There are two main selection processes in evolution, natural selection and sexual selection. Natural selection determines evolutionary fitness with characteristics related to survival. Sexual selection is the process that determines who is able to successfully reproduce. In the language of evolutionary theory, the probability that an individual will successfully reproduce by passing on her or his genes is called fitness. Classical (Darwinian) fitness is based on an individual's reproductive success in passing on traits to future generations through direct decedents.
An adaptation is an anatomical structure, a physiological process, or a behavior pattern with an inherited basis that reliably emerges at the appropriate time(s) in an organism's life, and helped to solve some problem in the ancestral environment. Initially, an adaptation begins as a mutation in a single individual (although it is theoretically possible to have multiple points of origin) and since it is necessary for or enhances reproductive success, it will pass to more individuals in the next generation. Over time, an adaptation that continues to benefit fitness may spread to the entire breeding population.
The environmental context may be critical to the development and/or expression of an adaptation. The qualities of pre-natal events and characteristics of the physical and social environment during development can lead to context-specific adaptations, such as the type of mating strategy exhibited (Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998).
Adaptations can be distinguished from other characteristics related to natural selection. Random effects, or "noise," are features produced by mutations that do not solve an adaptive problem. These random effects do not contribute to or detract from the functional design of the organism, so they have no impact on survival or reproductive fitness. A spandrel is defined as an incidental by-product of an adaptation, analogous to the static electricity produced by a cathode ray tube on a television screen. Exaptions are defined by evolutionary psychologists as a feature which performs a function it was not originally selected for. Exaptions can be co-opted structures that were either adaptations or spandrel by-products of adaptations (Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998).