Tough Times on Land


Evolution and Chance:

Dispersal of Parental Units by a variety of mechanisms:

Passive: (slowly alters gene pool over time)

Active: (catastrophe --> "overnight" change in ecosystem)

Survival of the fittest works in well-defined, slowly evolving ecosystems (like the ocean); On long time scales, the role of random catastrophe is more important

Mass Extinctions in Geological History:

Mass extinctions mean:

Timescale of extinction is crucial in identifying its cause:

Evidence from Fossil Record: 100-150 million year extinction periodicity ???

Since the orbital period of the Sun around the center of the Galaxy is about 250 million years then every 100-150 million years the earth is in a spiral arm which is a region of high supernova density. It is possible the earth could be subject to high radition from these supernova.

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