Advantageous Cancer in Swordtail Fish
The black pigment associated with swordtails (Xiphophorus sp.) is cancerous, and should a fish be born with black on their fins, it will almost certainly die of cancer. However, despite its seemingly negative effects have not been bred out within wild populations, as would typically happen to a inhibiting trait.
A study has revealed that female swordtails are more likely to mate with males that exhibit the cancerous traits than one that does not. Swordtails with the most striking fins, colours and patterns are more likely to be chosen as mating partners. As a result, the cancerous black pigment gene is passed on to progressive generations.
While it may seem strange that a detrimental attribute is encouraged, cancer may actually benefit the population by ensuring a constant renewal of fish. Swordtails develop fast, reaching sexual maturity at around 4 months, and are prolific breeders. The cancer, which starts killing at around 7 months, reduces the amount of resource competition. The mating preference for cancerous males ensures that they breed before they die, so their genes can be passed on, also keeping a wide genetic diversity within a group.
Based on research by Andre A. Fernandez and Molly R. Morris (Mate choice for more melanin as a mechanism to maintain a functional oncogene)
Photo: Valentin Hintikka on Flickr