Origin of the Goldfish
The goldfish (Carassius auratus auratus) you see in pet shops are the result of centuries of selective breeding.
The ancient Chinese have been rearing wild Prussian carp (Carassius gibellio) as food fish for thousands of years. During the Tang Dynasty (618-907), it was popular to raise carp in ornamental ponds. Natural genetic mutation produced individuals that had a yellowish hue to them, which was preferred over the natural silver coloration.
By the Song Dynasty (960-1279), the domestication of goldfish was established. Further genetic mutations caused for stronger oranges, reds and yellows to appear. Because these fish were kept in the safety of a pond, the conspicuous metallic colors did not disadvantage their survival.
The Ming Dynasty (1368-1644) saw the raising of goldfish indoors. This led to the development of many fancy strains, greatly changed from their wild shape. Protruding eyes, double tails and many unusual traits were emphasized through selective breeding, leading to many unique varieties of the one species.
Goldfish are still being developed today, with around 300 breeds recognized in China. Some varieties have been so physically changed that they are unable to survive outside of the aquarium. If introduced to the wild, domesticated goldfish can hybridize with certain species of carp. Within three generations, the hybrid fish revert back to their original coloration.
Images: Viridiflavus, Piet Spaans, Adityamadhav83, ぱたごん on Wikimedia Commons