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. Over time, an adaptation that continues to benefit fitness may spread to the entire breeding population. Adaptations can be distinguished from other characteristics related to natural selection. 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.