GENETIC DRIFT. “We are promoting the idea that the power of random genetic drift imposes a lower bound to the degree to which natural selection can reduce the mutation rate. This drift-barrier hypothesis seems to support a number of previously disconnected observations, including the increase in the mutation rate with reductions in effective population size, the magnified error rates associated with DNA polymerases and repair enzymes used only infrequently in replication, and the extraordinarily high rates of base misincorporation into transcripts.”
https://biodesign.asu.edu/mechanisms-evolution/research