T he term species is defined as a group of organisms which interbreeds under natural circumstances, producing viable, fertile offspring, and which is reproductively isolated from other groups.This definition is known as the Biological Species Concept.hapter 4: Speciation and Phylogeny What is a species? The term species is defined as a group of organisms which interbreeds under natural circumstances, producing viable, fertile offspring, and which is reproductively isolated from other groups.