Whales evolved their enormous size because they were not constrained by living on land. A large volume of food was available during their evolution, and the bigger animals were more likely to survive predator attacks.
It is impossible for animals to reach whale-like proportions on land. Even small whales are much bigger than the largest land mammals, and the blue whale is bigger than any dinosaur in history. High body mass puts stress on the animal’s muscles and organs, but this stress is reduced if the animal is aquatic. Water supports a whale’s body and allows it to function at a colossal size without battling gravity.
Large whales are not vulnerable to predator attacks due to their sheer size, and this is one factor that influenced their development. Natural selection is the process of certain traits being inherited by future generations. For example, small prehistoric whales were at risk from predators. The larger whales would have been more likely to survive an attack, and therefore the bigger whales would have passed on their genes to the next generation. As this process continued, whales got larger.
Another element that determined their size was the amount of available food. When baleen whales like minke and blue whales were evolving, the amount of krill would have been enough to support their rapid growth.
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Image from flickr.com/photos/irschick