JusticemakerMusic has gotten to evolution of the riff, in a pretty basic way, down to a science. Watch as he walks you through a few decades of riffs and how they all play into each other! It's pretty cool, if not interesting to see how he pretty seamlessly blends each genre together.
Next week, he'll be covering stoner rock, where he'll just hit the low E string and then walk away for an hour or so.