question: What heterotachy states about molecular evolutionary processes?
answer: Functional constraints may account for heterogeneity in the evolutionary rates among different sites of amino acid or DNA sequences. Apart from variations of substitution rates among different sites (spatial variation), heterotachy states that there are variations of substitution rates of a given site throughout time. Heterotachy is the within-site evolutionary rate variations across different lineages over time. Heterotachy (temporal rate variation) occurs with varying severity because the intensity of purifying selection and adaptive forces acting at a given position of a homologous amino acid or DNA sequence are not the same in different species. Heterotachy may mislead phylogenetic inferences.