Evolution is a slow process, often taking thousands or even millions of years. But it can be quick when it needs to be. Take the case of the common carp (Cyprinus carpio). The species, like most fish, is covered in scales, but medieval European monks selectively bred carp centuries ago to produce ones with fewer scales, thus making them easier to gut and cook. Starting in 1912, these “mirror carp”—so named for their smooth sides—were introduced to Madagascar, where no carp existed, for fish farming, and they quickly spread throughout the island. However, by the late 1950s, people observed the mirror carp’s descendants “degenerating” into more scaled fish. In a study, published today in the Proceedings of the Royal Society B, researchers collected nearly 700 wild and farmed carp from across Madagascar and studied their scale patterns and genotypes. Roughly 65% of the wild carp, which mostly descended from the original mirror carp released into the wild, were fully scaled, despite all still carrying the genetic mutation that originally turned their ancestors into mirror carp. The newly scaled carp had evolved back their scales through different genes in 100 years, or roughly 40 carp generations, the team concludes. Scaled carp have been shown to survive better in the wild than mirror carp, likely because of the protection that scales offer from predators and parasites, and their reappearance shows just how much pressure a creature’s environment can put on an animal to quickly evolve.