What is stress?

Figure 1. Adaptations and tradeoffs. The theory of allocation predicts that the evolution of adaptations to a particular environment necessitates a loss in fitness in other environments. This prediction has been explored by testing the running speed of lizards at various temperatures. Interestingly, while critical maximum temperatures are positively correlated with the animals’ normal temperature in the field, consistent with adaptation, lizards that are optimized for performance at high temperature do not necessarily have compromised function at low temperatures, at odds with allocation theory [2]. (Image: Wikipedia.) What is stress?