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Biogeography and diversification rates in hornworts: The limitations of diversification modeling

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Hornworts comprise ca. 220 species and are among the oldest landplant lineages, even though their precise phylogenetic position remains unclear. Deep within-hornwort divergences, highly uneven species numbers per genus, and the assumed high stem age together suggest a history of changing diversification (i.e., speciation minus extinction) rates. To study the geographic distribution of modern hornworts and their patterns of species accumulation, we generated a mitochondrial and plastid DNA matrix for 103 species representing all major groups and then applied molecular-clock dating, using a different calibration approach than in earlier work. We used the BAMM software to fit rate-variable and constant-rate birth-death diversification models to the dataset, and we also inferred ancestral areas to a time depth of 55 Ma (Early Eocene). We analyzed diversification rates for all hornworts and separately for species-rich subclades. Under BAMM's variable-rates model (which fits the data better than a constant-rate birth-death model, but still assumes that each species has the same speciation and extinction probability regardless of its age), hornworts have gradually increasing rates of speciation and a constant background extinction rate. No shifts in diversification rate could be detected. The implausible finding of a constant background extinction rate illustrates the limitations of diversification modeling especially as regards extinction rates.
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Keywords: BIOGEOGRAPHY; DIVERSIFICATION MODELING; EXTINCTION RATES; GEOGRAPHIC DISJUNCTIONS; HORNWORTS

Document Type: Research Article

Affiliations: 1: Systematic Botany and Mycology, Department of Biology, University of Munich (LMU), Menzingerstr. 67, 80638 Munich, Germany;, Email: [email protected] 2: Systematic Botany and Mycology, Department of Biology, University of Munich (LMU), Menzingerstr. 67, 80638 Munich, Germany

Publication date: 2015-05-05

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