Since the time of the last major theoretical integration in evolutionary biology, the Modern Synthesis of the 1930s and 1940s, the biosciences have made significant advances, and today’s evolutionary biology operates with numerous concepts that were not part of the original synthesis. Since the retrospective subsumption of these novel concepts under the well defined Synthesis framework is impossible, propositions for a new and extended evolutionary synthesis are on the rise. One characteristic feature of such revised models is the shift from the population dynamic emphasis, favored by the traditional approach, towards a causal-mechanistic explanation of phenotypic complexity. The integration, for instance, of organizing relations between genes, cells, and tissues in development, as well as the interactions of these processes with physiological and environmental factors, entails a revised understanding of the evolutionary role of natural selection. It will be argued that not only the kind and number of theory elements have increased, but their inclusion also modifies the formal structure of evolutionary theory and expands its explanatory capacity.