Posted: Jun 26, 2010 9:10 am
The reason that they both have prominent dorsal fins is because of convergent evolution. The dorsal fin is absolutely necessary for (large) sea creatures for stability, and those organisms with it were much more likely to have offspring than those that didn't.
Two organisms need not be closely related to evolve many of the same structures, since Natural Selection will select for those traits for the sole fact that they are advantageous in a particular environment. Dolphins and sharks are not the only ones with convergent evolution either; the evolution of the wing on both birds and bats are another example, and The Ancestor's Tale by Richard Dawkins also gives numerous examples of convergent evolution, such as the marsupial mole and it's Eurasian counterpart, or the shrews that evolved independently on Madagascar.
Two organisms need not be closely related to evolve many of the same structures, since Natural Selection will select for those traits for the sole fact that they are advantageous in a particular environment. Dolphins and sharks are not the only ones with convergent evolution either; the evolution of the wing on both birds and bats are another example, and The Ancestor's Tale by Richard Dawkins also gives numerous examples of convergent evolution, such as the marsupial mole and it's Eurasian counterpart, or the shrews that evolved independently on Madagascar.