I have only stayed at one HGI (Bloomington, IN) with the Evolution room. As far as I can tell, it refers to the adjustable bed (because all the other amenities - flat screen TV, fridge, microwave -- were in the other rooms. At that property, the adjustment was 1-6. the adjustment was in an awkward place -- under the matress pad, inside a velcroed pocket). In two nights, I could not really get comfortable at any setting.

Note, at this property, regular and Evolution rooms were priced the same.