Diminishing-returns epistasis decreases adaptability along an evolutionary trajectory

Area of Science:

• Evolutionary biology
• Microbial evolution
• Genetics

Background:

• Adaptability in populations often declines over time in constant environments.
• Understanding this decline is key to predicting evolutionary outcomes.
• Potential causes include changes in mutation effect size or occurrence rate.

Purpose of the Study:

• To investigate the mechanisms behind declining adaptability in evolving populations.
• To determine whether changes in mutation effect size or rate explain this phenomenon.
• To elucidate the role of genetic interactions in adaptive trajectories.

Main Methods:

• Evolving populations of Escherichia coli from various points on a single evolutionary path.
• Analyzing the effect size and occurrence of beneficial mutations.
• Utilizing genome sequencing to identify genetic changes.
• Assessing the impact of genetic background on mutation effects.

Main Results:

• Declining adaptability is primarily caused by a decrease in the effect size of beneficial mutations.
• Negative genetic interactions significantly reduce the benefits of new mutations in fitter genotypes.
• Starting genotype similarity did not correlate with similarity in beneficial mutations acquired.
• Epistasis influences the magnitude of mutation effects more than their identity.

Conclusions:

• The decrease in beneficial mutation effect size, driven by epistasis, is the main driver of declining adaptability.
• Evolutionary trajectories are predictable due to global epistatic effects.
• This provides a general mechanism for predictable phenotypic evolution.