What are some of your favorite evolutionary lines?

I like the Magnemite in because I feel like it tells a little story. Magnemite isn't very strong and, as a magnet, is naturally attracted to other Magnemites. So three of them join up, but they aren't really acting as a singular unit yet and are still basically just three Magnemites stuck together. But after some time they are able to finally become one in a proper fusion, and are now greater than the sum of their parts.

Do you ever come up with small stories or explanations for evolution lines?