Abstract
Early animal domestication may have been driven by selection on tameness. Selection on only tameness can bring about correlated selection responses in other traits, not intentionally selected upon, which may be one cause of the domesticated phenotype. We predicted that genetically reduced fear towards humans in Red Junglefowl, ancestors of domesticated chickens, would be correlated to other traits included in the domesticated phenotype. Fear level was determined by a standardised behaviour test, where the reaction towards an approaching human was recorded. We first selected birds for eight generations for either high or low fear levels in this test, to create two divergent selection lines. An F3 intercross, with birds from the eighth generation as parentals, was generated to study correlations between fear‐of‐human scores and other unselected phenotypes, possibly caused by pleiotropy or linkage. Low fear‐of‐human scores were associated with higher body weight and growth rates, and with increased activity in an open field test, indicating less general fearfulness. In females, low fear‐of‐human scores were also associated with more efficient fear habituation and in males with an increased tendency to emit food calls in a mirror test, indicating increased social dominance. Low fear‐of‐human scores were also associated with smaller brain relative to body weight, and with larger cerebrum relative to total brain weight in females. All these effects are in line with the changes observed in domesticated chickens compared to their ancestors, and we conclude that tameness may have been a driving factor underlying some aspects of the domesticated phenotype.
Highlights
Animal domestication has been defined as a process in which humans are responsible for the selection of traits that cause changes in a population,[1] as such it is a special case of evolution driven by humans
To shed light on the role of tameness in the evolution of domesticated phenotypes in chickens, we studied an F3-intercross between two lines of Red Junglefowl, previously selected during eight generations for divergent scores in a “fear-of-humans (FOH) test”
We found that body weight and growth rate was correlated with FOH-score, in that less fearful birds were bigger and grew faster
Summary
Animal domestication has been defined as a process in which humans are responsible for the selection of traits that cause changes in a population,[1] as such it is a special case of evolution driven by humans. Natural selection still plays a relevant role.[2] Individuals that cannot cope with the captive environment are not likely to thrive, regardless of human intervention. Early domestication in animals is thought to have been initially driven largely by selection on tameness.[3,4] An interesting aspect of selection for tameness, whether done consciously or not, is that other traits that are not selected upon can change due to being genetically correlated.
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