Encyclopedia > Molecular clock

Article Content

Molecular clock

The molecular clock technique, which researchers use to date when two species diverged, deduces elapsed time from the number of minor differences between their DNA sequences.

This technique is based on Allan Wilson[?] and Vince Sarich[?]'s observation that rare spontaneous errors in DNA replication cause the mutations that drive molecular evolution, and that the accumulation of evolutionarily neutral minor differences could be used to measure time, if the DNA replication error rate 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.

this is a stub article -- a proper treatment needs to be written

References:

Vincent M. Sarich and Allan C. Wilson. "Immunological time scale for hominid evolution". Science 158, 1967, p. 1200-1203.