The evolution of oscillatory behavior in age-structured species. 
A major challenge in ecology is to explain why so many species show oscillatory population dynamics and why the oscillations commonly occur with particular periods.  The background environment, through noise or seasonality, is one possible driver of these oscillations, as are the components of the trophic web with which the species interacts.  However, the oscillation may also be intrinsic, generated by density-dependent effects on the life history.  Models of structured single-species systems indicate that a much broader range of oscillatory behavior than that seen in nature is theoretically possible.  We test the hypothesis that it is selection that acts to constrain the range of periods.  We analyze a nonlinear single-species matrix model with density dependence affecting reproduction and with trade-offs between reproduction and survival.  We show that the evolutionarily stable state is oscillatory and has a period roughly twice the time to maturation, in line with observed patterns of periodicity.  The robustness of this result to variations in trade-off function and density dependence is tested.