1. Arnold, A. J., & Fristrup, K. (1982). The theory of evolution by natural selection: A hierarchical expansion. Paleobiology, 8, 113–129.
2. Axelrod, R. (1984). The evolution of cooperation. Basic Books.
3. Brandon, R. N. (1988). Levels of selection: A hierarchy of interactors. In H. C. Plotkin (Ed.), The role of behavior in evolution (pp. 51–71). MIT Press.
4. Brandon, R. N. (1990). Adaptation and environment. Princeton University Press.
5. Damuth, J. (1985). Selection among species: A formulation in terms of natural functional units. Evolution, 39, 1132–1146.