Abstract

Adaptation is a defining property of living systems. It occurs when a population of organisms becomes better suited to its environment. The phenomena that people find most fascinating about biological systems are, in general, the result of adaptive processes. Examples include the mammalian central nervous system, the flight of birds and insects, photosynthesis, and the human hand. However, despite the centrality of adaptation for biology, there is no generally agreed quantitative way to describe the degree to which an organism is adapted. Here, we address this situation by proposing a quantitative measure of adaptation. We then provide evidence that the proposed measure can also serve to estimate biological information, to assess biological complexity, and to help resolve questions about the ‘units of selection’ and ‘the major transitions in evolution’.