DAVIDOWITZ, G.*; ROFF, D.A.; NIJHOUT, H.F.; University of Arizona; University of California, Riverside; Duke University: The physiological regulation of simultaneously selected life history traits.

Integrating physiology and selection experiments can be a powerful tool when asking one of the central questions of evolutionary biology: why do traits respond to selection the way that they do? We present such an approach in a simultaneous selection experiment on body size and development time in the tobacco hornworm Manduca sexta. Three physiological factors control body size and development time in the tobacco hornworm: growth rate (GR), the critical weight (CW) that measures the timing of the onset of the cessation of juvenile hormone secretion (which initiates the processes leading to pupation), and the time interval between attainment of the critical weight and secretion of the molting hormone 20-hydroxyecdysteroid (the interval to cessation of growth, ICG). These three physiological factors explain over 95% of the variation in these two life history traits. Knowledge of the physiological mechanism allow us to make explicit predictions, based on physiological first principles, as to how these life history traits should respond to simultaneous selection. Selection lines in each of the four combinations of body size and development time differed significantly from the initial population and from the control after ten generations of selection. Initial results support our predictions and show that the response to selection of the BIG/SLOW line was mainly due to an increase in CW and ICG, of the SMALL/FAST line to a decrease in CW and ICG, of the BIG/FAST line to an increase in GR, and of the SMALL/SLOW line to a decrease in GR. These results demonstrate the power of a combined physiological and selection approach to the study of life history evolution.