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appear to be sufficient to explain complexity, at least as it relates to the problem of the major transitions between the different kinds of evolutionary individuals.

The theme of this article, which could well be the theme of this Colloquium, is that evolutionary biology can explain complexity. I will consider the problem of explaining the “major transitions” between the different kinds of evolutionary individuals that make up the familiar hierarchy of life: genes, bacteria-like cells, cells-in-cells (eukaryotic cells), multicellular organisms, and societies (Maynard Smith and Szathmáry, 1995). Evolutionary individuals are integrated and indivisible wholes with the property of heritable variation in fitness so that they may evolve adaptations at their level of organization. Being wholes, evolutionary individuals may be thought to be irreducibly complex, but this has not been the case during evolutionary history; a series of stages, each advantageous in itself, may be shown to exist allowing evolution to get from one kind of individual to another. The evolutionary concepts we use to understand evolutionary transitions in individuality involve fitness and its reorganization, fitness tradeoffs (especially the cost of reproduction to survival) and their roles in life-history evolution, and kin selection and altruism and their roles in social evolution. We focus on the transition from unicellular to multicellular life, but the points made apply more generally to the other transitions (Michod, 1999).

Our understanding of life is being transformed by the realization that evolution occurs not only through the standard processes operating within populations, but also during evolutionary transitions in individuality, when groups of individuals become so integrated that they evolve into new higher-level individuals. Indeed, the major landmarks in the diversification of life and the hierarchical organization of the living world are consequences of a series of evolutionary transitions: from genes to gene networks to the first cell, from prokaryotic to eukaryotic cells, from cells to multicellular organisms, from asexual to sexual populations, and from solitary to social organisms. Such transitions require the reorganization of fitness, by which we mean the transfer of fitness from the old lower-level individual to the new higher level, and the specialization of lower-level units in fitness components of the new higher-level individual. It is a major challenge to understand why (environmental selective pressures) and how (underlying genetics, population structure, physiology, and development) the basic features of an evolutionary individual, such as fitness heritability, indivisibility, and evolvability, shift their reference from the old level to the new level.

The evolution of multicellular organisms is the premier example of the integration of lower-level individuals (cells) into a new higher-level



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