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					| Rates of Evolution |  
					| Lecture 10 2/27/98
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					| |  | Today, we continue our investigation of evolution by focusing on the rates of speciation. | 
 |  | We will use this discussion as a back drop for looking at the Cambrian Explosion | 
 |  | Earths history shows periods of slow evolution punctuated by periods of rapid evolution | 
 |  | The rate of evolution is dependent on at least three different things | 
 |  | rate of environmental change | 
 |  | Once structures and/or behaviors evolve it is difficult to change them radically | 
 |  | Seven major animal groups appear in short period of geologic time | 
 |  | Additional groups appear, but do not survive long | 
 |  | By comparison, the rest of the earths history is one of slow evolution | 
 |  | Speciation typically occurs when a small population is isolated from the main gene pool | 
 |  | Selection that each population experiences varies. | 
 |  | Over time genetic variability between the two populations grows to a point of where they can no longer interbreed | 
 |  | Two primary factors govern the rate of speciation | 
 |  | a positive force, providing new genetic material | 
 |  | a negative force, removing mutations that are not beneficial | 
 |  | Selection can be viewed as competition between individuals | 
 |  | Competition between members of the same species | 
 |  | Competition between members of different species | 
 |  | new species form in a "filled" environment | 
 |  | leads to normal rates of evolution | 
 |  | new species form in an empty environment | 
 |  | competition between species is minimal but increases with time. | 
 |  | differences are preferentially selected for | 
 | The role of environmental change | 
 | A look back at the Cambrian Explosion | 
 |  | Development of the SuperWorms | 
 |  | Oxygenation of the upper sediments | 
 |  | Escape to the surface (Base of the Cambrian) | 
 
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