In the fossil record an enormous variety of animal and plant species are seen to appear and disappear in rock sequences that were laid down over long time spans. Successive species often show a sequence of development through time, indicating evolutionary trends or relationships.

Evolution by natural selection, first described in 1859 by Charles Darwin, is a powerful and unifying theory that proposes an explanation of how the diversification of living things occurred. It explains many different observations in biology and palaeontology.

What is evolution?

Evolution is a gradual process where living things give rise to different and often more complex or better adapted forms. It is a result of natural selection acting on the genetic variation among individuals of the same species.

Evolution results in the development of new species.

Rates of evolution

Evolution can proceed at very different rates - from hardly at all (stasis), to slow and steady (gradualism), to relatively sudden (punctualism). Examples of these different rates are shown by New Zealand marine fossils such as the snails Struthiolaria and Pelicaria.

The snail Struthiolaria calcar is an example of stasis over c. 10 - 15 million years:


Punctuated equilibrium is shown by the sudden appearance of Pelicaria nana alongside Struthiolaria praenuntia about 10 million years ago:

Punctuated equilibrium

Gradual evolution is shown by the transition between Pelicaria canaliculata and Pelicaria vermis:

Gradual evolution