The Evolution of Biological Individuality by Social Niche Construction

The biological world as we see it today has a part-whole hierarchical structure. For example, eusocial societies are made up of many organisms, multicellular organisms are made up of many cells, those cells contain numerous organelles and so on. This hierarchical organisation is thought to have evolved over a long period of time in a series of events known as ‘evolutionary transitions in individuality’. Evolutionary transitions present an interesting challenge for evolutionary theory because they involve changes in the hierarchical level at which the evolutionary process itself acts. This thesis is intended as a contribution to theoretical work aiming to explainsuch transitions in the hierarchical structure of life.Evolutionary transitions are extreme cases of the evolution of cooperation. Social evolution theory is the part of evolutionary theory that tries to explain the evolution of cooperation. It typically takes an externalist explanatory stance, explaining cooperative behaviour in terms of external factors (e.g. genetic relatedness) that make cooperation sustainable. In this thesis, I move from an externalist to an interactionist explanatory stance, in the spirit of Lewontin andthe niche construction theorists. I develop the theory of social niche construction, which has it that biological entities are both the subject and object of their own social evolution. That is, the niche in which social behaviour occurs is not entirely externally defined but is partly modified by the organisms in it. Then, cooperation and the social niche modifier traits supporting it can each evolve as evolutionary responses to the other. This claim is supported by detailed argumentand by simulation modelling.Some important social niche modifiers enabling cooperation (e.g. life-history bottlenecks) havethe side-effect of raising the hierarchical level at which the evolutionary process acts. This isbecause modifier traits acting to align the fitness interests of lower-level units (e.g. cells) in a collective also diminish the extent to which those units are bearers of heritable fitness variance, while augmenting the extent to which collectives of such units (e.g. multicellular organisms) are bearers of heritable fitness variance. So while there is no selection-for evolutionary transitions in individuality, there is selection-of the sufficient conditions for transitions to occur. My explanation for evolutionary transitions is couched only in terms of evolutionary self-interest ofthe lower-level units, so avoiding many of the problems that befall alternative accounts