Molecular evolution and the decline of purifying selection with age

Area of Science:

• Evolutionary biology
• Molecular evolution
• Genetics

Background:

• Life history theory posits that the intensity of selection decreases with age.
• This age-related decline in selection is predicted to influence the evolution of genes based on their expression timing.

Purpose of the Study:

• To investigate the relationship between a gene's age of expression and its patterns of molecular evolution.
• To test predictions derived from life history theory regarding selection efficacy across the lifespan.

Main Methods:

• Comparative analysis of gene expression timing and molecular evolutionary patterns in humans, mice, mosquitoes, and fruit flies.
• Development of an evolutionary model to explain observed patterns.
• Gene Ontology (GO) enrichment analysis.
• Analysis of disease-associated genes and mutations in humans.

Main Results:

• Genes expressed later in life accumulate nonsynonymous mutations faster, show higher polymorphism, and have shorter evolutionary lifespans compared to early-expressed genes.
• Early-expressed genes exhibit conserved functional enrichment across species, unlike late-expressed genes.
• Late-expressed genes in humans are associated with cancer and dominant disease mutations.
• The effective strength of selection (N_e s) decreases, and the proportion of beneficial mutations increases with gene expression age.

Conclusions:

• Findings support the hypothesis of diminishing purifying selection efficacy with age, consistent with theories of senescence.
• Establishes a link between life history theory, gene expression timing, and molecular evolutionary dynamics.
• Highlights species-specific functional constraints on early-expressed genes versus more relaxed constraints on late-expressed genes.