Evolution under intensive industrial breeding: skull size and shape comparison between historic and modern pig lineages

Domestication and subsequent human-induced selection has enhanced profound changes in animal morphology. On modern domestic pigs, those transformations encompass not only overall increases in body size but also modifications in skull morphology. While skull morphological differences between modern domestic pigs and wild boar are relatively well-documented, less understood is the variation and underlying mechanisms associated with intensive breeding. In this study, we investigated the rate and direction of phenotypic change of skull morphology using a unique dataset that includes two lineages of German domestic pig that were subjected to similar intensive industrial selection pressures throughout the twentieth century, alongside contemporaneous populations of German wild boar. Size and shape variation of 135 specimens was quantified through geometric morphometrics, with 82 three-dimensional landmarks. We find expected differences in skull shape between wild and domestic pigs, but also convergence between the two domestic lineages through the century of directed breeding, despite population segregation. Our results suggest that cranial morphologies have rapidly responded to selection pressure that is independent of genetic isolation. This also suggests that pig morphotypes quickly reflect human agency and impact upon domestic animal phenotypes, revealing a pathway to investigate early human breeding activity in ancient and historical contexts.