This program focuses on macroevolution and the processes that explain it. A key question of macroevolution is, 'What distinguishes a species?' This leads to taxonomy and discussion of the two main approaches to classification: evolutionary systematics and cladistics. Examples are given of homologies and analogies, and ancestral and derived traits, keys to classifying by these different approaches. In the second segment, two different species concepts are mentioned, the biological species concept, which is the most commonly used definition, and the ecological species concept, which places emphasis on niche occupation and natural selection.