Explain why visible structures, such as fins, beaks, and tails, look so different on different organisms.

Morphological changes to function appendages arise because of different requirements this was termed by Darwin as adaptive radiation. For example darwins finches - each birds beak was slightly different in shape - this allowed them to exploit a different food source, thereby reducing intra specific competition. As for fins, these vary depending on the swimming requirements of the organism; balance, agility or speed may be selected for.

That would be a working theory.