[Rduke55]

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But you need species - using Mayr's definition of reproductively isolated, etc. - to really get at evolution because once gene flow stops between populations then they each develop unique paths and evolutionary histories.

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Mayr's definitions are becoming increasingly dated, the inaccuracies are highlighted by interpecies sex. Which gene pools are allowed to mingle with others is a highly dynamic process.

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Can you elaborate?

Also, I'm not saying that there aren't problems with defining "species" or that transitional organisms or exceptions can't be found - this is biology after all.