I was thinking of a scenario in which a population of snakes gets stranded on an island in which they are literally the only animals. This hypothetical island does not even have insects for the snakes to eat. The island does have dense forests of trees that produce fruit. All the fish and marine life in the water around the island is poisonous to the snakes so eating marine animals is NOT an option for the snakes. The snakes before getting stranded on this hypothetical island have only ever eaten small animals and have never eaten fruit before. Could the snakes on this hypothetical island survive entirely on the fruit that the trees produce? What effect would eating only fruit have on the evolution of the snakes on this hypothetical island? Would the snakes be able to rip fruit from trees on the island or would they have to eat fruit that fell from the trees.
Could the snakes on this hypothetical island survive entirely on the fruit that the trees produce?
The simple answer is "no." A very emphatic "no." Snakes are obligate carnivores. There is no such thing as a vegetarian snake. Snakes recognize fruit as "not meat" and won't consider eating it.
All the snakes will die. They will either starve to death or die trying to eat the poisonous fish in the water. They don't have reasoning to 'try' other things, nor is their body likely able to cope with eating materials other than animal protein.
If they were really lucky, one of the fruit might be high in protein and smell enough like a rat or mouse they might try it, and maybe survive. But you would need to invent a fruit along these lines.
Your scenario is problematic and as others have mentioned...everyone dies...errr every snake dies.
The problem with your scenario is that your ecosystem is not scientifically plausible. To have an environment you need each progressively more complex creature to make it all work. You can't just have big trees that bear fruit without, bacteria and other cellular organisms, insects, reptiles, birds etcetera.
It is hypothetically possible that you could stop at the insect level and still have big trees but it seems unlikely.
You need pollinators, though you could argue that this is done via wind...it's not foolproof but if there is no competition among trees I suppose it's possible.
You need soil cultivators, insects and smaller life play a major role in making soil more than dirt/rock. They make it able to support plant life. Without them there simply aren't going to be any nutrients in the soil to make things grow which is the main problem.
Snakes are supposedly all obligate carnivores. Monitor lizards are their closest lizard relatives (and are pretty close) and they are also obligate carnivores – except for the ones that eat fruit instead.
How could frugivory evolve out of obligate carnivory? I bet it had to do with eating frugivorous insect larvae. Lots of protein rich bugs make their homes and lay their eggs in fruit, and a lizard picking those out might have done better just eating the fruit with it.
Could this happen with a snake? Snakes definitely eat worms and bugs and I would not put it past one to eat some bites of fruit as well. There might be an occasional mutant which enjoys fruit for whatever reason, like this whip snake caught on film.
If it could get enough protein from the fruit diet (maybe from the larvae in them? Maybe from seeds that it could digest?) a snake stuck on this island might found a new line of frugivorous serpents.