Galapagos is an odd book. (Of course, Vonnegut is an odd man too.) I would describe it as primarily science fiction but with an element of fantasy as well.
The fantasy element is that the story is told first-person by a ghost who chose to stay in the world just to see what would happen. He has the opportunity to move on to an afterlife on several occasions, offered by the ghost of his father, and finally accepts after describing to the reader all the changes he has witnessed. I suppose some scientific explanation could be found for this, but it seems a stretch.
The science fiction comes into play in what happens in the world. The story focuses on a group of people planning a cruise to the Galapagos islands in a time when the world is falling apart. The cruise is canceled due to riots, however the people who had already arrived end up being forced onto the ship to flee the mobs, only to find looters have already removed everything of any value. They set sail anyway to escape death at the hands of rioters.
All the electronic navigation equipment was stolen, along with food, tools, and everything else that wasn’t bolted down securely enough, and the ship gets off course and crashes on one of the smaller islands. Without tools there is no way to repair the ship, and the people are marooned on the island. This actually turns out to be lucky for them since a new disease is spreading through the populated world that makes women sterile, and in only a generation humanity has died out everywhere except the island.
This is where it starts to get pretty strange. The ghost, who has followed and observed all the seemingly unrelated events that culminated in the survivors coming to the island, observes how this small group of humans survives in such an inhospitable environment, how they manage to continue the species, and how they evolve over time. His assessment of the whole situation is that “the only villain in the story is the over-sized human brain”.