Species concept Definitions

Biological species: A group of interbreeding natural populations that do not successfully mate or reproduce with other such groups (and, some would add, which occupy a specific niche)
Cohesion species: The smallest group of cohesive individuals that share intrinsic cohesive mechanisms (e.g. interbreeding ability, niche)
Ecological species: A lineage which occupies an adaptive zone different in some way from that of any other lineage in its range and which evolves separately from all lineages outside its range
Evolutionary species: A single lineage of ancestor–descendant populations which is distinct from other such lineages and which has its own evolutionary tendencies and historical fate
Morphological species: The smallest natural populations permanently separated from each other by a distinct discontinuity in heritable characteristics (e.g. morphology, behaviour, biochemistry)
Phylogenetic species: The smallest group of organisms that is diagnostically distinct from other such clusters and within which there is parental pattern of ancestry and descent
Recognition species: A group of organisms that recognize each other for the purpose of mating and fertilization