Figure 1.

Transitions in evolution. Evolution is change through time in biological organisms, and it can be categorized in four levels: (1) baseline evolution, or normal evolutionary changes in characters over time within a lineage; (2) novel taxa, or the appearance of new lineages, usually through cladogenesis and speciation; (3) novel adaptive zones, or significant new adaptations which open up new ecological structures and opportunities; and (4) major transitions, or transitions where new biological processes emerge, or new units of selection, and there is increased complexity. (Online version in colour.)