THE ROLES OF DEVELOPMENT, ECOLOGY AND ENVIRONMENT IN EVOLUTIONARY INNOVATION: THE CAMBRIAN RADIATION

Evolutionary radiations can be driven by new ecological opportunities, changes in the physical environment or by new genetic and/or developmental possibilities. Each element of this triad has been invoked to explain the magnitude of the Neoproterozoic-Cambrian metazoan radiation, but with no consensus on the relative significance of each element. Developmental innovation has received considerable attention, driven in part by the recognition of the extensive conservation of a variety of transcription factors and signaling molecules between Drosophila and vertebrates. The conserved elements include the well-known Hox and Pax-6 systems and a much broader array of genes. At the limit, these genes suggest a protostome-deuterostome ancestor (PDA) was a morphologically complex animal possessing eyes, segmentation, appendages, heart, complex nervous system, respiratory structures, and the complex morphogenetic pathways needed to produce them. In 2003 Eric Davidson and I suggested these highly conserved regulatory sequences are better understood as elements of a developmental toolbox associated with vectoral patterning, cell type specification and other more general developmental tasks, but in many cases without morphogenetic patterning. This view of the PDA suggests it may have been a much simpler animal than the extreme developmental position. If so, such animals would be unlikely to be preserved as trace or body fossils, and the PDA could substantially pre-date 555 Ma. The existence of a PDA with a diverse developmental toolkit, but evidently without many of the complex morphogenetic pathways apparent in Cambrian organisms suggests: 1) many of these morphogenetic pathways are clade-specific, as suggested by some developmental data; 2) that since the PDA predates 555 Ma, developmental innovation was likely not a significant factor in the breadth and rapidity of the Metazoan radiation; and 3) that ecological interactions played a key role.