results in the prevention of the development of abdominal legs
(Ronshaugen et al., 2002). Although transcription factors change
slowly, their effect on the development body plan is of equal
importance to that of cis-regulatory elements, since the variation of
transcription factors directly changes the topology of the gene
regulatory network. To test the theory of Davidson and Erwin, we
calculated the speed of evolution of regulatory genes. We found that
those genes which determine the phylum and superphylum characters
evolve slowly, while those genes which determine the classes,
families, and speciation evolve more rapidly. These observations
provide support for Davidson and Erwin's theory.