In this paper Brown and Wilson defined character displacement as follows: "Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the populations of that species are similar to the other species and may even be very difficult to distinguish from it. In the area of overlap, where the two species occur together, the populations are more divergent and easily distinguished, i.e., they 'displace' one another in one or more characters. The characters involved can be morphological, ecological, behavioral, or physiological; they are assumed to be genetically based."

MacArthur and Wilson showed that the species richness of an area could be predicted in terms of such factors as habitat area, immigration rate and extinction rate.

Integration of biological and evolutionary theory with the study of social behavior and social organization of animal populations.