A Multi-level Selection Theory of Evolutionary Transitions in Individuality

The organization of the living world is hierarchical—lower level units group together and cooperate to form higher level units of organization (genes, chromosomes, bacterialike cells, eukaryotic-like cells (cells in cells), multicellular organisms and societies). How this came about is far from clear. What is clear is that the major landmarks in the diversification of life have involved transitions between these hierarchical levels and that cooperation has been a dominant theme. Throughout this paper, we illustrate our approach to evolutionary transitions using the transition from single cells to multicellular organisms. This transition has occurred multiple times in all major groups including bacteria, fungi, protists, plants and animals. Although we focus on the case of multicellularity, we believe the issues we discuss apply to the other major transitions including the origin of bacteria-like cells, eukaryotic-like cells (as discussed briefly at the end) and societies. To be able to adapt to their environment, evolutionary individuals must be units of selection, and satisfy Darwin’s conditions of heritability and variation in fitness. Using a multi-level selection framework, it is possible to understand how Darwin’s properties emerge at a new level (for example, the multicellular organism) from the evolution of interactions in groups of lower level individuals (for example, cells). In this way, we may understand, in a general sense, the origin and maintenance of the biological hierarchy. The properties of fitness, heritability and variation are all interrelated, although for purposes of discussion we consider each factor separately.