On the Causes of Evolutionary Transition:Transversion Bias.

Area of Science:

• Molecular Evolution
• Biochemistry
• Genetics

Background:

• A common observation in molecular evolution is a preference for nucleotide transitions over transversions.
• This pattern is often attributed to selection acting on proteins, assuming transitions cause more conservative amino acid changes.

Purpose of the Study:

• To directly test the hypothesis that nucleotide transitions are more conservative than transversions in proteins.
• To investigate the role of selection in the observed bias of transitions over transversions in molecular evolution.

Main Methods:

• Compiled data from 8 published studies encompassing 1,239 single-nucleotide mutations causing amino acid replacements.
• Analyzed fitness effects of mutations using individual or paired growth experiments.
• Assessed the conservativeness of transitions versus transversions.

Main Results:

• Only one of the 8 studies found a significant difference, suggesting transitions are more conservative.
• Across all studies, transitions were only slightly more conservative than transversions (53% vs. 50% null expectation).
• The observed effect of transitions being more conservative is small and insufficient to explain the several-fold bias seen in evolution.

Conclusions:

• The hypothesis that transitions are inherently more conservative and drive evolutionary bias is not well-supported by direct evidence.
• Selection on proteins appears to play a minimal role in the observed preference for transitions over transversions in molecular evolution.