THE FIVE CAUSES OF MORPHOLOGICAL INNOVATION

Previously I have presented a framework for understanding how diversity and disparity flow from the interaction between developmental systems and the ecological and environmental needs of organisms. In short, morphogenetic rules define a morphospace. Each morphology in that morphospace can be assigned (in principle) a fitness based on how well the needs of the organism are met. Thus, the morphospace can be contoured based on the fitnesses, forming an adaptive landscape. Using this framework, I argue that five fundamental processes are responsible for all morphological innovation: 1) Addition of new morphogenetic rules (e.g., the origin of the bilaterian developmental system, or the ability of fish to make jaws). This increases the dimensionality of the adaptive landscape. 2) Changing the roughness of the landscape by increasing the number of needs each organism must meet. I believe the Cambrian explosion was driven by roughening, with the invention of adult-body/adult-body ecological interactions. 3) Increasing the size of the landscape, without increasing its dimensionality or roughness, by increasing the number of states the morphogenetic rules may take. The radiation of angiosperms is characterized by this mechanism. 4) Simply shifting the position of the peaks in the landscape without changing its dimensionality, roughness, or size. These shifts can be minor, (due to ecological changes) or major (e.g., with switches between the terrestrial and marine realms). 5) Re-population of a stable landscape after extinction. This produces iterative evolution. Macro-evolution occurs when there is a major change in the adaptive landscape, whether the change is an increase in the landscape’s dimensionality, roughness, size, or even its re-organization due to a major switch in environmental conditions. Micro-evolution simply refers to the mechanisms (recombination, mutation, etc.) that come into play in the exploration of any landscape, whether changing or stable. The fossil record suggests that type 1 and 2 changes are rare, and that new landscapes are successfully explored in about a geological Period. Type 3 change is typically driven by co-evolution, and can persist for 100 myr or more. Type 4 and 5 constitute background evolution and extinction, and the repopulation of the adaptive peaks is relatively fast.