Modules, kinds, and homology

Developmental modules are best conceptualized as homeostatic property cluster natural kinds. As is true in other fields of biology, an individual may instantiate properties of various natural kinds. Through their dissociability, developmental modules can be recruited to function as evolutionary modules. The proper analogy to developmental modules, atoms, or biological species depends on the scope over which specific developmental modules allow generalizations. The nature of the relationship between developmental modules, evolutionary modules, and taxic (phylogenetic) homology are explored. Similarity of gene expression patterns and developmental pathways as captured by biological homology may support hypotheses of taxic homology, but not the other way around. (C) 2005 Wiley-Liss, Inc.