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9Evolutionary Transitions in Individuality: Multicellularity and Sex Richard E. Michod Reorganization of fitness during Evolutionary Transitions in Individuality (ETIs) Levels of Selection and Levels of Complexity The challenge of explaining evolutionary transitions (ETs) was initially posed by Maynard Smith in two papers (1988, 1991) and later in a more systematic and comprehensive way in his book with Szathmáry (1995). The list of “levels of complexity” first offered by Maynard Smith (1988, table 2) focused on the levels of selection: replicating molecules, replicators in compartments, prokaryotic cells, eukaryotic cells, multicellular organisms, demes and social groups, species, and groups with cultural inheritance. Maynard Smith noted that it was debatable whether the last three were levels of selection; indeed, the level of species was left out of his list in the later paper and sex was added as an ET in the book with Szathm áry. Maynard Smith used the levels of selection as the defining framework of evolutionary transitions between different levels of complexity. The problem posed by Maynard Smith in both papers (1988, 1991) was “How did natural selection bring about the transition from one stage to another, since at each transition, selection for ‘selfishness’ between entities would tend to counteract the change . . . how is it that selection at the lower level does not disrupt integration at the higher level?” (1988, 222–223). It is this question that my colleagues and I have been trying to answer, using a combination of mathematical models and a model experimental system, the volvocine green alga. Transfer of fitness and ETIs I have referred to Maynard Smith’s original list as involving evolutionary transitions in individuality (ETIs; see Michod 1999). Let us first distinguish between ETIs and ETs in the sense of major events in the history of life. There are a number of immensely important events without which life as we know it would be vastly different, including such major events as the origin of the genetic code, language, oxidative photosynthesis, and the Cambrian Explosion. Understanding these events is critical for the field of evolutionary biology and for understanding life on earth. These events are not, however, ETIs. ETIs comprise 170 Richard E. Michod a common set of problems and solutions involving levels of selection and the integration of evolutionary units. Samir Okasha, at the KLI workshop, suggested ETIs constitute a natural kind, a natural grouping of phenomena involving common problems and sharing common solutions. In addition to the familiar levels of selection, the list of ETs offered by Maynard Smith and Szathmáry (1995, 6) include, sex, the genetic code, and language; language was included because of its importance to cultural evolution and inheritance, and sex because members of a sexual population cannot reproduce without a mate. For many workers, sex and language are the odd members of the list. I argue here that sex belongs on the list, because the mating pair is the new level of selection, a new kind of individual as far as fitness is concerned. I am not so sure about the genetic code and language; I won’t say anything more about them. What are the essential properties of an ETI in the original levels-of-selection sense of Maynard Smith? During a transition from a lower to a higher level, we expect the fitness of the higher level to increase and the fitness of the lower level to decrease. Fitness may be defined as expected reproductive success at either the cell or group level. This expectation is illustrated in the panels in figure 9.1, which give the output of two completely different kinds of mathematical models of the origin of multicellular individuals from groups of cells. Panel A gives the results of a two-locus population genetic modifier model of the evolution of conflict mediators. Conflict mediators are genetically encoded properties that reduce the opportunity for within-group selection and/or enhance the opportunity for between-group selection. Panel B gives the results of an optimization model of the evolution of division of labor among cells when selection is at the cell-group level. These two models are vastly different, and are intended to study different aspects of ETIs’ yet both models illustrate an ETI—the decrease in fitness at the lower (cell) level and the increase in heritable fitness of the new higher (multicellular group) level. Cooperation among cells is central to evolutionary transitions in individuality and to the results shown in figure 9.1. Cooperation may be costly, as in the case...

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