A complex terminology developed around the evolutionary concept of adaptation. One definition of adaptation is synonymous with adaptive evolution and denotes a process of population change driven by natural selection resulting in individuals better able to survive and reproduce in a particular environment (compared to the population before the process began). This places adaptation at a hypothetical end point in a temporal sequence and can be difficult to use because the phenotypic composition of earlier, non-adapted populations is unknown. However, resurrection experiments can provide comparative data on populations before and after adaptation. The process of adaptation is a corollary to the well-described precepts of natural selection found in textbooks. Adaptation also denotes any trait that increases an organism's fitness relative to individuals lacking it. This 'all or none' concept of adaptation limits its usefulness because most traits are quantitative. Leaf pubescence, a quantitative trait showing phenotypic and genetic variation in plant species, is used to illustrate adaptive value and significance. Historical application of adaptation has ranged from individuals to populations to species and encompassed molecular and phenotypic traits. Although exaptation and maladaptation have some heuristic utility, 'pre-adaptation' is best avoided because it does not reflect adaptive evolution.