The evolution of methods for establishing evolutionary timescales
Philip C. J. Donoghue, Ziheng Yang
Published 20 June 2016. DOI: 10.1098/rstb.2016.0020
Tip-calibration relies upon a molecular sequence alignment from living species, a morphological character set for living and fossil species, and a prior topology (a); total-evidence dating co-estimates topology and timescale. Branch lengths are estimated in a Bayesian MCMC approach based on both data types for living lineages and based on morphological data alone for the extinct lineages; these are calibrated to time based on the age of the fossil species (b). The divergence time estimates and inferred rates of molecular and morphological evolution are based on a consensus of the MCMC analysis (c)
The fossil record is well known to be incomplete. Read literally, it provides a distorted view of the history of species divergence and extinction, because different species have different propensities to fossilize, the amount of rock fluctuates over geological timescales, as does the nature of the environments that it preserves. Even so, patterns in the fossil evidence allow us to assess the incompleteness of the fossil record. While the molecular clock can be used to extend the time estimates from fossil species to lineages not represented in the fossil record, fossils are the only source of information concerning absolute (geological) times in molecular dating analysis. We review different ways of incorporating fossil evidence in modern clock dating analyses, including node-calibrations where lineage divergence times are constrained using probability densities and tip-calibrations where fossil species at the tips of the tree are assigned dates from dated rock strata. While node-calibrations are often constructed by a crude assessment of the fossil evidence and thus involves arbitrariness, tip-calibrations may be too sensitive to the prior on divergence times or the branching process and influenced unduly affected by well-known problems of morphological character evolution, such as environmental influence on morphological phenotypes, correlation among traits, and convergent evolution in disparate species. We discuss the utility of time information from fossils in phylogeny estimation and the search for ancestors in the fossil record.
This article is part of the themed issue ‘Dating species divergences using rocks and clocks’.
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