A generation of hooded snakes are born. their hoods are used to make them seem bigger in order to scare off their predators, such as hawks.
most of the snakes have normal sized hoods, but a random mutation occurs in several snakes, causing those snakes to have larger hoods.
Not all of the snakes will survive long enough to acheive reproductive success. Many will be killed by the hawks before they can mate.
changes in gene pool
The snakes with larger hoods have an advantage over those with normal sized hoods. The larger hoods make them seem bigger and so it is easier for them to scare off the hawks, meaning they are less likely to be eaten than the others. this also means that the large hood is a beneficial trait to have.
It is time to mate and reproduce. The snakes with larger hoods are more likely to survive and reproduce than those with normal sized hoods. because of natural selection, a lot of the large hooded snakes suvived, and there are more baby snakes with larger hoods than there were in the previouse generation.
Now that there are more large hood genes in the gene pool, and because those with the large hood phenotype are more likely to survive, the large hooded snakes become more frequent in each generation. Eventually the majority of the population will have large hoods.