Just to stir the pot somewhat...

It seems most of the discussion here in regards to evolutionary models is about what some might  call 'directional selection' (my wife's term) or what I more loosely refer to as a constant evolutionary model but we should also keep in mind that there is the 'punctuated' model of evolution.  Both of these are of course only models and my best guess is that often in the real world how things really work out fall somewhere in between these two very different evolutionary models.  

As loose examples... selection by beekeeper might be view as 'directional selection' while the introduction of both the mites into the US could be viewed as 'punctuated selection'.  

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