the consequent adaptive benefit in predator-free environments. If, however, one inspects the earlier part of stickleback evolutionary history and inquires about the initial gain of the pelvic armature in the stickleback lineage, one is immediately confronted with the question of how the genetic network that specifies the pelvic armature evolved. One hypothetical scenario is given in Fig. 4.3. Although the details of this scheme may well prove wrong and the picture is, at best, highly schematic, it illustrates the fact that this evolutionary change must have been a multistep process of network construction, involving several (possibly many) mutations and, presumably, either multiple or continuing selection pressures (from predation).

Although three traits in three organisms create far too small a sample from which to draw firm general conclusions, one can offer a few tentative generalizations. For new or modified evolutionary traits (e.g., finch beaks, bat wings), knowing the genetic network, with its component modules, is essential for understanding in depth what the phenotypic change actually involves. Single-gene stories, however informative in themselves, cannot provide comparable understanding. For loss of traits, however, as in the stickleback example, knowing the genetic networks responsible for those traits may be unnecessary. It is sufficient to know which genes

FIGURE 4.3 A tentative evolutionary scenario for the evolution of the genetic network underlying the pelvic armature of sticklebacks. The essential general feature is that it involves a stepwise (gradualistic mode) process of construction of the network. A particular aspect to note is that Pitx1 is seen as having been recruited at an intermediary step, after evolution of the basic network/module structure for constructing skeletal elements, permitting it to have well defined regional control effects and its loss of expression to affect only that region.