[Paper Review] Interference limits resolution of selection pressures from linked neutral diversity

Benjamin H. Good, Aleksandra M. Walczak+2 more|arXiv (Cornell University)|Jun 5, 2013

Evolution and Genetic Dynamics39 references3 citations

TL;DR

This paper introduces a coarse-grained coalescent framework that approximates the effects of many weakly selected mutations in linked genomic regions by focusing on the variance in fitness within a linkage block, rather than individual selection coefficients. It demonstrates that neutral diversity patterns under interference selection are largely insensitive to individual fitness effects, revealing a fundamental limit in resolving individual selection pressures from sequence data alone.

ABSTRACT

Pervasive natural selection can strongly influence observed patterns of genetic variation, but these effects remain poorly understood when multiple selected variants segregate in nearby regions of the genome. Classical population genetics fails to account for interference between linked mutations, which grows increasingly severe as the density of selected polymorphisms increases. Here, we describe a simple limit that emerges when interference is common, in which the fitness effects of individual mutations play a relatively minor role. Instead, molecular evolution is determined by the variance in fitness within the population, defined over an effectively asexual segment of the genome (a ``linkage block''). We exploit this insensitivity in a new ``coarse-grained'' coalescent framework, which approximates the effects of many weakly selected mutations with a smaller number of strongly selected mutations with the same variance in fitness. This approximation generates accurate and efficient predictions for the genetic diversity that cannot be summarized by a simple reduction in effective population size. However, these results suggest a fundamental limit on our ability to resolve individual selection pressures from contemporary sequence data alone, since a wide range of parameters yield nearly identical patterns of sequence variability.

Motivation & Objective

• To address the challenge of predicting genetic diversity when multiple selected variants interfere in linked genomic regions.
• To develop a simplified, efficient coalescent framework that captures interference effects without tracking individual mutations.
• To investigate the limits of inferring individual selection pressures from contemporary sequence data under interference selection.
• To identify when and why traditional methods based on effective population size fail in the presence of widespread interference.
• To explore the implications of parameter degeneracy for inference in empirical population genomics.

Proposed method

• Propose a coarse-grained coalescent model that replaces many weakly selected mutations with a smaller number of strongly selected mutations having the same fitness variance.
• Define an effective selection strength and effective mutation rate based on the variance in fitness within a linkage block.
• Use a central limit theorem-like argument to show that diversity at neutral sites depends primarily on fitness variance, not individual selection coefficients.
• Validate the model through simulations and compare predictions to empirical data, such as the D. melanogaster dot chromosome.
• Identify degenerate parameter sets—different combinations of Ns, NU, and NR—that produce indistinguishable diversity patterns.
• Apply the framework to infer global properties like fitness variance and linkage scale, which are more robust than individual selection coefficients under interference.

Experimental results

Research questions

• RQ1 Can we accurately predict genetic diversity in regions with high densities of selected polymorphisms without tracking each individual mutation?
• RQ2 To what extent do individual selection coefficients influence neutral diversity when interference is common?
• RQ3 What are the implications of parameter degeneracy for inferring selection pressures from sequence data?
• RQ4 How does the variance in fitness within a linkage block determine patterns of neutral diversity under interference selection?
• RQ5 Can a coarse-grained coalescent model based on fitness variance outperform traditional methods that assume independent sites or reduced Ne?

Key findings

• Neutral diversity patterns under interference selection are primarily determined by the variance in fitness within a linkage block, not by individual selection coefficients.
• A wide range of parameter combinations—especially differing numbers of weakly selected mutations—produce nearly identical patterns of genetic diversity, revealing a fundamental resolution limit.
• The model accurately predicts diversity and frequency spectra for the D. melanogaster dot chromosome, even when underlying parameters differ substantially.
• Simulations show that highly resolved frequency spectra still fail to distinguish between parameter sets with the same fitness variance, indicating intrinsic uncertainty in inference.
• The framework suggests that global properties like fitness variance and linkage scale are more reliable targets for inference than individual selection coefficients in interfering populations.
• Weak purifying selection across many sites can mimic signals of recurrent sweeps or demographic changes, challenging prior interpretations of genomic diversity patterns.