Transformationalism, taxism, and developmental biology in systematics

Issues concerning transformational and taxic comparisons are central to und erstanding the impact of the recent proliferation of molecular developmenta l data on evolutionary biology. More importantly an understanding of taxism and transformationalism in comparative biology is critical to assessing th e impact of the recent developmental data on systematic theory and practice . We examine the philosophical and practical aspects of the transformationa l approach and the relevance of this approach to recent molecular-based dev elopmental data. We also examine the theoretical basis of the taxic approac h to molecular developmental data and suggest that developmental data are p erfectly amenable to the taxic approach. Two recent examples from the molec ular developmental biology literature-the evolution of insect wings and the evolution of dorsal ventral inversion in vertebrates and invertebrates-are used to compare the taxic and transformational approaches. We conclude tha t the transformational approach is entirely appropriate for ontogenetic stu dies and furthermore can serve as an excellent source of hypotheses about t he evolution of characters. However, the taxic approach is the ultimate arb iter of these hypotheses.