When does natural selection take place?

Although many studies of form and function find a correlation between performance and adaptive specialization, others fail to discern such a tight link despite careful monitoring and observation. This inconsistency among studies raises the question of when, how often, and how effectively natural selection and the organism's own activities operate to maintain or improve the adapted state. I suggest here that most organisms operate well within the limits of their capacities (safety factors) most of the time and that interactions and circumstances that cause natural selection and test the body's limits come in discrete, intermittent events rather than as continuously present or chronic conditions. Everyday life without such events does not test performance limits and therefore does not usually result in natural selection. This perspective on selection as rare, intermittent testing by ecological agencies suggests that studies of selective processes and activity in the wild should focus on observing and measuring the intensity and frequency of selective events and responses, intense challenges stemming from agencies such as predators, competitors, mating-related rituals, and extreme weather.