What is adaptation and how should it be measured?

Adaptation is a defining property of living systems. It occurs when organisms become better suited to their 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 also present results of computer simulations which demonstrate that some changes in parameter values cause mean adaptedness and mean relative fitness to change in opposite directions. This indicates that adaptedness and relative fitness are distinct concepts. We suggest that the measure of adaptedness proposed in this work may help to resolve questions about 'units of selection' and 'major transitions in evolution'.