The Role of Adaptive Mechanisms in the Origin of Higher Levels of Organization

One of the most controversial questions in the study of major evolutionary changes is whether or not all macroevolutionary steps are fully adaptive and completely understandable in terms of the known principles of microevolutionary change. The answer appears to lie not in the search for new evolutionary mechanisms, but in the correct description and analysis of the events involved in the origin of new higher levels of organization, as has been stressed by several workers who are dissatisfied with the synthetic theory of evolution. Evolutionary adaptation is the relationship of a species to a specific set of environmental conditions; it connotes both the process and the state of being. An adaptation is a formfunction complex (= faculty) that has two components: it has a definite biological role that interacts with a selection force; and it has a relative degree of efficiency. The degree of adaptation, as a state of being, is defined as the ability of the feature to execute successfully its biological role with the minimal expenditure of energy. As a process, adaptation is defined as any evolutionary change that reduces this amount of energy. Preadaptation occurs when a feature, by virtue of its present form and functions, acquires a new biological role and thus gains a new adaptation. Key innovations are those adaptations focal to the exploitation of a new habitat. The model suggested for the sequence of events in the origin of new groups is an evolutionary change that: (a) involves a series of many steps, each representing an intermediate stage between the ancestral and descendent groups; (b) occurs in an intermediate adaptive zone; and (c) involves a number of phyletic lines. Each step involves preadaptations, key innovations and a postadaptational period with an adaptive radiation of forms. This radiation, in turn, may result in a new preadaptation leading to another step, and so on. If the evolving group in the transitional zone reaches a new major adaptive zone, a larger adaptive radiation may occur, marking the primary flowering of the new group. However, no single point can be cited as the origin of the group. The series of modifications involved in the evolution of a new higher level of organization is exactly the same as the continuous evolutionary change within the old adaptive zone, except for the steady change in niche relationships in the transitional zone. The adaptive mechanisms involved are identical for both series of changes; a search for new mechanisms is not needed.