AbstractIt is presumed that the evolution of morphological diversity in animals and plants is driven by changes in the developmental processes that govern morphology, hence basically by changes in the function and/or expression of a defined set of genes that control these processes. A large body of evidence has suggested that changes in developmental gene regulation are the predominant mechanisms that sustain morphological evolution, being much more important than the evolution of the primary sequences and functions of proteins. Recent reports1, 2 challenge this idea by highlighting functional evolution of Hox proteins during the evolutionary history of arthropods. BioEssays 24:775–779, 2002. © 2002 Wiley Periodicals, Inc.