In any spawn, there will be a certain number of mutations, that in nature are removed by predation, or some other means if the trait is not vigorous, or robust enough for the continuation of the species in its habitat.
In aquariums when a less than optimum survival mutation is noticed, it can be separated out, allowed to grow up unimpeded in a controlled, none competitive environment, and then bred with others of that, or a similar mutation to start a line.
This may occur naturally, and how a new species is created.
When natural habitat is somehow altered by ie a volcanic eruption or other natural phenomenon, and the specific trait favors the species survival in the altered habitat.