UC Berkeley
Dolphins and sharks both have a streamlined body shape with a triangular fin on the back and two side fins. However, the two animals also have many differences.
| Sharks | Dolphins |
 |
 |
| skeleton made of cartilage | skeleton made of bone |
| use gills to get oxygen from the water in which they swim | go to the surface and breathe atmospheric air in through their blowholes |
| don’t nurse their young | do nurse their young |
| don’t have hair | do have hair – they are born with hair around their “noses” |
Shark photo provided Dong Lin © California Academy of Sciences; Dolphin photo provided by M. Winning © California Academy of Sciences.
They may share the same basic shape, but underneath their skins, sharks and dolphins are very different!
Do you think that sharks' and dolphins' similarities (body shape, fin, and flippers) are homologies or convergent traits? (Click on the correct response.)
homologies convergent traits
Nice try, but maybe it would have helped if we had shown you the hints first. Dolphins and sharks are on distant branches of the tree of life. Sharks are closely related to rays, and dolphins are closely related to cows and other mammals. Streamlined bodies and fins are traits that dolphins and sharks evolved separately, both as adaptations for swimming.
This means that sharks' and dolphins' similarities (body shape, fin, and flippers) are convergent traits that each lineage evolved independently.
See some hints
Hint 1: Having a streamlined body shape, fin, and flippers allows aquatic animals to swim faster. Both sharks and dolphins swim after prey.
Hint 2: This tree shows where sharks and dolphins are positioned on the tree of life. They are not very closely related to one another. Is it likely that sharks and dolphins inherited their body shapes, fins, and flippers from the same common ancestor?
