Watson, Richard A. “Compositional evolution: interdisciplinary investigations in evolvability, modularity, and symbiosis.” PhD diss., Brandeis University, 2002.
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Conventionally, evolution by natural selection is almost inseparable from the notion of accumulating successive slight variations. Although it has been suggested that symbiotic mechanisms that combine together existing entities provide an alternative to gradual, or ‘accretive’, evolutionary change, there has been disagreement about what impact these mechanisms have on our understanding of evolutionary processes. Meanwhile, in artificial evolution methods used in computer science, it has been suggested that the composition of genetic material under sexual recombination may provide adaptation that is not available under mutational variation, but there has been considerable difficulty in demonstrating this formally. Thus far, it has been unclear what types of systems, if any, can be evolved by such ‘compositional’ mechanisms that cannot be evolved by accretive mechanisms. This dissertation takes an interdisciplinary approach to this question by building abstract computational simulations of accretive and compositional mechanisms. We identify a class of complex systems possessing ‘modular interdependency’, incorporating highly epistatic but modular substructure. This class typifies characteristics that are pathological for accretive evolution - the corresponding fitness landscape is highly rugged, has many local optima creating broad fitness saddles, and includes ‘irreducibly complex’ adaptations that cannot be reached by a succession of gradually changing proto-systems. Nonetheless, we provide simulations to show that this class of system is easily evolvable under sexual recombination or a mechanism of ‘symbiotic encapsulation’. Our simulations and analytic results help us to understand the fundamental differences in the adaptive capacities of these mechanisms, and the conditions under which they provide an adaptive advantage. These models exemplify how certain kinds of complex systems, considered unevolvable under normal accretive change, are, in principle, easily evolvable under compositional evolution.