Using natural laboratories to study evolution to global warming: contrasting altitudinal, latitudinal, and urbanization gradients.

Demonstrating the likelihood of evolution in response to global warming is important, yet challenging. We discuss how three spatial thermal gradients (latitudinal, altitudinal, and urbanization) can be used as natural laboratories to inform about the gradual thermal evolution of populations by applying a space-for-time substitution (SFTS) approach. We compare thermal variables and confounding non-thermal abiotic variables, methodological approaches and evolutionary aspects associated with each type of gradient. On the basis of an overview of recent insect studies, we show that a key assumption of SFTS, local thermal adaptation along these gradients, is often but not always met, requiring explicit validation. To increase realism when applying SFTS, we highlight the importance of integrating daily temperature fluctuations, multiple stressors and multiple interacting species. Finally, comparative studies, especially across gradient types, are important to provide more robust inferences of evolution under gradual global warming. Integrating these research directions will further strengthen the still underused, yet powerful SFTS approach to infer gradual evolution under global warming.