Final answer:
Cladistics is a method of visually distinguishing between homologous ancestral and derived characteristics. It rests on three assumptions: common descent, speciation occurs in pairs, and traits change over time. Reptiles and birds are placed in the same clade due to shared derived characteristics.
Step-by-step explanation:
Cladistics, or the use of cladograms, is a method of visually distinguishing between homologous ancestral and derived characteristics. Ancestral characteristics are found in the common ancestor of the species being classified, whereas derived characteristics are only found in the groups in question. By exclusively looking at derived characteristics, biological anthropologists can develop a clearer understanding of the relationships between the groups being studied.
Cladistics rests on three assumptions. The first is that living things are related by descent from a common ancestor, which is a general assumption of evolution. The second is that speciation occurs by splits of one species into two, never more than two at a time, and essentially at one point in time. The third assumption is that traits change enough over time to be considered to be in a different state.
For example, let's consider reptiles and birds. Reptiles and birds are placed in the same clade because they share a common ancestor and have derived characteristics that distinguish them from other groups. The presence of scales and the ability to lay eggs with hard shells are examples of derived characteristics that reptiles and birds share. By comparing the shared derived characteristics, scientists can determine the evolutionary relationships between different groups.