Why Does the Human Scalp Donor Zone Persist? An Evolutionary Hypothesis of Differential Follicular Preservation

Androgenetic alopecia removes hair in a highly reproducible topographic pattern, yet it consistently spares an occipito-parietal band of terminal follicles across populations and ethnicities. This preserved zone is the donor region on which hair restoration surgery depends, through the principle of donor dominance. The proximate basis of the pattern — the differential androgen sensitivity of follicles — is well characterised, but it explains how the boundary is enforced, not why the preserved zone occupies this particular region. Here I propose that the topography of preservation reflects differential regional selective pressure acting during the long pronograde phase of the lineage, when the spatial orientation of the cranium plausibly placed the occipito-parietal surface among the regions of greatest cumulative solar exposure. On this view the donor zone did not acquire resistance but conserved the ancestral programme of follicular cover with greater fidelity, under more intense purifying selection than other scalp regions. The hypothesis is scoped to the anatomical distribution of androgenetic susceptibility, complements developmental and molecular accounts rather than replacing them, and yields eight testable predictions.