Artículo de publicación ISIWhen we sent Alan Berryman a first draft of this essay for review, he sent us back an unpublished manuscript that discussed similar questions and possible solutions (Berryman 1999, 2002). Consequently, we decided to submit these two essays together in the hope of stimulating ecologists to think about how they define and use the population concept. The population concept is central to ecology and evolution. The definition of a population represents a fundamental problem for understanding ecological dynamics, and it has wide implications for applied issues such as management and conservation. Further, the population concept merges evolution and ecology, because interactions between conspecific individuals, as well as between species (enemies, mutualists, etc.) and the environment are reflected in survival and reproduction rates. These rates determine population dynamics and thus represent the basic elements of natural selection and microevolutionary forces (Futuyma 1986). Therefore, populations are faced with evolutionary and ecological forces, both operating through demographic processes (Caswell 1989, Futuyma 1986). Consequently, the way ecologists define populations may change drastically the theoretical framework used to understand nature, leading to divergent views on the dynamics of ecological systems, such as the non-equilibrium versus equilibrium paradigms (Berryman 1987)