1. According to the 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 isolation, which prevents gene flow between species. But in some cases, distinct species can occasionally interbreed, producing offspring. 2. Alternatively, the species concept identifies species based on physical traits or body form. This species concept can be applied to species, and it is the way most of the 1.8 million named species have been identified. 3. In the species concept, species are defined by their niche--the role they play in the environment. 4. The species concept defines species as the smallest group of individuals that share a common ancestor and form one branch on the tree of life.