HEMISPHERE-SCALE DIFFERENCES IN CONIFER EVOLUTIONARY DYNAMICS

Northern and Southern Hemispheres differ fundamentally with respect to the distribution of seas and landmasses, which influences broad climatic patterns and potentially impacts patterns of biological diversity in the two areas. Conifers present an excellent opportunity to explore how such broad geographic and climatic differences may influence evolutionary dynamics within and between groups of organisms, since most living species belong to lineages that have been broadly confined to the Northern or Southern Hemisphere through the Cenozoic. Based on a critical review of the conifer fossil record, we developed an age-calibrated phylogeny that samples approximately 80% of living conifer species for four chloroplast and nuclear genes. Our results show that most extant conifer species diverged recently during the Neogene within larger clades that were generally established during the later Mesozoic. However, clades that diversified mainly in the Southern Hemisphere contain a greater proportion of old lineages than their counterparts in the Northern Hemisphere. In contrast, most Northern Hemisphere conifer diversity is found in several diverse subclades characterized by very recent species divergences. Our tree topology and estimated divergence times are best fit by diversification models where Northern Hemisphere conifer lineages have higher rates of species turnover compared to Southern Hemisphere lineages. The abundance of recent divergences in northern clades may therefore reflect complex patterns of migration and range shifts during climatic cycles over the later Neogene, leading to elevated rates of speciation and extinction, while the scattered persistence of mild, wetter habitats in the Southern Hemisphere may have favored the survival of older conifer lineages.