Clusters of identical new mutations can account for the "overdispersed" molecular clock.

Germ-cell mutations may occur during meiosis, giving rise to independent mutant gametes in a Poisson process, or before meiosis, giving rise to multiple copies of identical mutant gametes at a much higher probability than the Poisson expectation. We report that the occurrence of these early premeiotic clusters of new identical mutant alleles increases the variance-to-mean ratio of mutation rate (R(u) > 1). This leads to an expected variance-to-mean ratio (R(t)) of the molecular clock that is always greater than one and may cover the observed range of R(t) values. Hence, the molecular clock may not be over-dispersed based on this new mutational model that includes clusters. To get a better estimation of R(u) and R(t), one needs measurements of the intrageneration variation of reproductive success (Nt/Ne(i)), population dynamics (ki), and the proportion of new mutations that occur in clusters (rc), especially those formed before germ-cell differentiation.