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The molecular clock is a technique in genetics, which researchers use to date when two species diverged, deduces elapsed time from the number of minor differences between their DNA sequences.
The Molecular Clock hypothesis is first attributed to Emile Zuckerkandl and Linus Pauling (1962) whom theorized that the quantity of amino acid differences in Hemoglobin between lineages roughly matched the known evolutionary rate of divergance based upon fossil evidence. Later Allan Wilson and Vince Sarich built upon this work and the work of Motoo Kimura (1968) observed and formailized that rare spontaneous errors in DNA replication cause the mutations that drive molecular evolution, and that the accumulation of evolutionarily "neutral" differences between two sequences could be used to measure time, if the error rate of DNA replication could be calibrated. One method of calibrating the error rate was to use as references pairs of groups of living species whose date of speciation was already known from the fossil record.
Originally, it was assumed that the DNA replication error rate was constant--not just over time, but across all species and every part of a genome that you might want to compare. Because the enzymes that replicate DNA differ only very slightly between species, the assumption seemed reasonable a priori.
As molecular evidence has accumulated, the constant-rate assumption has proven false--or at least overly general. Molecular clock users are developing workaround solutions using a number of statistical approaches including maximum likelihood techniques and later bayesian modeling.
The molecular clock technique is an important tool in molecular systematics, the use of molecular genetics information to determine the correct scientific classification of organisms.
See also
References
- ZUCKERKANDL, E., and L. PAULING, 1962 "Molecular disease, evolution, and genetic heterogeneity", pp. 189–225 in Horizons in Biochemistry, edited by M. KASHA and B. PULLMAN. Academic Press, New York.
- ZUCKERKANDL, E., and L. PAULING, 1965 "Evolutionary divergence and convergence in proteins", pp. 97–166 in Evolving Genes and Proteins, edited by V. BRYSON and H. J. VOGEL. Academic Press, New York.
- Vincent M. Sarich and Allan C. Wilson. "Immunological time scale for hominid evolution". Science 158, 1967, pp. 1200-1203.
- Kimura, Motoo (1968). "Evolutionary Rate at the Molecular Level", Nature Vol. 217, pp. 624-626.
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