The molecular clock analyses give broadly similar results regardless of clock model and tree prior, with the exception of the BSP tree prior, which gives an intriguingly recent tMRCA, and faster substitution rates than the other analyses.
Their evolutionary history is uncertain since their fossil record is patchy and molecular clock studies have given inconsistent results.
A strict molecular clock was assumed given the shallow levels of divergence, with a mitochondrial mean rate of 1.52%% divergence per million years, with a standard deviation of 0.5 % divergence, based on mtDNA calibrations from other squamates [ 69].
Our results reveal that in each of the two regions, the oldest interacting clades are indeed of matching age, at least within the error of molecular clock dating and given our incomplete species sampling, but the North American bird/plant assemblage is roughly half as old as the temperate South American one, yet has more than 3x as many bird-pollinated plant species.
Therefore, molecular clock mechanisms that allow a given species to anticipate the tidal change can be of critical importance to avoid exposure to life-threatening conditions.
Nevertheless, the young age here inferred for Maloideae, based on molecular data and the assumption of a molecular clock, has dramatic biogeographic implications given the distribution of the genera and the history of plate tectonic movements [ 67].
Given our criticism of some of the calibration points used in molecular clock studies, we present a more conservative, and better justified, set of points (Table 3).
Evidently, fossil ages do not directly represent clade ages and it is for this reason that in attempting to derive an evolutionary timescale, literal reading of the fossil record has given way to molecular clock methodology which uses statistical distributions to describe uncertainties in fossil calibrations.
Accurate molecular clock analysis thus can be expected to give an earlier divergence time than the date of the earliest relevant fossil.
Small differences between observed and estimated values suggest that the molecular clock assumption is (approximately) valid for a given dataset and tree topology, whereas large differences indicate that molecular clock is not applicable.
