Stronger Evidence for Relaxed Selection Than Adaptive Evolution in High-elevation Animal mtDNA

Area of Science:

• Evolutionary biology
• Molecular evolution
• Genomics

Background:

• Mitochondrial (mt) genes are central to energy production and metabolism.
• Adaptive hypotheses suggest high-elevation environments drive rapid mitochondrial DNA (mtDNA) evolution via positive selection.
• The role of relaxed purifying selection versus positive selection in mtDNA evolution remains under-tested.

Purpose of the Study:

• To test the hypothesis that high elevation drives positive selection on mtDNA.
• To investigate whether relaxed purifying selection explains elevated mtDNA evolutionary rates in high-elevation species.
• To examine the relationship between environmental factors, range size, body mass, and mtDNA evolutionary rates (dN/dS).

Main Methods:

• Calculated the dN/dS ratio, a measure of nonsynonymous substitution bias.
• Analyzed mtDNA from over 700 species of terrestrial vertebrates, freshwater fishes, and arthropods.
• Correlated dN/dS ratios with elevation, latitudinal range limits, range sizes, and body sizes.

Main Results:

• High-elevation taxa exhibited slightly higher dN/dS ratios than low-elevation relatives.
• Smaller range sizes predicted higher dN/dS ratios, indicating relaxed selection.
• Absolute elevation and latitude did not predict dN/dS; body mass positively correlated with dN/dS, suggesting relaxed selection due to smaller effective population size.

Conclusions:

• Elevated mt dN/dS in high-elevation species is more likely attributed to relaxed selection from smaller ranges and reduced effective population size.
• The findings challenge the assumption of positive selection driving mtDNA evolution at high elevations.
• Emphasizes the necessity of testing non-adaptive explanations for observed patterns in mitochondrial genome evolution.