Decomposing social environment effects on eco-evolutionary dynamics: from density regulation to frequency-dependent selection

The density and frequencies of interacting phenotypes create a type of environment which affects both phenotypic selection and population growth. Fluctuations in population density create temporal variation in population mean fitness, driving population dynamics, while fluctuations in phenotypic frequencies create variation in the relative fitness of phenotypes through frequency-dependent selection. Different modelling frameworks have been used to study these (social) environment effects and the eco-evolutionary dynamics produced by their interaction. However, the diversity and mathematical complexity of these models can represent an obstacle for empiricists aiming to study the social factors shaping the eco-evolutionary dynamics of natural populations. Here, we reformulate components of these models using generalized linear regression equations to provide a statistical decomposition of how different frequency- and density-dependent processes influence phenotypic selection, population growth, and the expected equilibrium density and mean phenotype of a population. We complement these results with individual-based simulations to illustrate how quantifying the different ways the social environment affects an individual's fitness can improve our understanding of the feedback that links the evolutionary dynamics of phenotypes with the carrying capacity of natural populations.