Life history evolution is fundamental to how evolution shapes the ecology of organisms. In turn competition between different life-history traits within the same species, and the interaction of their competition with environmental variability, has a major effect on evolutionary outcomes. Our goal is to unify the theory of life-history evolution and the theory of diversity maintenance in variable environments. We modify a framework for species interactions and community dynamics in fluctuating environments to study life-history evolution. The original framework expresses the long-term population growth rate of different species as a function of fluctuating environmental and competitive factors. Partitioned to distinct coexistence mechanisms, it allows for their identification and quantification. Community dynamics is then studied by standard invasibility analysis, asserting that species coexist if each of them has a positive long-term population growth rate at low density. This approach is applied to the new framework straightforwardly: instead of distinct species, we consider different life history strategies within the same species. As variations of a life history trait continually arise in the population, directional selection determined by environmental and competitive factors will lead to replacement of, or coexistence with existing types.
Results/Conclusions
The new framework is demonstrated by analyzing a specific model of desert annuals, with marked within-species variation of germination behavior with respect to environmental variability. A distinct feature of this model is that only within-year variation in water availability is allowed, in order to gain deeper understanding to the role of different trade offs on the relationship between coexistence mechanisms and the evolutionary outcomes. In the new framework we evaluate the properties of evolutionarily singular points by analyzing the higher order derivatives of the partitioned fitness function. Fitness maxima represent evolutionarily stable strategies, whereas fitness minima correspond to evolutionary branching points leading to diversification. In our particular example the coexistence mechanisms are fluctuation independent, mean fitness differences and the storage effect, which depends on within-year fluctuations of resource availability. We identify and quantify these mechanisms and classify the evolutionary outcomes depending on the types of trade offs functions and the strength of environmental variation. Our results indicate that convex trade offs and multiple resource pulses facilitate diversification. Moreover, we follow how the strength of equalizing and stabilizing coexistence mechanisms changes during directional selection in monomorphic and polymorphic populations. Our expectation is that the community average storage effect increases with diversification.
