hierarchy, then the nature of the gene tinkering process is seen to be much less haphazard than the process connoted by the term “bricolage,” having more built-in molecular constraints yet, at the same time, lacking the goal-directed nature of a process that is implied by the term “design.”
All of the above statements can be accepted, however, without embracing the idea that the network perspective is needed for experimental research in evolution. A case can be made that a sufficient understanding of the genetic basis of adaptive evolutionary changes emerges from classic quantitative trait locus and molecular single gene-based experimental perspectives and that neither the concept of networks nor detailed knowledge of particular networks is needed for actual progress. This position is seemingly bolstered by recent success in understanding the genetic foundations of several adaptive traits, work that has underlined the key importance of a small number of specific genes. These cases involve finch beak dimensions in Darwin’s finches, characteristics related directly to specific feeding adaptations (Abzhanov et al., 2004, 2006); the adaptive evolution of bat wings for flight (Sears et al., 2006; Weatherbee et al., 2006); and the adaptive loss of pelvic armature in freshwater sticklebacks (Shapiro et al., 2004; Colosimo et al., 2004).).
The adaptive radiation of Darwin’s finches, with their different kinds of beaks suitable for different feeding adaptations, is one of the classic instances of evolutionary divergence due to natural selection (Lack, 1947). Abzhanov et al. (2004) have identified two key gene activities that are associated, respectively, with beak depth and width, on one hand and beak length on the other. The first characteristic, beak depth and width, is correlated with and evidently determined by an elevated level of activity of bone morphogenetic protein 4 (BMP4) during a critical phase of beak development. In contrast, finch beak length is evidently linked to a specific elevation of calmodulin activity during beak development (Abzhanov et al., 2006).).
The analysis of the genetic basis of bat wing evolution bears some similarity. A key component in the evolution of the distinctive wings of bats is the elevation in activity during a key phase of forelimb development in the embryo of another TGF-β activity, also a member of the BMP family, BMP2, which promotes the selective growth of the metacarpals to extend the key digits (Sears et al., 2006). Making a batwing, however, involves more than just exaggerated digit length; it also involves suppression of the waves of apoptosis that eliminate interdigital material in tetrapods with distinct digits. In the case of bats, the maintenance of