The oceans of the early Muricene continue to run red with algae. The mats that line the coast measure more than 20 centimeters thick and, in some places, reach tens of kilometers out to sea. Beneath this crimson photosynthetic mass, however, one might now spot flashes of other colors; blues, greens, yellows, purples, all shimmering with an appealing iridescent gleam. These are the marine mosquitofish, a group composed of roughly two dozen species and subspecies within the ever-growing Apterran branch of the Gambusia genus. Each distinguishes itself from its cousins with a unique pattern; where one might have bright orange fins, its neighbor might possess black spots and stripes along its sides. Males in particular show an incredible diversity both between and within species, while their mates, though beautiful in their own right, retain a primarily silver-grey color. The selective pressures driving this explosion of color are twofold.
All these groups, being incredibly closely related from a genetic standpoint, need a way to distinguish themselves from each other in order to prevent hybridization. By having unique colorations, members of a group can ensure they breed only with conspecifics. This allows each species to maintain its own separate gene pool despite still being theoretically compatible with other populations in their environment. In turn, the distinct groups can specialize into a variety of niches, permitting a diversity of forms that would, without these anti-hybridization adaptations, take several million years longer to emerge.
Within each species, though, there is another, more immediate force that drives the development of these flashy patterns. To put it simply, brightly-colored Gambusia are sexy. Any male that can afford to devote a portion of his precious energy into otherwise useless display structures must be quite fit indeed. Consequently, any females who have a genetic predisposition to prefer males with bright colors will end up mating only with partners who themselves possess strong genes. As a result, females evolve over time to be pickier, choosing only the most gorgeous, ornately-patterned mates. Males, in turn, get fancier with each subsequent generation, becoming ever-prettier to prove their fitness above their marginally duller neighbors.
In this way, both sexes are selected toward greater extremes, but this process must end eventually. Generally speaking, the feedback loop ends when males reach a point where any further display structures would reduce their overall fitness to an excessive degree. For instance, females of many species have come to prefer males with long, flowing fins, a trait that reduces foraging success and increases the risk of predation. Males with such fins live shorter lives but are more reproductively successful. However, there is a certain threshold beyond which any further increase in fin length (or preference of fin length) reduces the survival rate so drastically that any reproductive advantage is negated. A male with fins so long that he can't swim properly, or a female who exclusively prefers such males, will soon see its genes removed from the population. In this way, each species reaches its own equilibrium; males are showy but not to the point of complete self-destruction, while females are choosy but not ridiculously so.
Beyond the simple favoring of bright colors and fancy ornamentation, other preferences have also emerged within the various female minnow populations. Avoiding inbreeding is a necessity, as a homogenous population is susceptible to disease, poor adaptability, and increased chances of extinction. As a result, females often go for males who look significantly different from themselves, as this is a sure sign that a partner is not one of their own close relatives. Of course, there are no mirrors in the oceans of Apterra (nor would these tiny-brained livebearers be capable of comprehending their own reflections anyway), so females often use the appearance of other members of their school as a proxy. As such, any male that looks noticeably different from his neighbors has a serious advantage, as females will recognize his uniqueness and flock to him. He had better be careful not to stray too far from the norm, though, as this would run the risk of females failing to recognize him as a member of their species. This supports a state of equilibrium in the diversity of colors within every species: quite a bit of variety exists in any given population, but extreme outliers are as uncommon as boring-looking males.