Heterotachy refers to variations in lineage-specific evolutionary rates over time. In the field of

molecular evolution Molecular evolution describes how Heredity, inherited DNA and/or RNA change over evolutionary time, and the consequences of this for proteins and other components of Cell (biology), cells and organisms. Molecular evolution is the basis of phylogen ...

, the principle of heterotachy states that the substitution rate of sites in a

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

can change through time. It has been proposed that the positions that show switches in substitution rate over time (that is, heterotachous sites) are good indicators of functional divergence. However, it appears that heterotachy is a much more general process, since most variable sites of homologous proteins with no evidence of functional shift are heterotachous. The

covarion {, class="wikitable" align="right" , - ! ! colspan=4 , Rate = 1 ! colspan=4 , Rate = 0 , - , , A1 , G1 , C1 , T1 , A0 , G0 , C0 , T0 , - , A1 , – , α , β , γ , δ , 0 , 0 , 0 , - , G1 , α , – , γ , β , 0 , ...

hypothesis is a specific form of heterotachy. Some studies have proposed functional divergence models that are also heterotachous. Additionally, some mixture models that do not explicitly account for rate-shift, but site-partitions evolving at different relative substitution rates across lineages are mathematically heterotachous. Failure to take heterotachy into account in phylogenetic reconstructions may lead to incorrect

phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...

s. Thus Zhong et al. (2011) say that heterotachy is one of the reasons for variability in reconstructions of the origin of

gnetophyte Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three relict ...

References

{{Reflist Molecular evolution