Taxonomy, the science of classifying organisms, is based on phylogeny. Phylogeny attempts to establish the evolutionary history of a species, and taxonomy classifies organisms by this history.
Key to the study of phylogeny is the theory that different types of modern species are descended from a common ancestor. Such records are determined by hypotheses of relationship supported by evidence from fossil records, DNA testing and research from the fields of palaeontology, comparative anatomy, comparative embryology and molecular genetics. The results are organized in charts that resemble standard family trees and form the basis for the scientific "tree of life." Taxonomy classifies and names organisms based on their place within this tree.
Early forms of taxonomic classification grouped organisms based solely on shared physical characteristics and had no real scientific basis. While some modern hypotheses originated in the same way, such ideas are tested to determine whether those similarities are based on an organism's adaptation to its environment or if they are the result of inheritance from a common ancestor.
The earliest forms of taxonomy based on shared physical characteristics, regardless of ancestry, have evolved into modern cladistics. This science also assumes evolutionary relationships, but does not assume that characteristics shared by two related species necessarily existed in the organism from which the species originated.