Evolution occurs whenever a population of individuals changes over time.
Evolutionary rates vary widely over time, among species, and among traits. Some species have changed rapidly in appearance over tens to hundreds of thousand years. Drosophilid flies on Hawaii Others have remained similar in appearance over tens to hundreds of million years. Darwin's finches of the Galapagos islands and the honeycreepers of the Hawaiian islands have undergone rapid morphological evolution in large part because the islands are relatively free of competitors. In fairly static environments, the rate of evolution slows down.