Evolution and plasticity of physiological traits in the collembolan Orchesella villosa at fine spatial scales within the city

Divergent natural selection caused by spatial environmental variation can lead to local adaptation and evolutionary divergence between populations, even those within close proximity to one another. This, however, is only one possibility among other outcomes, such as the evolution of adaptive phenotypic plasticity or local maladaptation. Cities, as fragmented landscapes with high environmental variability across microgeographic scales, present an excellent venue to explore these possible outcomes. Here, we use common garden experiments to explore the potential for microgeographic divergence of physiological traits among populations of a collembolan, Orchesella villosa, living within the footprint of a single city. We assessed if intra-urban variation in the intensity of the heat island effect and soil salinity has led to evolutionary divergence in heat tolerance and salinity tolerance, respectively. While we found little variation in salinity tolerance among populations, there was evidence for both plasticity and evolutionary divergence among populations in response to variation in the urban heat island. Although the adaptive nature of these specific responses is not known, we suggest that cities, and human modified habitats in general, promote evolutionary divergence at small spatial scales by creating barriers to dispersal and imposing divergent selective environments within previously contiguous habitats.