A turkey today is not the turkey of yesteryear.

For decades, animal breeders have been transforming the genomes of turkeys to make them grow larger. Since 1960, the weight of turkeys has gone up about a quarter of a pound each year. The average weight of a turkey has gone from 15.1 pounds in 1960 to 31.1 pounds in 2017.

Average Turkey Weight 1960-2016

Average turkey weight at slaughter, according to the USDA (Alexis Madrigal)

And most of that change has been genetic. In one study of a representative strain of turkeys, poultry researchers fed the same diet to turkeys from 2003 and to a control group of turkeys that were representative of that strain’s genetic pool from 1966. On average, the 2003 females grew to 33 pounds. Their 1966 cousins only got to 16.3 pounds.

The 2003 turkeys also grew much faster, reaching the same saleable weight twice as fast as their 1966 cousins. Even when they are raised under identical conditions, these are different animals with different genomes. Imagine if the average human were 300 pounds and reached what we now consider adult size at age 10, all while eating less food per pound of human. This is what just 60 years of scientific turkey breeding has wrought.

“Farmers grow for what the consumer wants—and that includes meatier turkeys,” said Keith Williams of the National Turkey Federation. “The live weights of turkeys, due to that consumer desire, has continued to grow, and the amount of feed to raise each pound of turkey continues to be less.”

Less feed reduces farmers’ costs, and it has environmental benefits, too, as less corn and soybeans are required to meet the nation’s turkey demand. And that demand has gone up a lot. In 1950, America’s per-capita turkey consumption was just 5.5 pounds. Nowadays, that number is over 16 pounds, and the National Turkey Federation would like to see it grow to 20 pounds, which would beat the industry’s peak consumption of 18.2 pounds of turkey per American in 1996.

Turkeys are no different from other livestock. Chickens have undergone a very similar process. Cows used to produce 5,000 pounds of milk. Now they produce more than 20,000 pounds. Selective-breeding programs combined with the development of quantitative genetics have combined to create systems for morphing our livestock to better fit the market.

For all these breeding programs, the key technology has been artificial insemination. A given male with the kinds of traits that breeders are looking for can be mated to many, many, many females. One bull, Pawnee Farm Arlinda Chief, had 2 million great-granddaughters, and is responsible for 14 percent of all the genetic material in Holstein cows.

While these programs are analogous to humans’ original domestication of these animals, it is different enough in degree to become different in type. This is market-driven hyperdomestication.

And there are trade-offs. While turkeys’ overall mortality rates do not seem to be higher than earlier generations, they—like our ever larger chickens—do suffer some new kinds of health problems. Their bodies can struggle to hold up their weight, leading to leg problems. And sometimes breeding exclusively for size can have a negative effect on animals’ fitness and fertility, which was an unintended side effect. Poultry scientists now say “fitness traits must be measured and incorporated into selection objectives.”

In turkeys, pure body-weight growth has been decelerating. In the 1980s, turkeys put on an average of 0.25 pounds per year. In the 1990s, that shot up to 0.41 pounds. In the 2000s, the number was down to 0.33. And this decade, we’re looking at 0.22 pounds per year so far.

At some point, the growth of these birds will hit a market or a genetic limit, but it’s an open question which one they will reach first.