Tempo and mode of morphological evolution in Australian and New Guinea diving beetles from underground waters to high-elevation peat swamps.

Transitions into novel environments expose species to altered biotic and abiotic selection pressures, promoting phenotypic diversification and the emergence of distinct ecological niches. While phenotypic change is often attributed to adaptive selection, non-adaptive processes, including relaxed selection and genetic drift, may also contribute to morphological evolution. Over the past c.10 million years, Australian and New Guinea diving beetles (Limbodessus, Dytiscidae) have repeatedly diversified across three contrasting environments: above-ground in lowland-montane waters and high-elevation habitats, and below-ground in groundwater aquifers. The recurrent habitat transitions serve as natural "evolutionary experiments" to investigate different selection pressures. Using a phylogenetic comparative approach, we examined the tempo and mode of Limbodessus body-shape evolution to distinguish between adaptive and relaxed selection across ecotypes. Lowland-montane dytiscids exhibit convergent evolution and slow morphological rates of evolution, consistent with stabilising selection maintaining streamlined forms. Subterranean species present high morphological disparity and diffuse morphospace occupation, suggesting relaxed or neutral evolution. High-elevation species show rapid morphological evolution into novel morphospace regions, suggesting strong selection pressures distinct from those in lowland environments. Together, our findings emphasise the importance of relaxed selection and neutral processes in shaping phenotypic variability during ecological transitions.