The Elements of Evolution (3) Mass Extinctions

Mass Extinctions

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

The nature of the geological record is complicated, so it is not trivial to decipher it correctly. ~ paleontologist Michal Kowalewski

As a guide to extinction, the fossil record readily fools. Life forms have specific ecological requirements. They may disappear from a biome because it did not suit them, but still survive in other habitats. If a location within that region becomes a fossil dig site, it may indicate an extinction that did not occur.

(A habitat comprises the relevant aspects of an environment in which a species population lives. By contrast, a biome is an area where organisms live with similar conditions, both geographically and climatically. Habitat is the environment from the perspective of a species, whereas biome characterizes a similar environment for all species within it.)

Paleontologists have seldom been methodical enough to cross-survey disparate sites of the same geological age: a difficult and expensive proposition. Instead, finds have been typically taken at face value wherever found. The methodological rigor necessary to accurately assess extinction has largely been lacking.

Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges. ~ paleontologist Rafal Nawrot et al