[ESS] Erik Tamre (Dartmouth)

“Rate variability and Sadler effect in molecular evolution over geological time”

The paleontological record shows that evolution at the morphological level varies in rate over time, with periods of relative evolutionary stasis punctuated by rapid change. However, the question of rate variability has received less scrutiny at the molecular level. For example, molecular clock approaches rely on a tendency towards clock-like behavior in sequence evolution. The accuracy of this view over geological time is challenging to test because no sequence data survives from the geologically distant past. Molecular phylogenies can estimate the amount of sequence evolution between nodes of the phylogeny, but well-dated and taxonomically diagnostic fossils are required to tie the phylogeny to absolute time.

Paleontological advances and the increasing availability of precise radiometric dates for fossil occurrences now make it possible to measure substitution rates in the geological past for at least some clades. Here, we examine substitution rate variability across the evolutionary history of eukaryotes. We report considerable (over an order of magnitude) variability even in genes chosen for their clock-like behaviour and within particular eukaryotic clades such as Metazoa. Inferred high rates of molecular evolution align with known episodes of fast morphological evolution, such as within early Metazoa in the Ediacaran and Cambrian and within dinoflagellates in the early Mesozoic. We observe that the substitution rate variability creates a scale-dependent observation bias akin to the Sadler effect in rates of sediment accumulation: shorter branches of the phylogeny systematically yield higher inferred substitution rates. This is consistent with punctuated dynamics not just in morphological evolution, but also at the underlying molecular level.

Earth Science Seminar —

Lecture portion of the EAPS graduate-level class 12.571, covering current research in geophysics, geology, geochemistry, and geobiology. All members of the MIT community are welcome to join for presentations by guest speakers, held approximately every two weeks during the term.

Contact: earth-science-seminar-info@mit.edu