Genetic saturation is the reduced appearance, which occurs over time, of sequence divergence rate that results from reverse mutations, homoplasies and other multiple changes occurring at single sites along two lineages

Lineage (genetic)

A genetic lineage is a series of mutations which connect an ancestral genetic type to derivative type. In cases where the genetic tree is very bushy the order of mutations in the lineage is mostly known, examples are the order of mutations between E1b1b and E1b1b1a1a for the human Y-chromosome...

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Genetic saturation is a cause of reticulation in genetic trees, where diverging genetic branches might fuse back into one another before splitting. Two other causes are sequence errors and recombination.

Genetic saturation occurs most rapidly on rapidly evolving sequences, such as the hypervariable region of mitochondrial DNA, or in Short tandem repeat

Short tandem repeat

A short tandem repeat in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. The pattern can range in length from 2 to 5 base pairs and is typically in the non-coding intron region...

such as on the Y-chromosome.