Is evolution of domestication driven by tameness? A selective review with focus on chickens

Abstract

Domestication of animals offers unique possibilities to study evolutionary changes caused by similar selection pressures across a range of species. Animals from separate genera tend to develop a suite of phenotypic alterations referred to as "the domesticated phenotype". This involves changes in appearance, including loss of pigmentation, and alterations in body size and proportions. Furthermore, effects on reproduction and behaviour are typical. It is hypothesized that this recurring phenotype may be secondary effects of the increased tameness that is an inevitable first step in the domestication of any species. We first provide a general overview of observations and experiments from different species and then review in more detail a project attempting to recreate the initial domestication of chickens. Starting from an outbred population of Red Junglefowl, ancestors of all modern chickens, divergent lines were selected based on scores in a standardized fear-of-human test applied to all birds at 12 weeks of age. Up to the eighth selected generation, observations have been made on correlated effects of this selection on various phenotypes. The fear score had a significant heritability and was genetically correlated to several other behavioural traits. Furthermore, low-fear birds were larger at hatch, grew faster, laid larger eggs, had a modified metabolism and increased feed efficiency, had modified social behaviour and reduced brain size. Selection affected gene expression and DNAmethylation in the brains, but the genetic and epigenetic effects were not specifically associated with stress pathways. Further research should be focused on unraveling the genetic and epigenetic mechanisms underlying the correlated side-effects of reduced fear of humans.

Domestication and early experiences in chickens: Behavior, stress and gene expression

“…For example, in chickens, selecting for tameness over only eight generations have led to the low-fear line having larger growth and body size, laying larger eggs, having a modified metabolism and increased feed efficiency, and displaying a reduced brain size and a modified social behavior. Changes were also observed in gene expression and DNA methylation in the brain (Agnvall et al, 2017).…”

“…Selection experiments in modern times, on foxes (Gulevich et al, 2004), rats (Albert et al, 2008) and chickens (Agnvall et al, 2017), have confirmed that selecting only for tameness can in fact lead to the emergence of several of the traits included in the domestic phenotype. For example, in chickens, selecting for tameness over only eight generations have led to the low-fear line having larger growth and body size, laying larger eggs, having a modified metabolism and increased feed efficiency, and displaying a reduced brain size and a modified social behavior.…”

Morphological diversity of integumentary traits in fowl domestication: Insights from disparity analysis and embryonic development

“…Variation in the timing of egg tooth development and different developmental rates could be related to the level of activity (or degree of precociousness) between red junglefowl and chickens, but to establish this relationship more samples and statistical tests will be needed. Red junglefowls, in comparison with their domesticated counterparts, are more active and show higher fear response . Possibly, this behavioral difference related to the domestication process is coupled with morphological variation and early hatching.…”

“…Red junglefowls, in comparison with their domesticated counterparts, are more active and show higher fear response. 96 Possibly, this behavioral difference related to the domestication process is coupled with morphological variation and early hatching.…”

Biologia Futura: adaptive changes in urban populations

“…Increased tolerance towards humans is most often demonstrated by reduced flight initiation distance when approached by a human observer (FID, a widely accepted proxy for risk-taking), a pattern consistent among a high number of species (Blumstein 2019). Disturbance tolerance has a genetic basis, as indicated by selection experiments that successfully produced tolerant lines (Agnvall et al 2018) and by the heritability of FID in wild populations (Bize et al 2012;Møller 2014;Carrete et al 2016). Furthermore, disturbance tolerance appears to be linked to candidate genes (e.g.…”