Internalized Evolutionary Inertia on Body Plan Evolution

Despite the phenotypic diversity of animal species, the basic anatomical features, or body plan, of each animal phylum have been strictly conserved since their initial establishment in the early Cambrian. While this remarkable conservation could be explained by the conservation of the mid-embryonic phase (the developmental hourglass model) when the body plan is established, the underlying evolutionary mechanisms remained largely unclear. In this respect, recent studies have highlighted intrinsic properties in development, such as robustness, stability, and pleiotropic constraints, as potential contributors to its limitation of phenotypic diversifications. These findings suggest a potential mechanism of how phenotypic evolution is intrinsically limited or biased. In this review, potential developmental factors that contributed to the intrinsic limiting effects of animal embryogenesis against phenotypic diversification will be overviewed, with a particular focus on the general relationship between evolution and developmental processes.