Evolution, Rate of

a concept that defines the speed of the evolutionary process. There are two principal approaches to determining the rate of evolution of organisms, one based on the change in certain organs or structures and the other on the origin of new species, genera, and other taxonomic groups. With the first approach, the rate is determined by the change in the average size of certain characters. Here, the rate can be expressed in darwins, a unit introduced by the British biologist J. B. S. Haldane; 1 darwin is equivalent to a 0.1 percent change in the average value of a character in 1,000 years.

With the second approach, the rate of evolution is measured by the number of generations or years (usually expressed in millions) necessary for the development of a new form (association), or by the number of new taxonomic groups evolving within a given period of time. Rates of evolution may vary for different groups of organisms within broad limits.