What's up with slowbro and slowpoke?

If a Shelder has to bite a Slowpoke's tail for the Slowpoke to evolve, would the Slowbro un-evolve if the Shelder let go? If you look at some of the card's art, you'd notice that the only difference is the shell on Slowbro's tail. Nothing else. I am very, VERY confused. Please help me understand.

Edit : to all who answered, thanks! You were very helpful.