Mostly. Clown fish are born male. In the wild, a clown fish will look around for some place (an anenome, for most) to settle down. If it finds an anenome that does not have a resident population, it settles in and becomes a female--larger, etc. All subsequent clowns arrive, and are already male, so can begin breeding immediately. If something happens to the female, the dominant male will turn into a female, and the cycle continues. If you buy a clown that has been kept alone for a while, it's already begun the process. If you get the fish from a group and it's solo in your tank, it converts. If you get a pair, than the larger, dominant fish will convert.
This is a survival mechanism. If there's already a female, reproduction can begin as soon as the male arrives. If there's no fish, converting prepares for his arrival.