Genetic Variation Some sharks are a darker shade of blue (even grey). Some sharks' heads are shaped differently like the hammerhead shark.
Overproduction The "overproduction" of offspring - Producing more offspring than the environment can support because most of them will not survive
Competition The sharks are at the top of their food chain so they really don't deal with predators or other sharks.
Gene Pool Changes Changes within the gene pool due to higher fitness rates. Ex. Sharks are a blue or grey color to blend in with the water in order to capture their prey better than if they were a bright color