Heterotachy

Heterotachy, in the context of molecular evolution and phylogenetics, refers to the change in evolutionary rate of a site (amino acid position in a protein or nucleotide position in a DNA sequence) through time or across different lineages. This means that a particular site might evolve rapidly in one part of the phylogenetic tree and slowly in another, or vice versa.

Heterotachy is distinct from heterogeneously distributed rates among sites, where some sites are intrinsically more mutable than others, but those relative rates remain constant across the phylogeny. In heterotachy, the relative rates between sites change.

Detecting heterotachy is important because it can violate the assumptions of some phylogenetic models, leading to inaccurate tree reconstructions and biased estimates of evolutionary parameters. Statistical methods are used to identify sites that exhibit significant rate shifts across the phylogeny. These methods often involve comparing models that allow for heterotachy with models that assume rate constancy.

The causes of heterotachy are varied and can include changes in selective pressure, alterations in protein structure or function that affect the importance of specific residues, or differences in mutation rates along different lineages. Understanding heterotachy can provide insights into the adaptive processes shaping molecular evolution.