Posted by rpcman on March 04, 1999 at 11:19:28:
In Reply to: Not exactly posted by Carlos on March 03, 1999 at 17:36:11:
: blue said:
: My understanding is that, from fossil evidence, most scientists now believe that most speciation took place rapidly during small periods of time that were separated by long periods of time during which very little speciation occurred.
When a Gould says 'rapid' he means something very different from the word's normal usage. The Cambrian "explosion", for instance, took place over more than 35 million years. That is an average human reproductive timeframe multiplied by more than 2,000,000 and a reprodutive timeframe multiplied by billions for many of the short-lived, quickly-reproducing creatures that lived at the time. It is also more than 5X the amount of time it has taken us to diverge from our common ancestor with the Chimps (as evidenced by the fossil record and DNA comparisons).
Also when you say 'very little speciation' you need to ask 'compared to what?'. Speciation has been taking place for billions of years. Certainly some periods show greater numbers (and greater causes). What else would one expect?
The following book has some interesting insights into the Cambrian "explosion":