principle 1: Genetic Variation Some Lion fish are born with Venom and some aren't. Venom is used as a defense mechanism against predators.
Principle 2: Overproduction and the struggle to survive. Lion fishes are new to the Atlantic oceans and are considered invasive species. They have little known predators and have venom as defense to those predators, they live very long and reproduce like crazy.
Principle 3: Survival and reproduction of the fittest Lion fish that do not have venom do not live long enough to have babies. Lion fish that do have venom get to live long enough to reproduce and pass on its venom-having genes
Principle 4: Changes in the gene pool Since the Lion fishes with venom glands reproduce more, it is very much more likely to find most of the population having venom glands and much less lion fish without the glands