Extinction is a natural event – perhaps 95% of all the species that have ever lived have gone extinct. By late 1700’s, as more fossil species were unearthed, catastrophism became the dominant explanation. In 1830’s, Lyell sought to explain geological phenomena as the result of the long, gradual action of natural forces – uniformitarianism.
In 1858, Darwin published his famous theory of evolution, also based on gradual processes of evolution and extinction. Whether evolution happens gradually, or is better described by periods of stasis interrupted by rapid episodes of change (punctuated equilibrium) is debated today. There is no argument that extinctions occur both gradually and episodically, in mass extinctions. Life has radiated following these mass extinctions, as new forms have arisen, radiated, and given the world a new biota.
How does the present, human-induced wave of extinctions compare to historical events? If life has always recovered, indeed progressed, what is the fuss about? Extinction, even mass extinction happens, and life goes on.
The Geological Time Scale
History of the earth is divided into four eons, eons into eras, and eras into periods.
Fossil record becomes extensive after the Pre-Cambrian/Cambrian boundary, some 600 million years ago. A period is defined by the fossils of organisms that dominated in that period; the end of a period is marked by a transition in fossils.
Clearly, many ancient forms no longer exist. As species have appeared and others have disappeared, the overall trend in diversity has been strongly upward. Note also that this trend has periodic interruptions, called mass extinctions.
The average rate over the past 200 my is 1-2 species per year, and 3-4 families per my. The average duration of a species is 2-10 million years (based on last 200 million years).
Roughly 20 are recognized, of which 5 are major and the largest is the Permean, in which 50% of all marine families and 80+% of all marine species went extinct. How do we recognize a mass extinction?
What causes mass extinctions?
The K-T Extinction event
Replacement by Descent
One species splits into two -- the ancestor is lost, but two descendent species replace it; or a species simply evolves gradually over time into something different from its ancestor.
Invasion of the Biologically Superior Life forms
Evolution occurs independently in different parts of the world, driven by chance appearance of new life forms, and environmental change. Unique mutations appear, unusual genetic combinations arise, and environmental pressures that drive natural selection differ from place to place. Speaking metaphorically, life continually experiments with new body plans and new adaptations. Many are failures, but others are better than anything else on the playing field at that point in time. The North American – South American faunal interchange illustrates what happens when two faunas collide.
How Do Historic and the Present-day mass Extinction Compare?
Transparencies: 1. Geological time scale 2. Marine families since Cambrian 3. the six (?five?) mass extinctions 4. NA-SA interchange