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Evolutionary dynamics from deterministic microscopic ecological processes: A toy model for evolutionary processes

The central goal of a dynamical theory of evolution is to abstract the mean evolutionary trajectory in the trait space by considering ecological processes at the level of the individual. In this work, we develop such a theory for a new class of deterministic individual based models describing individual births and deaths, which captures the essential features of standard stochastic individual-based models and become identical with the latter under maximal competition. The key motivation is to derive the canonical equation of adaptive dynamics from this microscopic ecological model, which can be regarded as a "toy model" for evolution, in a simple way and give it an intuitive geometric interpretation. Another goal is to study evolution and sympatric speciation under "maximal" competition. We show that these models, in the deterministic limit of adaptive dynamics, lead to the same equations that describe the unraveling of the mean evolutionary trajectory as those obtained from the standard stochastic models. We further study conditions under which these models lead to evolutionary branching and find them to be similar with those obtained from the standard stochastic models. We find that though deterministic models result in a strong competition that leads to a speed up in the temporal dynamics of a population cloud in the phenotypic space as well as an increase in the rate of generation of biodiversity, it does not seem to result in an absolute increase in biodiversity as far as total number of species are concerned. Hence, the "toy model" essentially captures all the features of the standard stochastic model. Interestingly, the notion of a fitness function does not explicitly enter in our derivation of the canonical equation, thereby advocating a mechanistic view of evolution.