The Effects of Insularity on Beak Morphology in a Hybrid and a Non-Hybrid Species

Abstract

Despite an increased recognition that hybridization is an important evolutionary force, we are only starting to investigate the potential for adaptation in hybrid species. Historically, hybridization has been viewed as maladaptive, but transgression or increased genetic or phenotypic variation could contribute to hybrid species’ potential for local adaptation. Investigating phenotypic variation in response to different environmental factors within a hybrid species is neccessary to address whether hybrid species can use hybridization-derived variation to adapt locally. Previous studies comparing island and mainland populations of both Italian and Spanish sparrows have indicated that insularity is an important factor affecting the size and shape of the beak in both parallel and non-parallel ways. Here I investigate the potential to adapt to insularity in the homoploid hybrid Italian sparrow (Passer italiae) and one of its parent species, the Spanish sparrow (Passer hispaniolensis). I focus on differences in beak morphology between the two species and also between insular and mainland individuals. I found significant differences in beak size between insular and mainland individuals, with island individuals having smaller beaks than their mainland conspecifics. For beak shape, I found differences between Italian and Spanish sparrow, as well as between island and mainland individuals. I also present an alternative method for investigating beak and head dimensions, namely 3D scanning. I argue that this is a valuable method for future research as it captures more of the skull anatomy, which can reveal important connections between beak and other anatomical structures.