New rocks for molecular clocks
In the sixties Linus Pauling and Emile Zuckerkandl noticed that hemoglobin amino acid sequences varied in a way that was proportional to the divergence of species. So they figured that such differences could be used to as a kind of clock to tell how long ago that two protein sequences diverged. The technique was gradually developed by other people over time, and is now most often applied to DNA sequences. In order to calibrate the clock, corresponding sequences of two species for which the date of divergence is known from the fossil record are compared.
A recent BioEssay paper argues that the commonly used calibration date, where mammals and birds diverged, is not known with enough certainty to be used, because:
(a) there are not enough fossil localities from the early Late Carboniferous that predate the earliest known evidence of the split,
(b) the same is true for the number of fossil localities immediately after the first appearance of amniotes and
(c) there are still several long ghost lineages in the phylogeny of early amniotes and their sister taxa. Thus, the fossil record simply does not have the required resolution for bracketing with confidence the evolutionary event associated with the mammal-bird split, as there is currently no closely related fossil taxon that provides a reasonable maximum age.
They also feel that multiple dates should be used, and have selected four dates for calibrating the molecular clocks:
Lungfish-Tetrapods 419-408 Ma
Bird-Lizard 257-252 Ma
Crocodile-Bird 251-243 Ma
Alligator-Caiman 71-66 Ma
A recent BioEssay paper argues that the commonly used calibration date, where mammals and birds diverged, is not known with enough certainty to be used, because:
(a) there are not enough fossil localities from the early Late Carboniferous that predate the earliest known evidence of the split,
(b) the same is true for the number of fossil localities immediately after the first appearance of amniotes and
(c) there are still several long ghost lineages in the phylogeny of early amniotes and their sister taxa. Thus, the fossil record simply does not have the required resolution for bracketing with confidence the evolutionary event associated with the mammal-bird split, as there is currently no closely related fossil taxon that provides a reasonable maximum age.
They also feel that multiple dates should be used, and have selected four dates for calibrating the molecular clocks:
Lungfish-Tetrapods 419-408 Ma
Bird-Lizard 257-252 Ma
Crocodile-Bird 251-243 Ma
Alligator-Caiman 71-66 Ma
The main problem, which they admit, is that the primary reason why the mammal-bird split is used is because of the large amount of genetic data that is available for birds and mammals. Until genomic sequence data is available for representatives of the different phylogenic groups, the scheme will be difficult to use. Once that data is available it will be interesting to see whether the new data causes any changes to the family tree.
Miller, J. & Reisz, R. BioEssays. 27:1069-1075. (2005)
Miller, J. & Reisz, R. BioEssays. 27:1069-1075. (2005)
posted by blogloggingsuperstar @ 4:31 PM
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