Natural reproduction in single-sex environment

Besides asexual reproduction, there is another type of reproduction similar to it, however, it requires fertilization.
Known as "protogyny" meaning the change of gender, a species of fish called "wrassess" are able to do so. Their female can morph into "intial phase" males or "terminal phase" males. When any of their males leave the group or die out, these females will morph into males and take their place.
In some other species of fish called the "parrotfish", they are also able to change gender. As they start maturing, hormones are released, changing them into male or female due to chemical signals. They are borne as either males of females, and when there comes a need to replace a male or a female, they would change gender as they mature.
In some species of frogs, this also happens. Chemical signals which detect the number of its own kind act accordingly, changing their set of reproductive organs from male to female, or vice-versa.
Through genetic engineering, humans might be able to obtain this wonderful evolutionary measure, in the future!