The Aquan civilization was an example of forced evolution. Long before the 23rd century, the Aquans were air-breathers, but massive planet-wide earthquakes flooded the entire planet. They were forced to adapt to be able to breathe underwater. Fortunately, their medical science technology was advanced enough to induce a necessary mutation, called a surgo-op, to ensure the survival of the entire species.
Encounter with the Federation
In 2269, the crew of the USS Enterprise visited Argo and discovered the underwater society, which at this time was experiencing ideological divisions between generations, and relations between the younger and older members of Aquan society were strained. On an away mission, Spock and Captain Kirk were temporarily altered to become water-breathers. Fortunately, Doctor McCoy discovered that the venom of the sur-snake contained an antidote, and they were returned to normal. (TAS: "The Ambergris Element")