The Physiological Regulation of Simultaneously Selected Traits

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Almost all biological traits have the potential to evolve in response to selection. Some traits may not respond to selection as might be expected, however, because they are negatively correlated with other traits. Such phenomena generate the tradeoffs that are the basis of life history evolution. Typically in studies of life history evolution, one focal trait is allowed to experience selection, and correlations between it and other traits are used to estimate the tradeoffs. However, selection tends to act on multiple traits simultaneously, necessitating studies of simultaneous selection on multiple traits rather than on correlations with one focal trait. Few studies have examined how the physiological or developmental mechanisms that underlie life history traits respond to selection on the focal traits and no study to date has examined the physiological regulation of simultaneous selection on multiple life history traits. This study will address these issues with studies of the well-known tobacco hornworm, Manduca sexta (Lepidoptera: Sphingidae). This is one of very few species in which the physiological mechanisms regulating two important life history traits, body size and development time, are known, providing essential background for this study. A series of six directional selection experiments will be run for ten generations. These will include all four possible combinations of simultaneous selection for increased and decreased development time and body size as well as selection to increase body size and selection to decrease development time. The three physiological factors that determine these traits (critical weight, PTTH delay time, and growth rate) will be measured before and after selection. This will be the first study to examine how the regulating physiological factors, constrain or engender, the evolution of multiple life history traits in response to simultaneous selection.