Organic evolution is descent with modification. Evolutionary tree A branching diagram depicting the genealogical relationships of taxa in time. The points at which lineages split represent ancestors to the taxa at the terminal points of the tree. Several evolutionary trees may be consistent with a single cladogram. When all the members of a lineage or taxon die, the group is said to be extinct.
Extinction is irreversible.
Gradualism A model of evolution that assumes slow, steady rates of change. Charles Darwin's original concept of evolution by natural selection assumed gradualism. Contrast with punctuated equilibrium. Hierarchy Nested sets of categories. Used in biosystematics to describe the groups within groups pattern of organization of the Tree of Life.