Re: Fossil calibration

>Dating the deep divergence times between higher taxonomic levels, however, still seems to be a serious problem, as the "molecular clock" almost never seems to run very clock-like. A calibration by means of well-dated fossils is required for every evolutionary divergence point, with the big problem being a run-away propagation of statistical and systematic errors.
>In a recent paper, Graur D., Martin W., "Reading the entrails of chickens: molecular timescales of evolution and the illusion of precision", Trends in Genetics 20 (2004), 80-86, wrote (their abstract):
>"For almost a decade now, a team of molecular evolutionists has produced a plethora of seemingly precise molecular clock estimates for divergence events ranging from the speciation of cats and dogs to lineage separations that might have occurred ~4 billion years ago. Because the appearance of accuracy has an irresistible allure, non-specialists frequently treat these estimates as factual. In this article, we show that all of these divergence-time estimates were generated through improper methodology on the basis of a single calibration point that has been unjustly denuded of error. The illusion of precision was achieved mainly through the conversion of statistical estimates (which by definition possess standard errors, ranges and confidence intervals) into errorless numbers. By employing such techniques successively, the time estimates of even the most ancient divergence events were made to look deceptively precise. ...
>I wonder whether Graur and Martin describe a general problem, or whether they are just pointing to some excesses. How do you evaluate the situation?<

The problem is very widespread in molecular clock papers, although a few do use multiple calibration points across many lineages and nonlinear models of rates of change. At worst, some molecular clock papers simply lift a rate from another molecular clock paper. Barely better is lifting a date or two from another molecular clock paper. Many confounding factors are also known. As an example, the 18S gene has major differences within the bivalve family Cardiidae, with groups that diverged during the past 65 million years, whereas the one crassatellid and astartid that have been sequenced show no divergence over ca. 300 million years. Some molecular biologists seem clueless about the fossil record, if not about organisms in general.

>Even more, however, I would be interested in some reliable fossil dates (including realistic error margins and references to the primary literature) which could reasonably be used as bounds for divergence time estimates with respect to higher taxonomic levels.<

Michael Benton, The Fossil Record II, is the most comprehensive source for that sort of thing that I know of.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL 35487-0345 USA
    bivalve@mail.davidson.alumlink.com

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

                 
Received on Tue Mar 30 00:25:58 2004