The domestic turkeys that will grace Thanksgiving dinner tables across the United States are genetically distinct from their wild ancestors, researchers say.

All commercial turkey lines descend from a South Mexican turkey subspecies that is extinct in the wild, but they show less genetic diversity than their forbears, they said.

"Ancient turkeys weren't your Butterball," said Rob Fleischer, head of the Smithsonian Conservation Biology Institute's Center for Conservation and Evolutionary Genetics.

"We set out to compare the genetic diversity of the domestic turkeys we eat today with that of the ancestral wild turkey from South Mexico," he said in a Smithsonian release Monday. "Some of what we found surprised us."

The domestic turkey that ends up on the dinner table exhibits less genetic variation than not only its ancestral wild counterparts, which were first domesticated in 800 B.C., but also than other livestock breeds, such as domestic pigs or chickens, Fleischer said.

The genetic traits involved are those responsible for body size and breast muscle development, the very features that determine the likelihood of a consumer buying a turkey, the researchers said.

"Few people know that the commercial turkeys served at Thanksgiving descended from Mexico, where they were discovered during the Spanish Conquest and transported to Europe," said Julie Long of the U.S. Department of Agriculture's Agricultural Research Service in Beltsville, Md. "During the next 100 years, Europeans created many different varieties of the domesticated turkey."

That domestication had a genetic impact, researchers said.

"It is often the case that selection in domestication reduces the level of variation," Fleischer said.