An open question in animal evolution is why the phylum- and superphylum-level body plans have changed
so little, while the class- and family-level body plans have changed so greatly since the early Cambrian.
Davidson and Erwin (Davidson and Erwin, 2006; Erwin and Davidson, 2009) proposed that the hierarchical
structure of gene regulatory networks leads to different observed evolutionary rates for terminal properties
of the body plan versus major aspects of body plan morphology. Here, we calculated the speed of evolution
of genes in these gene regulatory networks. We found that the genes which determine the phylum and
superphylum characters evolve slowly, while those genes which determine the classes, families, and
speciation evolve more rapidly. This result furnishes genetic support to the hypothesis that the hierarchical
structure of developmental regulatory networks provides an organizing structure which guides the evolution
of aspects of the body plan.