Natural selection weakens progressively throughout adulthood, creating a 'selection shadow' where mutations that cause harm only in later life accumulate unchecked. This evolutionary framework explains why genetic variants beneficial in youth persist despite costly effects in old age, and why longevity pathways remain conserved across species despite a 100-fold variation in maximum vertebrate lifespan. The research synthesizes how shared genetic architectures underlie multiple age-related diseases through this evolutionary lens. This evolutionary perspective represents a paradigm-shifting framework for understanding aging biology. Rather than viewing aging as a design flaw, it emerges as an inevitable consequence of selection's diminishing power over time. For longevity research, this insight suggests interventions should focus on pathways where evolutionary trade-offs create the greatest vulnerability in later life. The framework also explains why some individuals age more successfully than others—they may carry fewer late-acting deleterious variants or more protective alleles. Practically, this evolutionary lens could guide precision medicine approaches by identifying genetic variants with age-specific effects, potentially enabling targeted interventions before harmful late-life consequences manifest. The integration of comparative genomics with human genetic data offers unprecedented opportunities to test these evolutionary predictions.