I read this book in the 80s, a paperback off my parents’ bookshelf, so I assume it was 1960s era. I can remember how it was set up, but as the story evolved the characters find an overgrown forest, (and I think the hull of the ship?) They find out that the forest is overgrown hydroponics, that they are descendants / evolved from the original crew (maybe a mutiny or accident?). Some of the original crew descendants still run the ship and are aware of the native-like group living apart from them. One of the characters who is described as slow and tall is actually one of the bridge crew infiltrating the native group, he is shorter than normal and had to practice to make himself move and talk fast —because the native group is quick and small, and he succeeded only somewhat in matching them.