Molecular dating rates vary

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According to the molecular clock hypothesis, the rate of molecular evolution is constant over time and across lineages (1).The molecular clock can be used to generate a rate of molecular change per unit of time that can be applied across a phylogenetic tree to estimate ordinal times for phylogenetic nodes.Although the molecular clock hypothesis posits that the rate of molecular change is constant over time, there is evidence that rates vary among lineages.Some of the strongest evidence for variable molecular rates comes from the primates; e.g., the “hominoid slowdown.” These rate differences are hypothesized to correlate with certain species attributes, such as generation time and body size.Our analysis does not support a model where body weight or age at first reproduction strongly influences rates of molecular evolution across mitochondrial and nuclear sites.

There is evidence that aspects of an organism's phenotype correlate with its substitution rate, especially GT (3, 7, 16–18) and BS (19–22). The GT hypothesis posits that the rate of molecular evolution is inversely correlated with GT; e.g., species with longer GTs will exhibit slower rates of molecular evolution.Few studies have tested between these two competing hypotheses.This is partly because of the strong correlations between BS, basal metabolic rate, and GT; e.g., mammals with relatively larger BSs usually have longer GTs and a lower basal metabolic rate.Currently, there is no consensus as to whether the GT or BS hypothesis is a better fit to the observed variation in evolutionary rates (ref. The GT hypothesis assumes that most germ-line mutations occur during DNA replication.Species with longer generations go through fewer DNA replications and accumulate fewer mutations due to replication errors.

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