Body size diminution under domestication: Unconscious selection in primeval domesticates

The unique habitats that were created around permanent settlement sites encouraged certain animals to invade and rapidly colonize the newly opened anthropogenic niches. Most of the species became facultative or obligatory commensals, with extensive morphological and behavioral changes, and in a few cases, with full speciation taking place. The approach outlined here is based on the model of K- and r-selection, and favors the view that changes in body size (as well as other traits) observed in animals undergoing domestication were due to spontaneous morphogenetical responses to the special anthropogenic milieu. The observed reduction in body size of most domesticates is a function of changing reproductive strategies in the face of changes in selection pressures. It is suggested that changes in body size under domestication reflect a shift along the continuum from the selection for individual viability toward selection for higher reproductive rate. The shift in body size occurred as a response to the unique habitats that induced new conditions of food and water availability, relaxation in competition/predation pressures, low species diversity and hence low interspecific competition, but a significant increase in intraspecific competition due to the artificial decrease in the niche volume of each individual, by permitting higher population densities. The relieved selective pressures and high intrapopulation competition for resources associated with domestication set in motion a cyclical reaction of accelerated maturation, increased reproductive capacity with a tendency for larger litter sizes, and shortened generation time, leading tooverall size reduction in the population, and other traits connected with r-selection strategy.