Polyploidy has been recognized as an important factor in the evolution of plant species, but its role in vertebrates has only recently been acknowledged. Ohno proposed that genome doubling occurred in the common ancestor of all vertebrates, with a tetraploid event about 500 million years ago. Recent studies show that polyploid events are more common than previously thought, especially in amphibians and reptiles. However, polyploidy has not been a major factor in the progressive evolution of these groups. Polyploidy has also been observed in fish, with some primitive fish species appearing to be tetraploid. There are close relationships between diploid and tetraploid forms in several fish species. Three cyprinid species, the barb, the carp, and the goldfish, appear to be tetraploid. Some unisexual goldfish strains are triploid or tetraploid. Recent polyploid events have been found in two genera of livebearing fishes in Mexico. These fish have been studied extensively and provide useful model systems for examining polyploidy. There are only two known cases of polyploidy in fish giving rise to an entire family. The suckers of the family Catostomidae are a large evolutionary group that apparently share a common tetraploid origin. The Salmonidae are the second family of fish that apparently share a common tetraploid origin. There are three major salmonid taxa classified as subfamilies.Polyploidy has been recognized as an important factor in the evolution of plant species, but its role in vertebrates has only recently been acknowledged. Ohno proposed that genome doubling occurred in the common ancestor of all vertebrates, with a tetraploid event about 500 million years ago. Recent studies show that polyploid events are more common than previously thought, especially in amphibians and reptiles. However, polyploidy has not been a major factor in the progressive evolution of these groups. Polyploidy has also been observed in fish, with some primitive fish species appearing to be tetraploid. There are close relationships between diploid and tetraploid forms in several fish species. Three cyprinid species, the barb, the carp, and the goldfish, appear to be tetraploid. Some unisexual goldfish strains are triploid or tetraploid. Recent polyploid events have been found in two genera of livebearing fishes in Mexico. These fish have been studied extensively and provide useful model systems for examining polyploidy. There are only two known cases of polyploidy in fish giving rise to an entire family. The suckers of the family Catostomidae are a large evolutionary group that apparently share a common tetraploid origin. The Salmonidae are the second family of fish that apparently share a common tetraploid origin. There are three major salmonid taxa classified as subfamilies.