My son recently got Spore Origins by EA from the Apple Store, and it is his current favorite electronic game. He was so enchanted, in fact, that he agreed to pay for the upgraded app from his own chore money. He's evolved a few creatures already and enjoys planning out what they need to have to advance through the next level. I recommend it for any budding evolutionists.
For more on evolution, we'll check out Evolving Planet at the Field museum.