Size and shape changes in relation to the phylogeography of West Eurasian and North African house mice (<i>Mus musculus</i>)

Abstract Understanding how morphological variation reflects phylogeographical structure is fundamental to studying microevolutionary processes. This study examines ventral skull morphology in 410 house mice (Mus musculus) from 22 populations across Eurasia and North Africa using landmark-based geometric morphometrics. Size and shape variation are analysed in relation to phylogenetic lineages and colonization history, focusing on three major subspecies: M. m. domesticus, M. m. musculus, and an eastern group (ORIENT) including Iranian, Pakistani, and Indian populations. Skull size increases significantly in colder climates, suggesting persistent climatic selection despite the species’ commensal habits. Introgressed populations show larger residual sizes, probably due to heterosis. Although shape differences among genetic groups are significant, morphological separation between domesticus and musculus is weak, expressed primarily along the third relative warp axis. The greatest divergence is observed between ORIENT and the Danish hybrid zone (DKHYB), consistent with a colonization axis from South Asia to Northern Europe. Shape variability is highest in ORIENT and DKHYB populations, parallelling elevated genetic polymorphism. Robertsonian fusions have limited effect on mean shape but increase morphological variance. These findings support a model of northwestward expansion from South Asia and highlight the value of combining morphometric and genetic data to reconstruct evolutionary history.