To understand the basis of biostratigraphy, a discussion of evolutionary theories is necessary. The irreversible process of evolution allows for the independance of biostratigraphy vis a vis lithologies.

Although contested, evolution will continue as always. Through the preachings of creationism and the teachings of Darwinism, two major theories have become generally accepted as scientific models which depict evolutionary progressions. The traditional belief of phyletic gradualism has had to recently give some of its austerity to the methods of allopatrick speciation presented by Eldredge and Gould (1972).