Any two biologic groups — populations, species, or groups of species — that are separated in time but connected by evolution provide us with the opportunity to measure a rate of evolution.

Rates are interesting because they vary greatly. Some species have survived for millions of years without appreciable change. On the other hand, some species have arisen from other, quite different species within just a few thousands or tens of thousands of years: very brief intervals of time. Similarly, some groups of organisms have diversified rapidly, producing many new species during brief intervals of geologic time, whereas others have added new species very slowly.