During Stalin’s Great Terror in Russia, 1937-38, the geneticist Dmitry Belyaev pursued his studies despite the regime’s persecution of geneticists. In 1959, he made a groundbreaking experiment in Siberia by domesticating wild foxes. The experiment aimed to reveal whether selection for friendliness alone could lead to domestication traits. Belyaev and his protégée, Lyudmila Trut, divided a population of foxes into two groups. They bred one group based on friendliness toward humans, selecting foxes that approached or showed no fear. The other group was bred randomly, regardless of behavior. Over generations, the friendly foxes exhibited significant changes, both behaviorally and physically. They developed traits common in domesticated animals, such as coat color variations, floppy ears, curled tails, smaller teeth, and different skull shapes. These foxes also became more sociable, bred more frequently, and had higher serotonin levels, a neurotransmitter linked to lower aggression.

This study showed that domesticated traits like physical changes and reduced aggression were by-products of selecting for friendliness. These findings challenged the traditional view that domestication requires direct human intervention in breeding for specific traits. Researchers found genetic links between the foxes’ friendly behavior and certain genes. For example, a gene on chromosome 15 was associated with friendly behavior in foxes and similar regions in dogs.