So today some descendants of the ancestral chordates have poison fangs, some have teeth, and some have both. There is clearly a strong association between digestive enzymes and poisons, but the theory of evolution does not depend upon this exact evolutionary chain of events having taken place. This merely demonstrates that there is at least one plausible mechanism, so evolution might have occurred using it or some alternative mechanism unknown to people. The failure of people to suggest a plausible mechanism for the evolution of every biological feature does not mean that evolution is a poor theory. It merely means that people are intellectually limited.
Evolution is still the theory that explains the existence of species with the least assumptions.