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