Abstract
BackgroundA positive relationship between diversification (i.e., speciation) and nucleotide substitution rates is commonly reported for angiosperm clades. However, the underlying cause of this relationship is often unknown because multiple intrinsic and extrinsic factors can affect the relationship, and these have confounded previous attempts infer causation. Determining which factor drives this oft-reported correlation can lend insight into the macroevolutionary process.ResultsUsing a new database of 13 time-calibrated angiosperm phylogenies based on internal transcribed spacer (ITS) sequences, and controlling for extrinsic variables of life history and habitat, I evaluated several potential intrinsic causes of this correlation. Speciation rates (λ) and relative extinction rates (ε) were positively correlated with mean substitution rates, but were uncorrelated with substitution rate heterogeneity. It is unlikely that the positive diversification-substitution correlation is due to accelerated molecular evolution during speciation (e.g., via enhanced selection or drift), because punctuated increases in ITS rate (i.e., greater mean and variation in ITS rate for rapidly speciating clades) were not observed. Instead, fast molecular evolution likely increases speciation rate (via increased mutational variation as a substrate for selection and reproductive isolation) but also increases extinction (via mutational genetic load).ConclusionsIn general, these results predict that clades with higher background substitution rates may undergo successful diversification under new conditions while clades with lower substitution rates may experience decreased extinction during environmental stasis.
Highlights
A positive relationship between diversification and nucleotide substitution rates is commonly reported for angiosperm clades
For the non-phylogenetic correlation, r2 = 0.32, slope = 91.18, F1,11 = 5.16, P = 0.04 (Figure 1a). This positive relationship suggests that the correlation between diversification and rates of molecular evolution is due to a process of enhanced speciation rates in faster-evolving lineages, rather than decreased extinction
Plant species from clades characterized by high nucleotide substitution rates tend to both speciate and go extinct at higher rates than species from more slowlyevolving clades
Summary
A positive relationship between diversification (i.e., speciation) and nucleotide substitution rates is commonly reported for angiosperm clades. Hypothesized causes of the positive relationship between molecular evolution and diversification may be divided into two categories. Hypotheses for extrinsic causes suggest that aspects of a clade's ecology (its habitat or traits adapted to habitat) affect rates of both molecular evolu-. Increased generation time can cause mutations to accumulate faster over absolute time than they do in organisms with a slower life history [7,20,34] Shorter generation times are positively correlated with species richness of clades [8], possibly because shortened life cycles and/or enhanced mobility associated with smaller size allow individuals to colonize more extreme habitats. Environmental energy directly affects both species-richness and faster evolution [4]
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