CBSE Class 10 Science 2017 · Paper 31/1 · Question 15

(1) Organisms are classified on the basis of their similarities and differences in characteristics, as groups within groups. (2) Characteristics are inherited, so the more characteristics two species have in common, the more closely they are related and the more recently they had a common ancestor; for example, the lion and the tiger share more characteristics, and a more recent common ancestor, than either shares with the dog. (3) Hence each group in a classification is a set of species descended from a common ancestor: the classification of organisms reflects their evolutionary relationships. Characteristics inherited from common ancestors, such as the homologous forelimbs of vertebrates, are used both to classify organisms and to trace their evolution.