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Seph · 09-11-02

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Molecular Clock

Phenotypic evolutionary changes are highly erratic. Some lineages may change rapidly within a short time, while others appear not to have changed their phenotypes for many millions of years. Early protein sequence comparisons suggested that, at the molecular level, amino acid replacements might accumulate at a much more regular pace. Indeed, if a large proportion of nondeleterious mutations were neutral in terms of their effect on fitness, their fate in the gene pool would be determined by random genetic drift and fixations could be expected to occur at more or less regular intervals. It would, however, be a stochastic regularity, rather like that leading to the expectation of obtaining 50% heads in coin flipping: the more times you flip a coin, the closer the match between the observed and the expected result. The supposition that molecules evolve at an approximately constant rate is termed the molecular clock. According to the molecular clock hypothesis, the number of amino acid replacements (and by extension also the number of nucleotide substitutions) found between proteins (nucleic acid segments) of two species is proportional to the time elapsed since the divergence of these species from their common ancestor. If the hypothesis holds true, it should be possible to use proteins and nucleic acids for determining not only relationships among organisms, but also their divergence times from shared ancestors. The clock, of course, has to be calibrated by nonmolecular (geological, palaeontological) events, but once this is accomplished the clock can be applied to groups for which no fossil record is available. See also:Molecular clocks

The molecular clock hypothesis, first proposed in 1965, has been controversial. Nevertheless, a certain regularity in the accumulation of amino acid replacements can no longer be doubted. However, proteins differ in the rate at which they diverge. Some, such as fibrinopeptides, which are fragments of proteins participating in blood clotting, evolve rapidly, while others, such as histones, which form the spools for winding DNA strands in a chromosome, evolve very slowly. The reason for these differences is that proteins are differentially constrained in their evolution by their functions. Some, such as the fibrinopeptides, can change considerably and still remain functional, while most amino acid replacements in others, such as histones, would endanger the protein’s function. In the former, therefore, many more mutations have a chance of being fixed than in the latter, in which virtually all mutations are eliminated. Also controversial is the speed of the molecular clock in different evolutionary lineages. Some researchers have obtained evidence that in some lineages, for example rodents among the mammals, the clock runs faster than in others, such as primates. Other researchers contest these findings. At the nucleotide level, some regions of the genome and some sites within a given region evolve faster than others. See also:Histones;Rodentia (rodents);Primates (lemurs, apes, monkeys)