Selection Limits to Adaptive Walks on Correlated Landscapes

Area of Science:

• Evolutionary biology
• Population genetics
• Molecular evolution

Background:

• Adaptation is influenced by mutation effects and genetic background, conceptualized by fitness landscapes.
• Complete fitness landscape analysis is often inaccessible due to combinatorial complexity.
• Understanding adaptation dynamics is crucial for predicting evolutionary trajectories.

Purpose of the Study:

• To develop a method for estimating adaptation time without complete fitness landscape knowledge.
• To analyze conditions enabling rapid adaptation in the weak-mutation regime.
• To investigate the role of selection strength and the number of sites under selection in adaptation.

Main Methods:

• Analysis of populations in the weak-mutation regime.
• Calculation of upper limits for time to reach a fitness peak across general landscape classes.
• Characterization of adaptation speed as a function of the number of selected sites.

Main Results:

• For additive landscapes, a critical selection strength is identified that facilitates reaching high-fitness genotypes, independent of mutation effect distribution.
• This critical strength scales with the number of selected sites, indicating a potential limit to adaptation due to increased deleterious mutation pressure.
• For unimodal landscapes, this critical strength is sufficient but not necessary; highly epistatic landscapes may allow rapid adaptation without this constraint.

Conclusions:

• The study provides a framework to assess adaptation speed by calculating upper bounds on time to fixation.
• A critical selection threshold exists for additive landscapes, limiting adaptation speed based on the number of selected sites.
• Epistasis can alter or remove these adaptation barriers, highlighting the importance of landscape topology in evolutionary dynamics.