Cooperation and Conflict in the Evolution of Complexity

The basic problem in an evolutionary transition in complexity is to understand how a group of individuals becomes a new kind of individual, having heritable variation in fitness at the new level of organization. We see the formation of cooperative interactions among lower-level individuals as a necessary step in evolutionary transitions; only cooperation transfers fitness from lower levels (costs to group members) to higher levels (benefits to the group). As cooperation creates a new level of fitness, it creates the opportunity for conflict between the new level and the lower level. Fundamental to the emergence of a new higherlevel individual is the mediation of conflict among lower-level individuals in favor of the higher-level unit. We define a conflict mediator as a feature of the cell-group (the emerging multicellular organism) that restricts the opportunity for fitness variation at the lower level (cells) and/or enhances the variation in fitness at the higher level (the cell-group). There is abundant evidence that organisms are endowed with just such traits and numerous examples are reviewed here from the point of view of a population genetic model of conflict mediation. Our model considers the evolution of genetic modifiers that mediate conflict between the cell and the cell-group. These modifiers alter the parameters of development, or rules of formation, of cell-groups. By sculpting the fitness variation and opportunity for selection at the two levels, conflict modifiers create new functions at the organism level. An organism is more than a group of cooperating cells related by common descent and requires adaptations that regulate conflict within itself. Otherwise their individuality and continued evolvability is frustrated by the creation of withinorganism variation and conflict between levels of selection. Conflict leads to greater individuality and harmony for the organism, through the evolution of adaptations that reduce it. INTRODUCTION Life is organized hierarchically and the major transitions in biological complexity have involved transitions between the levels in the biological hierarchy: genes, gene networks, chromosomes, bacteria-like cells, eukaryotic cells with organelles (cells within cells), multicellular organisms, and social organisms (Buss, 1987; Maynard Smith, 1988; Maynard Smith & Vida, 1990; Maynard Smith, 1991; Maynard Smith & Szathmáry, 1995; Michod, 1999). These transitions in the basic units of life, what we term evolutionary individuals, share two common themes: (i) the emergence of cooperation among the lower level units in the functioning of the new higher level unit and (ii) regulation of conflict among the lower level units. Evolutionary individuals are units of selection, and must satisfy Darwin's principles of heritability and variation in fitness, which may apply at different levels in the hierarchy of life (Lewontin, 1970). Because of the hierarchical nature of selection we take a multi-level selection approach to the origin of multicellularity and to evolutionary transitions generally. The multilevel selection approach to evolutionary transitions seeks to understand how a group of pre-existing individuals becomes a new evolutionary individual, possessing heritable fitness variation at the group level and protected from within group change by conflict mediators. We focus here on the unicellular multicellular transition, but the principles we describe are applicable to all the major evolutionary transitions in the hierarchy of life. The origin of multicellular organisms from unicellulars is the premier example of the integration of lower level units into a new higher level. This transition has occurred multiple times in all the major groups of life. The transition to a new higher-level individual is driven by cooperation among lower-level individuals. Only cooperation trades fitness from the lower level (its costs) to the higher level (the benefits of cooperation for the group) (Table 1). Because cooperation exports fitness from lower to higher levels, cooperation is central to the emergence of new evolutionary individuals and the ____________________________ Copyright © 2002, American Association for Artificial Intelligence (www.aaai.org). All rights reserved. From: AAAI Technical Report SS-03-02. Compilation copyright © 2003, AAAI (www.aaai.org). All rights reserved.