I think I may have an idea on what's holding up the evolution...it takes certain shapes to do well in later stages, but those shapes often fail at earlier stages. In other words...the bikes get gradually more and more succesful, but as they reach new and more demanding territory the shapes that got them there prevent them from making significant progress. So a shape goes 210...they tweak it in an effort to get it past that point, but that shape dies at 80.