Variation in reproductive success due to variation in the ability to acquire mates.

Different definitions are available dependent upon the characters being used to define a species. See also biological species concept, morphospecies concept and phylogenetic species concept.

Individuals with intermediate trait values have greatest fitness. E.g. selection on body mass at birth in humans. Mortality rate is lowest at intermediate body mass and the optimum mass has the lowest mortality rate, which is close to the population average.

Two species or populations sharing the same geographic region so that there is an opportunity to interbreed. See also allopatric and parapatric.

The science of classification of organisms according to resemblances and differences.

Two widespread populations are divided by the emergence of an extrinsic barrier, e.g. the formation of a mountain belt.