The evolutionary scaling of cellular traits imposed by the drift barrier

Significance Owing to internal homeostatic mechanisms, cellular traits may experience long periods of stable selective pressures. Nonetheless, drift and mutation still conspire to generate significant variation in mean phenotypes among phylogenetic lineages. Provided there are classes of mutations with sufficiently small effects, even in the face of constant selection, variation in genetic effective population sizes will result in gradients of mean phenotypes with respect to organism size across the tree of life. Mutation is an important determinant of such patterns, even in the absence of directional bias. Thus, a substantial amount of variation in cellular features may be a simple consequence of lineage-specific differences in the power of drift rather than a reflection of adaptive divergence.