The correct answer is evolution (homology).Concept:Classification: It is the systematic grouping of organisms based on structural or functional similarities or evolutionary history.Evolution: It can be defined as a naturally occurring slow, continuous and irreversible process of change. It is the gradual change of living organisms from pre-existing organisms since the beginning of life into various other forms. Homology: It refers to the similarity of the structure, physiology, or development of different species of organisms based upon their descent from a common evolutionary ancestor.Explanation:The modern system of classification of living beings is based on the principles of evolution (homology). Earlier scientists classified organisms on the basis of their habitat (ecology) into aquatic, and terrestrial. Then, Aristotle classified plants into herbs, shrubs, and trees according to their heights or their external appearance i.e. morphology.Then, anatomy or internal structure was emphasized for classifying organisms, such as cell structure, number of cells, components of the cell wall, etc. With advancements in technology, scientists are now able to study the evolutionary lineage of different organisms by studying their DNA. It has been observed that Birds and Reptiles are made into different categories according to traditional classification systems.But their evolutionary lineage suggests that they are more closely related to reptiles and they should be in the same group. During evolution, new species arose from pre-existing ones.The accumulation of variations in the DNA or the genetic material is one of the reasons for evolution. Thus, by tracing the similarities in DNA, scientists can tell how closely related two species are, and how they can be classified into different groups.