Definitions for "Molecular clock"
(1) The rate at which mutations accumulate in a given genomic segment. (2) The hypothesis that, in any given gene or DNA sequence, mutations accumulate at an approximately constant rate in all evolutionary lineages as long as the gene or the DNA sequence retains its original function. The extent to which the clock applies to all genes and all organisms is controversial.
The theory that macromolecules diverge from one another over evolutionary time at a constant rate, and that discovering this rate gives insight into the phylogenetic relationships of organisms.
The hypothesis that nucleotide or amino acid substitutions occur at more or less fixed rates over evolutionary time, like the slow ticking of a clock.