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Introduction: key points
- Evolution acts primarily by selection on heritable variation among a population of organisms.
- Selection maximizes fitness (reproductive success) and not necessarily health or longevity.
- Health and disease are influenced by an individual’s evolutionary and developmental background in the context of their current environment.
- Concepts of health and disease are altered by taking an evolutionary perspective.
Biological responses over different time-scales
Organisms face challenges over a number of timescales. These include immediate physical challenges, such as intraspecific competition for energy and reproductive opportunity or attack by predators (themselves engaged in energy harvesting), and changes in the environment. The latter may be short-term, such as daily temperature fluctuations, medium-term such as seasonality of food availability, or long-term such as climate change causing disappearance of preferred food sources or habitat. Organisms have evolved a hierarchy of responses to these challenges. Some classical homeostatic responses are very rapid and highly reversible (e.g., those mediated by the central nervous and endocrine systems) over seconds to hours, and some involve structural change or long-term readjustment of set-points for homeostatic feed-back systems (called rheostasis) and these operate over hours to years. Many of these longer-term but within-lifetime effects are initiated in early life through the processes of developmental plasticity. Yet other responses are beyond the timescale of individual lifetimes and involve natural selection resulting in genetic change over several generations.