For example, Slowpoke now requires a Shellder in the active party to evolve. Upon hitting Lv. 37, it will evolve into Slowbro, unless it's holding the King's Rock, in which case it evolves into Slowking. The older evolutions methods would no longer work as of Gen. VI.
Another possibility could be turning Nidoran into a single Pokemon line, with obvious gender differences. (Which would still maintain each gender having different stat arrangements and moves.)