For the past 500,000 years, Guppies have changed their color dramatically, but yet an orange patch on males has remained unchanged. It’s all about romance. Or maybe it’s just about doing what you have to do to get the ladies. Females like the orange patch, which contains two pigments. The first are carotenoids, which are yellow and aren’t produced by guppies themselves. They can only get them from the algae that they eat. The second is drosopterins, which are red and are produced in the body.
Guppies should have huge variations in the color of their patch, depending on how much algae they eat. Why has it stayed the same hue of orange? Researchers at UCLA learned that female guppies are drawn strongly to males with intermediate levels of drosopterins; so females desire males with the orange patches. Even when the females were presented with a wider variety of colors, they chose orange. To ensure males are able to mate and have offspring, they must do whatever they have to in order to be orange and please the females.
“To human eyes at least, as the proportion of carotenoids in the spots goes up, the spots look yellower, and as the proportion of drosopterins goes up, the spots look redder. By maintaining a very similar ratio of the two pigments across sites, the fish maintain a similar hue of orange from site to site. What is maintaining the similar pigment ratio across sites and across populations? The reason for the lack of variation is that genetic changes counteract environmental changes. The males have evolved differences in drosopterin production that keep the hue relatively constant across environments. As a result of [lead author] Kerry [Deere]’s experiment, we now have good evidence that female mate choice is responsible for this pattern.”
As far as the scientists are concerned this is the first evidence for a hypothesized agent of countergradient sexual selection. Men and guppies alike ultimately always give in to the needs of their mates.