Does evolution in body patterning genes drive morphological change - or vice versa?

Increased understanding of the regulation of body patterning genes in devel opment, especially the homeotic genes, has led to the revival of ideas sugg esting that "saltational" modes of evolution are important. However, such m odels are problematic on the grounds of functional continuity and populatio n genetics, and the more dramatic scenarios rely on an overinterpretation o f the taxonomic hierarchy. This article proposes an alternative model for t he evolution of Hox gene expression, stressing the need for incremental fun ctional integration. One surprising implication of the model would be that mutations in Hox genes and their regulators have virtually no primary role in driving morphological evolution. Rather, morphological change through mi croevolutionary adaptation comes first, with Hox expression shifting only a fterwards, presumably to make the building of the new body pattern more eff icient or more stable. Such a model has affinities to Waddington's "genetic assimilation" but invokes discrete rather than continuous shifts in contro l of a particular morphology. BioEssays 21:326-332, 1999. (C) 1999 John Wil ey a Sons, Inc.