**Covarion**
**Definition**
Covarion, short for „concomitantly variable codons,” is a model in molecular evolution describing how the evolutionary rate of a nucleotide or amino acid site can change over time. It accounts for the phenomenon where some sites in genetic sequences switch between being variable and invariable during evolution.
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**Covarion Model in Molecular Evolution**
The covarion model was introduced to better represent the dynamic nature of molecular evolution, where the evolutionary rate at a given site is not constant but can vary across different lineages or time periods. Traditional models often assume that each site evolves at a fixed rate, but empirical data suggest that some sites may alternate between periods of conservation and variability.
**Mechanism and Applications**
In the covarion framework, sites can switch between „on” states, where they are free to accumulate substitutions, and „off” states, where they are effectively conserved. This switching can be influenced by functional constraints, structural changes, or environmental factors. The model improves phylogenetic inference and evolutionary analysis by capturing these rate shifts, leading to more accurate reconstructions of evolutionary relationships.
**Significance in Phylogenetics**
By incorporating covarion dynamics, researchers can better understand molecular evolution’s complexity, especially in proteins and coding DNA sequences. The model has been applied in various studies to explain heterotachy—the variation in evolutionary rates across lineages—and to refine evolutionary trees.
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**Meta Description**
Covarion is a molecular evolution model describing how genetic sites switch between variable and invariable states over time. It enhances phylogenetic analyses by accounting for changing evolutionary rates at individual sites.