Transformationalism, taxism, and developmental biology in systematics.

Issues concerning transformational and taxic comparisons are central to understanding the impact of the recent proliferation of molecular developmental data on evolutionary biology. More importantly, an understanding of taxism and transformationalism in comparative biology is critical to assessing the impact of the recent developmental data on systematic theory and practice. We examine the philosophical and practical aspects of the transformational approach and the relevance of this approach to recent molecular-based developmental data. We also examine the theoretical basis of the taxic approach to molecular developmental data and suggest that developmental data are perfectly amenable to the taxic approach. Two recent examples from the molecular 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 that the transformational approach is entirely appropriate for ontogenetic studies and furthermore can serve as an excellent source of hypotheses about the evolution of characters. However, the taxic approach is the ultimate arbiter of these hypotheses.