Oak told Ash to ask Westwood about Slowpoke evolving into Slowbro as both Professor's were coincidently working on the same Mystery. Westwood invited Ash, Brock and Misty into his home and lectured them about his current analysis about Slowpoke's evolution. Jessie's Shellder (temporary) bit onto Westwood's Slowpoke causing it to evolve into Slowbro and leave Jessie, therefore solving the evolution mystery.
Slowpoke was Westwood's Pokémon he analyzed to complete his study of evolution. Though Westwood attempted to protect it from evolution, Jessie's Shelder bit Slowpoke's tail and evolved. Still, Westwood managed to complete his study.