Biologists have several different ways to define species. a. according to the biological species concept, a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring. the separation between species is maintained by reproductive isolation, which prevents gene flow between species. but in some cases, distinct species can occasionally interbreed, producing hybrid offspring. b. alternatively, the morphological species concept identifies species based on physical traits or body form. this species concept can be applied to asexual and extinct species, and it is the way most of the 1.8 million named species have been identified. c. in the ecological species concept, species are defined by their niche--the role they play in the environment. d. the phylogenetic species concept defines species as the smallest group of individuals that share a common ancestor and form one branch on the tree of life.