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However, many types of organisms have no fossil record and not all phenotypes fossilize, presenting a problem for dating biological events. If the amount of DNA sequence divergence between two species is known, then this information can be utilized to date their divergence. The molecular clock hypothesis asserts that for neutral alleles the rate of substitution is simply the mutation rate, or k = ||. If an absolute substitution rate expressed in fixations per time interval is available, multiplying that rate by a time gives an expected number of substitutions. The number of diverged nucleotide sites between two species also increases at twice the rate of substitutions since each lineage will experience substitutions independently. Bringing these two observations together gives the expected amount of divergence k = 2T|J,

between two species that diverged T time units ago. If divergence between two species as well as the rate of divergence is known, this relationship can be rearranged to solve for the unknown of time instead:

Absolute substitution rate A rate of molecular change estimated in substitutions per year based on the combination of a sequence divergence estimate from two taxa and an estimate of the time that has elapsed since those taxa diverged.

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