In nature, selection often fluctuates, but observed changes in traits may owe not only to selection but plasticity. Further, changes in traits due to selection and to plasticity do not have equal demographic effects: the cost of selection is paid through declines in population number or growth rate. Thus, observed trajectories of trait value and population number can depend on interactions between plasticity, evolution and environment. The extent to which these interactions affect observed trajectories depends on the demographic context. For example, it has long been known that natural selection interacts with density-dependence, which can dilute the effect of selection on population dynamics. Although recent methods decompose observed trait change into contributions due to evolution and plasticity, these methods have not yet fully accounted for the effect of selection on demography. In particular, including information about demographic change may sharpen inference of selection versus plasticity from time series.
I develop a framework to account for the joint effects of plastic and evolutionary change on population and trait dynamics. This framework focuses on the influence of selection by a seasonal environment on timing of life history events, accommodating mechanistic hypotheses concerning the form of selection. I demonstrate how the framework can be used to infer the action of plasticity and genetic evolution from time series in a case study of changes in migration timing of Pacific salmon (Oncorhynchus nerka).