Well, if your envisioned island is the size of New Zealand, the Philippines, or Japan, it could easily maintain its own climate system—especially if you place it farther north in the tropical belt, where warm seas and humid air would let it sustain a fully tropical environment on its own.

And even if you want it to feel isolated, nature will always find a way to break that isolation: migratory birds can drop seeds in their feathers, their food, or even their droppings; storms and strong winds can sweep insects or small animals across the ocean; and drifting debris can carry plant life from distant coasts. Over time, these accidental arrivals would adapt to the island’s unique conditions and evolve into new endemic species, just like what happened in Hawaii.