How do critters with deadly poisons manage to stay alive? From a fascinating NY Times article here by Sean B. Carroll, we learn about a particularly nasty poison called tetrodotoxin and how it operates in different species without killing them. The conclusion is also very intriguing:
Remarkably, some of the same channel gene mutations responsible for conferring partial resistance to tetrodotoxin have occurred in different snake species. Moreover, some of these and other mutations have also occurred repeatedly in puffer fish channels.
These precise parallels in channel evolution among species reveal a surprising facet of evolution that biologists had no inkling of before the ability to pinpoint adaptive changes in DNA — namely, that evolution is more reproducible than previously thought. The simple explanation for that profound insight is that given similar agents of natural selection (tetrodotoxin in this case), very different species living in different places on the planet will evolve similar or identical adaptations.
It follows then that evolution is somewhat predictable. Given the prevalence of tetrodotoxin-producing bacteria and the many known uses of the toxin as a defensive weapon strategy, we should expect to find more toxic animal species.