The role of unconscious selection in the domestication of sheep and goats

Abstract

It is proposed that many of the morphological, physiological and behavioural traits that characterize domestic sheep and goats and distinguish them from their wild ancestors (e.g. diminished sexual dimorphism; diminution in brain, body and horn size and changes in shape of the horns; changes in pelage coloration) were shaped, to a large extent, by unconscious selection. It is argued that once founder herds had been assembled and controlled by humans, the very transfer of these animals from their wild environments into the widely different human-made husbandry system caused automatically drastic changes in selection pressures. Several adaptations, vital for survival in the wild, lost their fitness under the new conditions and broke down. New traits (which characterize domestic caprines) were promptly and unconsciously selected for. Protection from predators, culling of young males, protection from the elements, and changes in land use and in food and water supplies are considered the main ecological factors introduced by humans at the start of caprine domestication.

We evaluate the shifts in the selection pressures brought about by each of these changes, and sketch their expected impact on the morphology, physiology and behaviour of the human-controlled herds. We also indicate which of the domestication traits (expected to be automatically selected for under the new set of conditions) are detectable in animal remains recovered from archaeological excavations.