Integrating experimental evolution and transcriptomics reveals the phenotypic and molecular architecture of the pace-of-life syndrome

Phenotypic traits often covary through shared genetic and environmental influences, shaping the evolution of integrated life-history strategies. In particular, correlations between behavioral and life-history traits form the core of the pace-of-life syndrome (POLS) hypothesis, yet direct experimental tests of its evolutionary and molecular foundations remain limited. Here, we combined experimental evolution and transcriptomic analyses to investigate the phenotypic and molecular architecture of POLS in the red flour beetle Tribolium castaneum . Using replicate lines artificially selected for increased or decreased locomotor activity, we quantified correlated responses in life-history traits and female reproductive performance and examined genome-wide divergence in gene expression. Lines evolving higher activity exhibited significantly reduced lifespan and elevated female reproductive output, whereas development time remained unchanged, revealing partial support for POLS predictions and a decoupling between activity and development. Transcriptomic analyses further identified multiple differentially expressed genes between activity-selected lines, suggesting the involvement of metabolic and regulatory pathways associated with behavioral and life-history divergence. By integrating phenotypic and molecular data under controlled experimental evolution, our study provides a robust experimental test of the POLS hypothesis. These findings reveal asymmetric and context-dependent evolution of pace-of-life traits and offer insights into how behavioral variation and life-history strategies are genetically integrated and maintained in natural populations.