The concept of evolutionary adaptation to environmental stresses will be clarified by studying natural genetic variation in the context of "bioenergetics", that is the flow and use of energy through and by living systems. Techniques will be employed from molecular biology and physiological ecology, as well as more traditional areas of population biology. An insect study system, on which much progress has already been made, will be used as a "model system" for examination of these processes. This work will yield understanding of how natural variation interacts with identifiable environmental stress patterns and other selection pressures, and thus of how energy flow through all living beings is shaped, over time, by natural environmental processes. This will allow test of, and stimulate extension of, new analytic theory of physiological organization now being developed. This work is thus of importance not only to fundamental understanding in population biology, but to a wide variety of applied-biological areas. It has already begun to have impact on the genetic aspects of conservation biology, and is starting to show major potential in genetic aspects of agriculture and preventive medicine.