Select ALL the correct answers. Some individuals of a particular species of sea bird underwent a genetic mutation that resulted in larger beaks. The larger size of beaks helped
these birds to break open dams and other mollusks for food. It also helped them defend themselves against predators. After many generations,
almost all of the birds in the population had big beaks. For which two reasons did this phenomenon occur?

A. natural selection

B. mechanical Isolation

C. prezygotic barriers to reproduction

D. high genetic variation in the species