Start with a 3 species model. Imagine a shallow, closed sea, in which there is a shark, a plesiosaur, and a bird. We need to explain why the shark and the bird

Start with a 3 species model. Imagine a shallow, closed sea, in which there is a shark, a plesiosaur, and a bird. We need to explain why the shark and the bird survive, but the plesiosaur goes extinct. An asteroid hits somewhere, causing environmental change. For the rest of this comment, I'm going to refer to temperature as a short-hand way of referring to any environmental change. The shallow, closed sea is undergoing temperature change, so the environment for bacteria and viruses is rapidly changing, therefore new species of bacteria and viruses are emerging at a high speed. The shark is safe because the shark has general purpose macrophages which fights all pathogens with equal efficiency -- the shark does not care whether a pathogen is new or old, because the shark has a disease fighting system which does not rely on the principle of immunity. Sharks are 350 million years old -- they evolved long before immunity existed. The plesiosaur and the bird are both in danger, because they have a system of disease fighting that does rely on immunity -- they are efficient at fighting off pathogens they've already been exposed to, but they are inefficient at fighting off pathogens that they have never been exposed to. The birds see half the flock grow sick and die, so some instinctual fear of disease kicks in and the birds fly away from the shallow, closed sea. The plesiosaur has no escape. Their disease fighting system is inefficient when dealing with new diseases, and they are in a bath of new diseases, from which they can not escape. They go extinct. Immunity is the best model I've seen so far for explaining which species went extinct and which species survived.