Hemisphere-scale differences in conifer evolutionary dynamics

Abstract

Fundamental differences in the distribution of oceans and landmasses in the Northern and Southern Hemispheres potentially impact patterns of biological diversity in the two areas. The evolutionary history of conifers provides an opportunity to explore these dynamics, because the majority of extant conifer species belong to lineages that have been broadly confined to the Northern or Southern Hemisphere during the Cenozoic. Incorporating genetic information with a critical review of fossil evidence, we developed an age-calibrated phylogeny sampling ∼80% of living conifer species. Most extant conifer species diverged recently during the Neogene within clades that generally were established during the later Mesozoic, but lineages that diversified mainly in the Southern Hemisphere show a significantly older distribution of divergence ages than their counterparts in the Northern Hemisphere. Our tree topology and divergence times also are best fit by diversification models in which Northern Hemisphere conifer lineages have higher rates of species turnover than Southern Hemisphere lineages. The abundance of recent divergences in northern clades may reflect complex patterns of migration and range shifts during climatic cycles over the later Neogene leading to elevated rates of speciation and extinction, whereas the scattered persistence of mild, wetter habitats in the Southern Hemisphere may have favored the survival of older lineages.

Fig. 1.

(A) Dated phylogeny for 489 extant conifer species with cycads as an outgroup. Dashed circles with dates in millions of years ago (Ma) indicate boundaries between the Paleozoic/Mesozoic and Mesozoic/Cenozoic. The 95% confidence intervals on node ages are colored according to the geographic range of extant descendant species. Gray indicates a node that joins extant taxa with both Northern and Southern Hemisphere distributions, red indicates a node joining only extant Southern Hemisphere taxa, and blue indicates a node joining only extant Northern Hemisphere taxa. (B) Scaled density distributions of node ages (in million years; My) for all Northern and Southern Hemisphere clades. (C) Median molecular branch lengths in Northern Hemisphere clades, Southern Hemisphere clades, and Northern Hemisphere clades that were resampled to 50% species sampling; error bars represent 95% confidence intervals based on median node ages for 10,000 resampled trees. (D) Observed median node ages for major Northern and Southern Hemisphere conifer clades.

Fig. 2.

Diversification dynamics of Northern and Southern Hemisphere conifer clades. (A) Lineage accumulation over the Cenozoic in Northern Hemisphere clades (Pinaceae, Cupressoideae) and Southern Hemisphere clades (Araucariaceae, Podocarpaceae, and Callitroideae). (B) Estimated turnover rates for Northern and Southern Hemisphere clades. Error bars represent 95% confidence intervals derived from fitting a constant diversity with turnover model to 1,000 different dated trees sampling the posterior age distributions from the BEAST analysis.