the outcomes of genetic recruitment events. The fact that most complex proteins are made up of distinct domains, with different kinds of domains shared between members of the same or even distantly related gene families (Alberts et al., 1989), constitutes a further layer of modular complexity and one that influences the behavior of those gene products that have this structure.