Every mass extinction marked a watershed in the evolution of life. ~ American geologist Jon Erickson
Earth has seen a staggering array of biota, beyond imagination in variety. This proliferation derives from evolutionary impulse to diversely adapt: either toward generalized hardiness, or, more often, to fit into environmental niches, to more adroitly exploit available energy resources.
Specialization proves risky in time, as environments change, often dramatically. The causes are various, but all ultimately involve changes in temperature and the availability of water.
There are 2 extents of extinction event: background and mass. Background extinction is the demise of a relatively few species. This occurs where adaptation fails, often in a rapidly changing biome. Mass extinction indiscriminately wipes out many species. The difference between the two is a matter of degree. Species are always going extinct.
Outside mass extinction, species diversity tends to hold as new opportunities arise, but, as the number of extinctions rises, speciation invariably declines for a time.
2 temporal vectors commonly lead to speciation via population separation: dispersal and vicariance. Speciation by dispersal happens when a subpopulation migrates outside the range of the main population, adapting to the new habitat to eventuate into a new species. Variation by vicariance occurs when a new geographic barrier arises, separating a population. Isolation precedes speciation in either case, but by geology in vicariance rather than overt dispersal behavior.
Many of the major biotic turnovers, extinctions, and radiations that had once been attributed to direct competitive replacements or adaptive breakthroughs are now seen as physically mediated. ~ American geophysicist David Jablonski
Background extinction and mass extinction are typically provoked by environmental changes such as climate. The rapidity and severity of change dictate the degree and duration of an extinction event.
There is no commonly accepted definition of the term mass extinction other than as a vague generic reference. ~ American paleontologist Norman MacLeod