P. Microlepsis

Perissodus microlepis is a species of cichlid fish found in Lake Tanganyika, one of the African Great Lakes. It’s a relatively small fish, usually only reaching a total length of 11 centimeters (about one hamburger diameter), and looks quite drab and unassuming. However, it has a rather grotesque diet: this fish, more commonly known as the scale-eating cichlid, feeds almost exclusively on the scales of other live fish, tearing them off the fish’s body with a peculiar, angled mouth. Some fish have mouths pointed towards the right (righty), while others have them bent towards the left (lefty). It’s not just the fish pouting—their skulls are actually shaped in that lopsided way!

​It has been shown that the fish’s predation tactics are affected strongly by their “handedness”—which way their mouths are bent towards. Righties preferred to rip scales from the left side of prey; lefties did the opposite and stripped scales from the right instead, as their angled mouths helped them produce more force in one particular direction during the attack. This is where the frequency-dependent selection comes in—the more of one particular morph there is, the lower the success rate of their attacks, and, by extension, their reproductive fitness. This is an example of negative frequency-dependent selection. ​

P. Microlepsis

​That was probably a little confusing, but an example will help. Say that a population of scale-eating cichlids contains 90% righties and 10% lefties. Prey fish around them will be attacked from the left side much more often, and they will, therefore, learn to be more alert to attacks coming from the left, leading to the righties not having as much success. The lefties, on the other hand, are not affected, and may even experience increased success as the prey fish are now hyper-focused on defending their left sides and leaving the lefties’ preferred right side unattended. As the lefties have relatively more success in feeding, they will likely be reproducing more successfully and passing on their lefty genes to create more lefty offspring; in a few generations’ time, the population’s morph composition may be oscillating closer to 50% - 50%. Thus, negative frequency-dependent selection actually serves to maintain morphological diversity in the species!​
This article was mainly focused on frequency-dependent selection in the context of the scale-eating cichlid, but many other species of animals, especially ones that display forms of mimicry, experience this form of selection as well. Do a little research and check them out if you find this intriguing, I promise it’s interesting!