What is thought to bring different organisms such as fish and dolphins to have evolved a similar body plan?

their very similar environments.

Explanation:

The type of evolution that is referenced here actually has a name - - convergent evolution. It's when two types of organisms/species have nothing in common ancestor-wise but still evolve similarly due to their similar environments. The similar body plan the question is referencing could include fins and other "fish features" as well. : -)