Who they will be transforming to then? We’ve got a lot of zombie of different kinds with no link to each other. The longest chain of evolution I can imagine is zombie->tough zombie->brute zombie->hulk zombie->Thriller. It would seem just strange to have a zombie transformed to, say, zombie cop. So yeah, despite having lots of enemies, they usually don’t have links to others.
Another way to make evolution (I suggested it a long time ago) is to not create new zombies, but strengthen existing. More HP, higher skill, higher speed and stuff. It might be done easily, but it’s not really that interesting. Without some, um, randomness evolution is not going to be interesting because after a while any player would learn the way it works. So yeah, without some randomness (of perhaps random enemy generator) this feature is doomed to be boring after a while.
Also it’s worth saying that even with zombie-lution zombies would not pose a threat for advanced character. Any player would know how to deal with some enemy if it’s not possible to just go and slash their throat with almighty katana, so that’s why random zombie generator would be far more better. If player have no idea how to deal with enemy - it becomes interesting to fight with this new kind and probably die the stupidest way. Just imagine random made a player a present - zombie who looks just like standard green Z, but is able to spit fire as tank drone. Now imagine how !!FUN!! things will be if the player is melee-based. That fits nicely with this idea (http://smf.cataclysmdda.com/index.php?topic=9734.0), but in other way - you learn about some kind of zombie as you see it using its abilities.
So, that being said, I consider this the best way to implement evolution.