Podocarp Evolution: A Molecular Phylogenetic Perspective

Abstract

Phylogenetic reconstructions of the relationships among extant taxa can be used to infer the nature of the processes that have generated contemporary patterns of biotic diversity. In this study, we present a molecular phylogenetic hypothesis for the conifer family Podocarpaceae based upon three DNA fragments that have been sampled for approximately 90 taxa. We use Bayesian relaxed-clock methods and four fossil constraints to estimate divergence times among the lineages of Podocarpaceae. Our dating analyses suggest that although the family is old (Triassic–Jurassic), the extant species groups are of recent evolutionary origin (mid-to late Cenozoic), a pattern that could reflect a temporal increase in the rate lineage accumulation or, alternatively, a high and constant rate of extinction. Our data do not support the hypothesis that Podocarpaceae have diversified at a homogeneous rate, instead providing strong evidence for a three-to eightfold increase in diversification associated with the Podocarpoid–Dacrydioid clade, which radiated in the mid-to late Cretaceous to the earliest Cenozoic, around 60–94 mya. This group includes a predominance of taxa that develop broad leaves and/or leaflike shoots and are distributed predominantly throughout the tropics. Tropical podocarps with broad leaves may have experienced reduced extinction and/or increased speciation coincident with the radiation of the angiosperms, the expansion of megathermal forests, and relatively stable tropical climates that were widespread through the Tertiary.