# Heterotachy

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'''Heterotachy''' refers to variations in lineage-specific evolutionary rates over time. In the field of [molecular evolution](/source/molecular_evolution), the principle of heterotachy states that the substitution rate of sites in a [gene](/source/gene) can change through time.<ref name="pmid11752184">{{cite journal |vauthors=Lopez P, Casane D, Philippe H | title = Heterotachy, an important process of protein evolution | journal = Mol. Biol. Evol. | volume = 19 | issue = 1 | pages = 1–7 |date=January 2002 | pmid = 11752184 | doi = 10.1093/oxfordjournals.molbev.a003973| doi-access = free }}</ref> 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](/source/covarion) hypothesis is a specific form of heterotachy. Some studies have proposed functional divergence models that are also heterotachous. Additionally, some [mixture models](/source/mixture_models) that do not explicitly account for rate-shift, but site-partitions evolving at different relative substitution rates across [lineages](/source/Lineage_(evolution)) are mathematically heterotachous.

Failure to take heterotachy into account in phylogenetic reconstructions may lead to incorrect [phylogenetic tree](/source/phylogenetic_tree)s. Thus Zhong et al. (2011) say that heterotachy is one of the reasons for variability in reconstructions of the origin of [gnetophyte](/source/gnetophyte)s.<ref>{{cite journal |last=Zhong |first=Bojian |year=2011 |title=Systematic error in seed plant phylogenomics |journal=Genome Biology and Evolution |volume=3 |pages=1340–1348 |doi=10.1093/gbe/evr105 |pmid=22016337 |pmc=3237385 |author2=Oliver Deusch |author3=Vadim V. Goremykin |author4=David Penny |author5=Patrick J. Biggs |author6=Robin A. Atherton |author7=Svetlana V. Nikiforova |author8=Peter James Lockhart  }}</ref>

== References ==
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Category:Molecular evolution

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