Rates of amino acid changes in fibrinopeptides, hemoglobin, and cytochrome c.
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Rates of amino acid changes in fibrinopeptides, hemoglobin, and cytochrome c.
Rates of amino acid changes in fibrinopeptides, hemoglobin, and cytochrome c. The three proteins show different rates of changes per unit time. However, for each protein, the rate of changes per unit time appears to be approximately constant. If these trends hold for other proteins and for all species comparisons, we can easily use the molecular differences to date species divergence. For example, if we found that the fibrinopeptides of two species show 110 changes per 100 residues (shown as the red dot in the plot), we can infer that those two species diverged approximately 120 millions of years ago.
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In the early days of molecular evolution, one of the most tantalizing findings was the observation that protein sequence change seemed to occur at a rate proportional to the time since the evolutionary divergence of species. The term 'molecular clock' was coined to describe the phenomenon, comparing the manner in which sequence changes occur to regular ticks of a clock. Is there really such a molecular clock? If so, why? Can we use molecular clocks to date evolutionary events? After several decades of study, we have answers to some of these questions. More importantly, in the process we have learned a lot more about how changes at the molecular level accumulate throughout the genome.
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