Guppies have been a separate species for 500,000 years, and during that time their color has changed drastically. But one thing has stayed the same: an orange patch on males. This is quite possibly the most romantic adaptation ever.
That's because females seem to like the orange patch, and male guppies have evolved and re-evolved that same patch multiple times. These patches are a mixture of two pigments, only one of which their body produces naturally. The first pigments are carotenoids, which are yellow and aren't produced by guppies, which means they can only get them by eating algae. The second set is drosopterins, which are red and are produced naturally in their body.
This means that wild guppies should have huge variations in the color of their streak, depending on how rich the local algae are in carotenoids. Life scientists would expect to see streaks vary from red to yellow, and yet the vast, vast majority of these streaks are just orange.
Researchers at UCLA decided to get to the bottom of this, and they quickly discovered that female guppies are drawn strongly to males with intermediate levels of drosopterins, which means they like males with the orange streaks. That held true even when the females were presented with a wider variety of colors than they normally see in nature. This means males with the preferred orange streaks likely have more offspring, but that alone doesn't explain why that color is so dominant.
As researcher Greg Grether explains, the male guppies actually do whatever it takes to keep the ratio of carotenoids and drosopterins exactly where it needs to be to produce that orange streak:
"A pattern I discovered 10 years ago, which was mysterious at first, is that in locations where more carotenoids are available in their diet, guppies produce more of the drosopterins. There is a very strong pattern of the ratio of these two kinds of pigments staying about the same.
"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."
Grether says that the more expected relationship between caretonoids and drosopterins would be for guppies to use the drosopterins to make up for any shortage of the caretonoids, but instead they're committed to exactly maintaining the right ratio. Since the experiments clearly indicate females prefer this look, it appears to be a strategy motivated by ensuring the best chance of having lots of offspring. Or, in other words, guppy males have been changing their appearance to make their females happy, and they've been doing that for half a million years. You know, I suddenly feel like those damn guppies are making all the rest of us dudes in the animal kingdom look bad...