Substantial global reduction in the of organisms.

An important component of the is that periodically the "slate" to some degree is wiped "clean". The result can be a substantial change in what sorts of organisms are dominant within ecosystems, since the previously dominant forms are not necessarily well equipped to the mass extinction

In addition, and often notably, is a substantial adaptive radiation of at least some of the forms that survive, as they invade what have become unoccupied niches (see, e.g., adaptive zone).

Mass extinctions tend to occur as a consequence of dramatic, global and/or events that target specific species types relative to other types. These can be a consequence of either terrestrial or instead disruptors. The result is a "roll of the dice" or "shaking of the " so far are life's diversity is concerned, that is, a substantial, stochastic event that is genetic drift-like but as occurring in a macroevolutionary rather than solely microevolutionary context.

Note that mass extinctions don't necessarily occur instantaneously, but do tend to occur over relatively short periods as measured over . The result is that the geological record shows abrupt changes in what species are present as fossils.

These abrupt changes, for example, can be seen particularly 250 as well as 65 million years ago, corresponding to the (250 mya) and the (65 mya), which were followed, on land, by the rise of dinosaur and the , respectively.