We generally do not want to call different populations that show clinal variation different species since the populations change gradually from area to area and individuals can reproduce with individuals in slightly different neighboring populations, so there would be no clear place to draw a line to distinguish species and overall the different populations can clearly affect one another by reproducing with one another. On the other hand, they reproduce with each other within the hybrid zone, and their offspring are healthy and fertile, so it appears that they are very similar and that they could affect each other, so based on this argument people would consider them as belonging to the same species. Examples such as this show why it can be difficult to categorize groups as species. We will discuss some of the main concepts of what a species is and consider how they would categorize groups from different examples as either the same or as different species. Biologists differ on whether or not they view species as fundamental evolutionary units.
Some biologists argue that evolution occurs primarily within populations.
There is evidence in a number of species that different populations evolve fairly independently from one another. These are the biologists who really care about species concepts. They do not, however, all agree on which concept of species is the best. In other words, organisms are considered to belong to the same species if they can potentially reproduce with each other and have offspring that are healthy and fertile. Otherwise, they are considered to belong to different species.