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Molecular Biology and Evolution, Vol 3, 205-221, Copyright © 1986 by Society for Molecular Biology and Evolution
PD Gingerich
Molecular clocks are routinely tested for linearity using a relative rate
test and routinely calibrated against the geological time scale using a
single or average paleontologically determined time of divergence between
living taxa. The relative rate test is a test of parallel rate equality,
not a test of rate constancy. Temporal scaling provides a test of rates,
where scaling coefficients of 1.0 (isochrony) represent stochastic rate
constancy. The fossil record of primates and other mammals is now known in
sufficient detail to provide several independent divergence times for major
taxonomic groups. Molecular difference should scale negatively or
isochronically (scaling coefficients less than 1.0) with divergence time:
where two or more divergence times are available, molecular difference
appears to scale positively (scaling coefficient greater than 1.0). A
minimum of four divergence times are required for adequate statistical
power in testing the linear model: scaling is significantly nonlinear and
positive in six of 11 published investigations meeting this criterion. All
groups studied show some slowdown in rates of molecular change over
Cenozoic time. The break from constant or increasing rates during the
Mesozoic to decreasing rates during the Cenozoic appears to coincide with
extraordinary diversification of placental mammals at the beginning of this
era. High rates of selectively neutral molecular change may be concentrated
in such discrete events of evolutionary diversification.
ORIGINAL ARTICLE
Temporal scaling of molecular evolution in primates and other mammals
Museum of Paleontology, University of Michigan, Ann Arbor 48109.
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