So why do sharks have the tail that goes up and down but whales have the tail that goes side to side. Is there an actual reason for it or is something that just kinda happened?
In: Biology
Evolution.
When animals evolved to live on land their skeletons evolved for this when they evolved legs. The spine needed to move up and down for walking on land rather than the side to side motion fish have as up and down is way better suited for walking on land than side to side motion.
Whales are animals that returned to the sea. They have kept the spine movement of up and down and so that’s how their tail works.
Evolution doesn’t plan and moving up and down works well enough so it seems there hasn’t been any pressure to change it back to how fish use it.
Edit:
As to why fish evolved side to side in the first place? Probably just chance. Either would have been fine. There doesn’t seem to be much of an advantage of one over the other.
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