LAUSR

ABSTRACT Birds exhibit remarkable variations in body size, making them an ideal group for the study of adaptive evolution. However, the genetic mechanisms underlying body size evolution in avian species remain inadequately understood. This study investigates the evolutionary patterns of avian body size by analyzing 15 body‐size‐related genes, including GHSR, IGF2BP1, and IGFBP7 from the growth hormone/insulin‐like growth factor axis, EIF2AK3, GALNS, NCAPG, PLOD1, and PLAG1 associated with tall stature, and ACAN, OBSL1, and GRB10 associated with short stature, four genes previously reported in avian species: ATP11A, PLXDC2, TNS3, and TUBGCP3. The results indicate significant adaptive evolution of body size‐related genes across different avian lineages. Notably, in the IGF2BP1 gene, a significant positive correlation was observed between the evolutionary rate and body size, suggesting that larger bird species exhibit higher evolutionary rates of the IGF2BP1 gene. Furthermore, the IGFBP7 and PLXDC2 genes demonstrated accelerated evolution in large‐ and medium‐sized birds, respectively, indicating distinct evolutionary patterns for these genes among birds of different sizes. The branch‐site model analysis identified numerous positively selected sites, primarily concentrated near functional domains, thereby reinforcing the critical role of these genes in body size evolution. Interestingly, extensive convergent evolution was detected in lineages with larger body sizes. This study elucidates the genetic basis of avian body size evolution for the first time, identifying adaptive evolutionary patterns of body size‐related genes across birds of varying sizes and documenting patterns of convergent evolution. These findings provide essential genetic data and novel insights into the adaptive evolution of body size in birds.