In evolution theory, phyletic gradualism, the proposition that evolution occurs at a slow and generally constant rate involving a large fraction of an entire species that gradually separates from the parent species (anagenesis). This is contrasted with a more recent theory prompted by discontinuities in the fossil record, the punctuated equilibrium model, the proposal that the rate of evolution is not constant but proceeds in spurts (of geological time), probably in response to environmental changes, and resulting in the sudden but rare generation of a new species (cladogenesis).