International audience There is little doubt that a better understanding of biological pheno mena taking place within natural populations must integrate the genetic and ecological approaches. In this essay, intended primarily for ecolo gists, I first reviewed the most usual models in population ecology based on the logistic equation and on the Lotka-Volterra models describing the dynamics, either of a single species population, or of a two-species system (competition or predator-prey), or else a multi-species system at equilibrium, that is when N = K. Then I mentioned some models used in population genetics to describe changes in allele frequencies and to define darwinian fitness. That way the origin of parameters that are fundamental to population ecology (r and K) and to population genetics (W) was reviewed. A fusion of these two disciplines is possible through the introduction of the concepts of r- and /(-selection. The latter selection mode is especially interesting because it permits one to develop models of density dependent selection, in which fitness is defined by ecological parameters (mostly K) that have different properties depending upon the genotype carrying them. In that view I cited several models of density dependent selection and discussed some of their actual or potential applications to important problems in population biology. So, topics apparently as diverse as isozyme polymorphism, random effects on colonizing populations, ecological energetics, and cyclic fluctuations may be examined both from a genetic and an ecological framework, thus demonstrating the validity of the suggestion made at the beginning of this paragraph. These first attempts at integration indicate the direction of the current that carries along the evolution of our ecological science. The vigorous renewal being operated at present started chiefly from theore tical considerations, and this must make it necessary for those who still consider theoretical models to be futile exercises when compared to gathering field data, to re-think their approach. As Levins (1966, 1968) has shown, there is no more dichotomy between theory and empiricism than between genetics and ecology when the goal of research is to explain how ecological systems work. It is of interest to note in passing that, when taken separately, genetics and ecology already possess an advanced theoretical structure, when compared with some other biological sciences. All one needs to do to be convinced of this is to consult recent texts : in genetics those of Wright (1968, 1969), Ewens (1969), Crow and Kimura (1970), and Kimura and Ohta (1971) ; in ecology, those of D’Ancona (1954), Bartlett (1960), Cohen (1967), Margalef (1968), and Pielou (1969). Yet integration is made difficult by the scarcity of common concepts, by the apparent difference between short term and long term kinds of evolution, by the great heterogeneity of both genetic and ecological systems, and by the technical obstacles to elaborating mathematical models. I chose to present here density dependent selection in some detail rather than to dilute the analysis by a more complete review including other developments, such as the concept of fitness set of Levins (1962, 1968), the evolution of life cycles (see Istock, 1967, 1970 ; Lewontin, 1965 ; Gadgil and Bessert, 1970), or mimicry (see Wickler, 1968 ; Pasteur, 1972 ; Williamson and Nelson, 1972). This choice was made because it seems to me that the selective mode called density dependent is the hinge between population ecology and population genetics. The models I reviewed, or others that will surely be invented in the future, will cer tainly oblige us to re-examine many mechanisms that are basic to the organization of communities, especially the evolution of territorial beha vior (Schoener, 1971, 1973), symbiosis (Roughgarden, 1971 b), the evolution of niche width (Roughgarden, 1972), and the significance of predation (S. A. Levin, 1972). Future work will have to combine increasingly the theoretical and empirical approaches of field experimentation. As these two sorts of talents are rarely found in a single individual, it is obvious that population biology will have to be done by researchers of diverse orientations acting as a team.