Abstract
Elevational gradients are frequently linked to dramatic biodiversity shifts, but the drivers and dynamics of alpine adaptation often remain unclear. A Batesian mimicry polymorphism in Aotearoa New Zealand stoneflies provides an ideal system for assessing the role of elevational gradients in selecting for melanic (dark) versus non-melanic (light) colour phenotypes. Although previous studies have proposed that melanism should be favoured in alpine ecosystems, we find the opposite pattern, with consistently reduced frequencies of dark phenotypes detected in upland versus lowland habitats. Specifically, parallel alpine reductions in frequencies of melanic (mimic) Zelandoperla (approx. 1500 specimens genotyped across 7 independent stream populations) are linked to reduced abundance of the noxious model Austroperla above the alpine treeline. Our findings highlight that shifting ecological interactions across environmental gradients can drive rapid evolutionary change over fine spatial scales.