“…The model (1.8)-(1.11) can be viewed as a reactive nonlinear equation of Fokker-Planck type, in the spirit of [21], with conservation of mass. Reaction-diffusion problems with conservation of mass were studied in [41,26,44,45,1,25,17], see also the references therein. On the other hand, after a change of variables, our problem fits into the framework of fitness-driven models of population dynamics, and might be applicable to some human societies.…”