Is There a Constant Rate of Molecular Substitution:A Critical Reappraisal of the Molecular Clock
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概要
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The current concept of the molecular clock is primarily based on the assumption that there is a constant rate of molecular substitution in all mammals. The calibration of the albumin clock of SARICH & CRONIN (1976) set 100 albumin immunological distance units (IDU) between two taxa equal to a most probable divergence time of 60 m, y. ago, based primarily on the early primate data. On this scale the marsupial-placental albumin immunological distance is about 210 units, corresponding to a period about 125m.y. ago, and the antiquity of the basic placental adaptive radiation is set at 90-100 m.y. ago. These values are will in accord with the paleontological data. According to SARICH & CRONIN, the catarrhine-platyrrhine divergence must have occurred about half as long ago as the beginning of of the prosimian-anthropoid split (70m.y. ago), that is, about 35m. y. ago. The conclusion that the catarrhine-platyrrhine divergence occurred about 35m. y. ago is in concordance with the paleontological records. However, the data of 70m. y. ago for the prosimian-anthropoid split does not agree with the initial assumption of a divergence date of 60m.y. ago given the substitution rate as 0.6m. y. per 1 IDU. Moreover, the paleontological records suggest that their divergence date is 50m.y. ago. These data do not support the assumption that there is a constant rate of molecular substitution. FITCH & LANGLEY (1976) described the relationship between nucleotide substitution and the divergence dates of mammals. The linear relationship between time elapsed and nucleotide substitution is beautifully observed, but all the primate pairs fall significantly below the line. From these results, FITCH & LANGLEY concluded that primates show a lower evolutionary rate of nucleotide substitution. However, they did not show the magnitude of the slowing down of its rate in primates. To correct the observed divergence of the prime data, the primate pairs were re-plotted according to the following method: the numbers for nucleotide substitution of each pair are unchanged, but the paleontologically estimated dates of divergence of each pair are shortened to one-half of the elapsed time estimates used by FITCH & LANGLEY. These new points are beautifully arranged closed to the original correlation line. Most of the new points are still below the line, but the older divergent pair (cercopithecids-pongids) is replotted above the line. This means that the divergence dates predicted by using the cor relation line represent less than half the elapsed time estimated paleontologically, except for the elder divergent pair. This evidence may indicate that the magnitude of the discrepancy between the paleontologically estimated dates and the dates predicted by the molecular clock depends on relative age of divergence among anthropoids: the more recent divergence date, the greater the magnitude of the discrepancy.
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