High Probability Distributions Research Articles

Orchid historical biogeography, diversification, Antarctica and the paradox of orchid dispersal

AbstractAimOrchidaceae is the most species‐rich angiosperm family and has one of the broadest distributions. Until now, the lack of a well‐resolved phylogeny has prevented analyses of orchid historical biogeography. In this study, we use such a phylogeny to estimate the geographical spread of orchids, evaluate the importance of different regions in their diversification and assess the role of long‐distance dispersal (LDD) in generating orchid diversity.LocationGlobal.MethodsAnalyses use a phylogeny including species representing all five orchid subfamilies and almost all tribes and subtribes, calibrated against 17 angiosperm fossils. We estimated historical biogeography and assessed the importance of different regions for rates of speciation, extinction and net species diversification. We evaluated the impact of particularLDDevents on orchid diversity by asking how many species evolved in the new range subsequent to those events.ResultsOrchids appear to have arisen in Australia 112 Ma (95% higher probability distribution: 102.0–120.0 Ma), then spread to the Neotropics via Antarctica by 90 Ma (HPD: 79.7–99.5 Ma), when all three continents were in close contact and apostasioids split from the ancestor of all other orchids. Ancestors of vanilloids, cypripedioids and orchidoids+epidendroids appear to have originated in the Neotropics 84–64 Ma. Repeated long‐ and short‐distance dispersal occurred through orchid history: stochastic mapping identified a mean total of 74LDDevents or 0.8 Ma−1. Across orchid history, Southeast Asia was the most important source and maximally accelerated net diversification; across epidendroids, the Neotropics maximally accelerated diversification.Main conclusionsOur analysis provides the first biogeographical history of the orchids, implicating Australia, the Neotropics and Antarctica in their origin.LDDand life in the Neotropics – especially the Andes – had profound effects on their spread and diversification; > 97% of all orchid species are restricted to individual continents.

Molecular structure of side-chain liquid crystalline polysiloxane in the smectic C phase from X-ray diffraction and molecular modeling

The structure of a side-chain liquid crystalline polysiloxane containing two oligo(oxyethylene) units and (R)-1-methylheptyl 4-(4-hydroxybiphenyl-4′-carbonyloxy) benzoate mesogens (PS221B) has been studied using X-ray diffraction patterns and molecular modeling. Ten periodic simulated cells with equilibrated structures are generated, each containing 35 mesogenic groups and a polysiloxane backbone. The simulated X-ray diffraction patterns are in agreement with those obtained experimentally. The calculated width of the sublayers of the polysiloxane backbone is about 5.63Å along the z-axis. The distributions of the dihedral angles of the polysiloxane backbone in the equilibrated structures indicate that the backbone has a higher probability distribution in the trans state. The distributions of the dihedral angles at the bonds in the aromatic core of the mesogenic groups reveal high probabilities of the trans and cis placements. The local order due to the interactions between the aromatic cores is in the range of 3–5Å. The order of the radial distribution function vanishes beyond this range.