Re: Fossil calibration

From: Peter Ruest <>
Date: Sat Apr 10 2004 - 05:11:49 EDT

David Campbell wrote:
> I don't know of anything as exhaustive as the Fossil Record volumes, but several references exist for individual groups. For example, there are newer Treatise on Invertebrate Paleontology volumes for several phyla.
> Nature, Science, PNAS, etc. seem rather enthusiastic about molecular biology, which tends to be much stronger on molecular than on biology. Thus, the review probably emphasized molecules rather than paleontology or statistics. I suspect that the situation is worse for molecular biology journals. The problem is not confined to this field-in general, beware of claims outside the area of expertise of the authors.<

PR: Agreed. I wonder that often molecular biologists apparently have not
been sufficiently aware of these problems with molecular "clocks". In
fact, the possibility of many of these deviations could have been
predicted on a theoretical basis.
> There are molecular clock studies that use more realistic models than a linear clock, as well as studies that have plausible error bars (very wide). Additional problems for clock models have recently been identified. For example, in an interval of evolutionary radiation, a larger proportion of the genetic variability will be preserved in various lineages than during ordinary background evolution. This will make the molecular clock dates for radiation events look too old. Redating evolutionary radiations has been a popular application of molecular clocks. Changes in environmental mutagens (radiation, ozone, etc.) become significant over very long time intervals.<

PR: Of course, it's only with today's greatly increased amount of
sequence data that the deviations could be tracked sufficiently to
determine estimates of these effects by statistical tests of more
realistic models.
> As an example, the 18S gene, with about 1800 bases, shows no difference between Eucrassatella and Astarte, despite a divergence at least in the Cretaceous, whereas species of Tridacna that diverged within the Cenozoic show very large differences (including a lot of extra bases).<

PR: Molecular biologists still have a lot to investigate. And so have
those on the fossil morphology side. Hlusko L.J., Integrating the
genotype and phenotype in hominid paleontology (Perspective), PNAS 101
(2004), 2653-2657, discusses some problems with constructing phylogenies
based on fossil evidence: Abstract:

"Competing interpretations of human origins and evolution have recently
proliferated despite the accelerated pace of fossil discovery. These
controversies parallel those involving other vertebrate families and
result from the difficulty of studying evolution among closely related
species. Recent advances in developmental and quantitative genetics show
that some conventions routinely used by hominid and other mammalian
paleontologists are unwarranted. These same advances provide ways to
integrate knowledge of the genotype into the study of the phenotype. The
result is an approach that promises to yield a fuller understanding of
evolution below the family level." Introduction: "...Within hominid
paleontology, these difficulties and limitations have generated three
widespread, often tacit, questionable presumptions: (i) most anatomical
traits are independent, (ii) most anatomical traits are adaptively
informative, and (iii) small-scale morphological change is almost always
parsimonious..." Hlusko suggests that the "quantitative trait loci"
which, in the phenotype, yield the morphological data on which fossil
phylogenies are constructed need to be interpreted on the basis of the
genotype determined by means of molecular data.

You are probably in a better position to judge whether such difficulties
occur in non-hominid paleontology, as well.

Thank you, David, for these additional remarks!


Dr. Peter Ruest, CH-3148 Lanzenhaeusern, Switzerland
<> - Biochemistry - Creation and evolution
"..the work which God created to evolve it" (Genesis 2:3)
Received on Sat Apr 10 05:12:33 2004

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