The Systems Approach: An Interface Between Development and Population Genetic Aspects of Evolution

The systems approach is an attempt to clarify the implications of morphological concepts such as homology and the natural system within the framework of the Modern Synthesis. The concept of homology implies the existence of invariant features of characters, most probably caused by developmental and functional constraints. The evolutionary history of homologues shows that the key events in transspecific evolution are changes in the system of developmental constraints: constraints caused by primitive developmental programs were overcome, leading to a higher flexibility of some characters, and new constraints acquired leading to the fixation of body design at a higher level of organization. Quantitative genetic theory shows that evolution of functionally coupled characters is highly dependent on an appropriate allocation of variance and thus depends on an appropriate pattern of developmental constraints. It is concluded that organismic evolution should be studied at two levels: a) the level of character evolution, and b) the level of the evolutionary modification of constraints.