For a section of DNA to be considered a molecular clock it must be susceptible to change through random mutations. Also the importance of neutral mutations (mutations that have no effect on phenotype's) in this measure of time is they are unable to have any effect on phenotype's however, they are able to build up in the DNA of different species at around the same rate, which enables researchers to use these mutation rates to estimate the amount of time that two species have been evolving independently from a shared ancestor.
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