Variation in human brains may facilitate evolutionary change toward a limited range of phenotypes.

Individual variation is the foundation for evolutionary change, but little is known about the nature of normal variation between brains. Phylogenetic variation across mammalian brains is characterized by high intercorrelations in brain region volumes, distinct allometric scaling for each brain region and the relative independence of olfactory and limbic structure volumes from the rest of the brain. Previous work examining brain variation in individuals of some domesticated species showed that these three features of phylogenetic variation were mirrored in individual variation. We extend this analysis to the human brain and 10 of its subdivisions (e.g., isocortex and hippocampus) by using magnetic resonance imaging scans of 90 human brains ranging between 16 and 25 years of age. Human brain variation resembles both the individual variation seen in other species and variation observed across mammalian species, i.e., the relative differences in the slopes of each brain region compared to medulla size within humans and between mammals are concordant, and limbic structures scale with relative independence from other brain regions. This nonrandom pattern of variation suggests that developmental programs channel the variation available for selection.