Table 24.1 Six Concepts of Species Compared 
Biological species concept Emphasizes reproductive isolation, the potential of members of a species to interbreed with each other but not with members of other species.
Morphological species concept Emphasizes measurable anatomical differences between species. Most species recognized by taxonomists have been designated as separate species based on morphological criteria.
Recognition species concept  Emphasizes mating adaptations that become fixed in a population as individuals "recognize" certain characteristics of suitable mates.
Cohesion species concept Emphasizes cohesion of phenotype as the basis of species integrity, with each species defined by its integrated complex of genes and set of adaptations. 
Ecological species concept  Emphasizes species' roles (niches), their positions and functions in the environment.
Evolutionary species concept  Emphasizes evolutionary lineages and ecological roles. 

From Campbell, Reece, Mitchell.  Biology, 5th Edition    (Menlo Park, CA:  Addison, Wesley Longman Inc.,  1999)  © 1999 Addison Wesley Longman