How to Quantify Urbanization When Testing for Urban Evolution?

Research in urban evolution requires that the features of cities are accurately captured for input into evolutionary models. Until recently, the evolutionary effects of cities have often been addressed using single sites, dichotomous urban–rural contrasts or, to a lesser extent, using urban gradients. However, urbanization does not produce a homogenous spatial continuum: cities are highly heterogeneous environments, with sharp and often non-linear environmental changes related to the amount of impervious surface, green vegetation, air pollution, light, noise, or contrasted temperature profiles. The comprehensive quantification of urban heterogeneity in space and time is essential for exploring the origins of organismal variation and adaptation in cities, and to best identify the strength and directionality of selective pressures and neutral processes occurring in populations of urban organisms. This chapter reviews frameworks that can be used to describe and quantify urbanization—these include classical ecological frameworks, the understudied temporal dimension of urban evolutionary biology, and the concept of replicated insight into urban-driven evolutionary processes. The chapter further discusses how axes of variation capturing the urban environment can be quantified with univariate and multivariate approaches, and presents quantitative results on how urbanization is captured in published studies of urban evolution. Finally, it discusses study design and statistical approaches of interest when testing for urban evolution: these include the question of model selection and variable fitting, spatial autocorrelation, and appropriate scale use in studies of urban evolution.