Hmmm I think the word evolve is pretty strong, I think I prefer nthe term innovation as a game evolving isn't always guaranteed.
If we look at the series, it spans over 26 years and 16 main games, it is one of the longest most notable/consistent series in gaming. However if we look at the games individually, i only see 4 or so games that have truly advanced the series or in a sense evolved. A Link to the Past took the original 2D formula and in my opinion nearly perfected it, it was also the first Zelda game to actually have a concrete story. Ocarina of Time of course for many aspects but most notably bringing Zelda into a 3D world. I would say to a certain extent Wind Waker as well, it showed us a completely new graphical nstyle as well as facial expressions. However WW was most notable for its differentiation in terms of overworld and exploration. The last game I would say it Twilight Princess, I may be biased here but let me explain. Don't get me wrong TP had many flaws but I thinknit is the perfect base for what could be nthe future of the series. The graphics in my opinion were as realistic as a Zelda game should get and with the dark themes it showcased in the story made the events seem realistic. I also thought that TP is the perfect example of how narrative and character development could work, what with its great characters like Midna and how effective the cut scenes were.
Ahh, went a bit off topic there but what I am essentially saying is that the Zelda series spans 25 years and 16 games yet there are only 3-4 games that I think have actually successfully advanced the series. Innovation in games is vital for the success in the long and I think Nintendo does see this, I mean each Zelda game most of the time is its own story, has its own graphical style and sometimes even controls. However this isn't always sussesssful, I mean take Skyward Sword, this is a game that changed the control scheme and graphical style. However do I think this is successful innovation to evolve the series? No, I doubt Motion+ controls will return in the future and neither do I think the impressionist art style will return.
This is the problem with the franchise, it is still trying to rethink the series. Zelda has the same formula of course but I think in order to evolve, instead of experimenting they need to in fact build up off the original formula. I think TP is the perfect base to work off, it is no where near perfect but essentially that games formula could turn out to be something special. Of course trying to improve the base formula isn't the sole way to evolve the franchise, innovations here and there may work and as well implementing features that are considered foreign to the series such as RPG elements.
Sales figures may not be the perfect figure to go off, comparing a series like Zelda to one like Cod simply won't work. Cod will always outsell Zelda in this situation (implying that the trend continues). Zelda should evolve, definitely however its the way it goes about it which is Key.