We are talking about a cell or a cell clone that accumulates mutations over a long period of time.

The idea is that genes change over time by accumulating mutations.

The inverted repeat regions are highly conserved among various plants, and accumulate few mutations.

Incipient cancer cells are thought to accumulate mutations in the genes that underlie these circuits.

Another view is that both copies are equally free to accumulate degenerative mutations, so long as any defects are complemented by the other copy.

Even functional mitochondrial genes accumulate mutations faster and more freely.

Each group will accumulate different mutations as well as be subjected to different selective pressures.

Thus it accumulates mutations faster than a functional single-copy gene, over generations.

Tissues made from a person's stem cells could therefore be rejected, because mitochondrial genomes tend to accumulate mutations.

This can lead to rapid speciation as the two populations separate and accumulate mutations.