Ichthyological terms, not general ones.
Used here on SF in reference to the theory of biological evolution whereby evolution occurs by a process of natural selection thus ensuring survival of the fittest. Includes some theories unconnected with the work of Charles Darwin and has been applied to a range of different philosophies over time, however.
In biology, Darwinian fitness or simply fitness of a biological trait describes how successful an organism has been at passing on its genes. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual.
Term introduced by Charles Darwin referring to the nonrandom mechanism of evolution by which certain biological traits become either more or less common in a population due to differential, environmentally-influenced reproduction. Better-adapted individuals tend to produce more offspring, thus the population evolves, and this process can result in populations that gradually become specialised to inhabit particular ecological niches and can eventually result in the emergence of new species.