The snake on the left that is green is a snake with no venom, the snake on the right that is brown is a snake that has lots of venom.
I have lots of venom in my fangs, it is enough to kill an adult.
The snake on the right has very venomous fangs that are good for protecting it self and hunting for food. The venom will kill anything that it bites. So if something attacks it,it can defend itself.
The snake on the left is a snake with no venom. There is no venom in its fangs to protect itself. If it has no venom then when it gets attacked by another animal, if it bites the animal ,the venom will not kill it
The snakes with the venom can kill the predators easily and defend itself and not die. The ones with no venom will be eaten more often and their type will die down. The ones with venom are the ones that are more fit. Their will be more of their type and less of the snakes with no venom.
Over time there were more and more snakes with venom because the ones with no venom died died from predator