“Diversity” is commonly defined as the number of kinds of organisms within an ecologic unit. “Kinds” are usually species but can also be genera, families, or higher taxa or even morphotypes. “Ecologic units” can be environmental patches, local communities, or whole ecosystems; in paleontology, the unit is often a complex of ecosystems or even the whole world ocean.

Diversity is interesting because it varies among taxa, places, and times, and study of this variation can lead to understanding of the processes that control the relative success of different groups of organisms. In the investigation of life's history, analyses of changing patterns of diversity have provided insight into the macroevolutionary dynamics of evolving clades and ecosystems and into the physical processes on earth that govern or limit evolutionary change. Because diversity represents the sum of all organisms in a clade or place, it provides a convenient index of evolutionary activity when mapped over time: when diversity is increasing, rates of evolution generally are high and new morphotypes, adaptations, and interactions often are appearing; when diversity is stable, rates of evolution usually are lower and changes in taxic structure of the ecosystem tend to be slow; when diversity is declining or fluctuating erratically, physical (or “extrinsic”) changes on earth, or even beyond earth, may be exerting dominant influence over evolutionary activity.