Genetics of growth predict patterns of brain-size evolution.

Experimental evidence is presented supporting a developmental model that explains the genetic basis for brain and body size associations. Evolutionary change in body size causes correlated change in brain size because some genes affect both traits. The commonly observed correlation between brain and body size results from genetic variation in growth determinants affecting both traits simultaneously during fetal and early postnatal growth. Later growth reduces brain-body correlation because of changes in the underlying causal components of growth in each trait. Brain-body size evolution shows a different pattern at higher taxonomic levels from that seen within and between closely related species because body-size evolution among higher taxa occurs primarily by change in early portions of growth, which share more genetic growth determinants with brain size.