Evolution of human commensalism in Passer sparrows - effects on skull morphology and brain size

Abstract

Human activities have historically influenced the evolution of other species. Dramatic modifications in land-use and extreme urbanization have increased not only the amount of species associated with human environments, but also the intensity of interaction among them. Hence, the evolution of these organisms can lead us to gain a deeper understanding of our own history. House sparrows (Passer domesticus) are a passerine bird species distributed worldwide due to their close association with anthropogenic environments. The species likely spread from the Middle East, with the advent of the Neolithic revolution less than 10Kya. This shift towards highly variable rural and urban niches has driven a change in diet preferences (mainly based on cultivated cereals such as wheat and barley), which has potentially triggered differences in skull morphology and biting mechanical advantage. Their adaptation to these unpredictable environments may be also reflected in an increase of relative brain size and signatures of selection for genes associated with skull morphology. In this project, we used 3D geometric morphometric approaches based on microCT scans to study skull morphological adaptations to anthropogenic niches in European and Iranian house sparrow subspecies, Spanish, Italian and tree sparrows. We used the subspecies P. domesticus bactrianus as a proxy of ancestral non-commensal ecology. Biting mechanical advantage was calculated to study feeding performance and relative brain size was examined to test whether larger brains tend to develop in commensal species. In addition, we performed genome scans in order to look for signatures of selection of candidate genes associated with craniofacial morphology. We identified significant differences in skull morphology and relative brain size between commensal and non-commensal groups. Estimates of biting mechanical advantage calculations showed a slight trend towards a more forceful bite in commensal species. At the genomic level, we detected strong signatures of selection for two candidate genes, which play a role in beak depth and length and craniofacial morphology. All these differences between commensals and non-commensals may shed light in understanding the adaptation of house sparrows to human-made environments.