Functional morphology of the vertebrate visual system an essay on the evolution of complex systems

Abstract

The existence of complex organ systems has always been a serious challenge to the synthetic theory of evolution. This essay considers two aspects of the problem in the case of the vertebrate visual system.The first is how to resolve the visual system into structural components which can be related to evolutionary processes. I use the visual system of the red-eared turtle Pseudemys scripta elegans to illustrate how a neural system can be decomposed into three nested levels of organization. The first level consists of individual neurons. Analysis at this level considers the types of neurons which are present and the interactions which occur between them. The second level consists of groups of neurons called modules. They form a unit structure which is repeated throughout some particular region of the system. The third level consists of a network which is composed of aggregations of neurons (nuclei) which are interrelated by bundles of axons. Several projections converge upon modules in many of the nuclei.The second aspect is the extent to which the vertebrate visuil system cin be accommodated into the synthetic theory. Analysis of genetic abnormalities provides evidence that some aspects of visual system structure are under genetic control. Analysis of variations in visual system structure indicates that interspecific variations occur frequently, particularly at the lower organizational levels. Serious difficulties arise however in attempts to establish causal relationships between visual system structure and the organisms behavioral adaptations. These difficulties stem from applications of the neurological concept of the localization of function.The available evidence is then, consistent with the hypothesis that the visual system evolved in accordance with the synthetic theory. It remains however, to establish more clearly the rehtionship between visual system structure and environmental demands. © 1980 by the American Society of Zoologists.