The concept of symmorphosis postulates rigorous congruence between structural design and functional demand in physiological systems. We consider here the evolutionary limitations of symmorphosis and, more generally, model of optimal design in physiology. Such models presume that evolutionary forces of selection are well-defined and invariant. Because selective pressures vary in space and time, the level of performance required from physiological systems is not necessarily constant and uniquely specified. Morphological structures frequently perform multiple roles, and optimal design for one role may reduce the effectiveness of other functions. Developmental and genetic constraints may also preclude optimal matching of physiological design with functional requirements. The complexity and interdependence of most morphological structures and physiological processes suggest that perfect rate matching of all constituent components is unlikely. Similar allometry of different components of a physiological cascade does not necessarily imply design according to symmorphosis, which specifies quantitative equality of maximum capacity at different levels of structural organization. Although the notion of optimal design has considerable utility as a null hypothesis, the extent of matching between form and function in organisms is best described as a continuum of imperfection.
