Biochemical polymorphisms: The unit of selection and the hypothesis of conditional neutrality

This paper has examined the dynamic aspects of the fitness of genetic variants in soluble proteins in the context of the unit of selection. The concept of conditional neutrality has been incorporated into this framework of the unit of selection to include those mechanisms which result in compensation for alterations in the physicochemical properties of an allozyme. Possible compensatory mechanisms have been suggested for single proteins, multiple enzyme systems and biochemical pathways. It is suggested that these homeostatic mechanisms have evolved as a result of selective pressures operating at the level of the selective unit. Following these considerations it is suggested that the molecular evolution of proteins can be viewed in a manner analogous to Wright's shifting balance theory of evolution, in that allozymes could enter into and oscillate between both neutral and selective phases. It is demonstrated that differences in the physicochemical properties of allozymes are a precondition for molecular evolution by selective processes. However, the possibility of selection acting upon allozymes depends upon the integration of the properties of individual allozymes with the homeostatic properties of the physiological processes of which the allozyme is a part. It is the development of such homeostatic aspects of biochemical systems that may result in a hierarchy of "selective units" which are important factors in ascertaining the role of allozymes in molecular evolution.