The Orcas live all over the world but mostly in the Arctic or Antarctica. In the sea they are in the highest food chain. The only predator of the Orcas also known as the Killer Whales are humans.
Change in Gene Pool
The Orcas stay together in a pact and only scatter to confuse the humans. Since the smaller Orcas are faster they can escape the nets of fisherman. The bigger Orcas can't really hide and get captured so they struggle to survive.
Now that the smaller whales are the better trait they can swim away from humans faster without being caught. Since being smaller is the beneficial trait it passes down to the next generation keeping them safe. Now that the bigger whales can't swim as fast they die off
The gene pool changed by now having more of the smaller Orcas rather than the big Orcas. The smaller Orcas have a better fitness to the water so its survival rate is increased and now the Orcas have more of an advantage to live in that environment.