modularity

Wang Z, Liao B-Y & Zhang J 2010 Genomic patterns of pleiotropy and the evolution of complexity. PNAS 107:18034-18039.

• we show that the fraction of traits altered appreciably by the deletion of a gene is minute for most genes
• the gene-trait relationship is highly modular
• the standardized size of the phenotypic effect of a gene on a trait is approximately normally distributed with variable SDs for different genes, which gives rise to the surprising observation of a larger per-trait effect for genes affecting more traits
• this scaling property counteracts the pleiotropy-associated reduction in adaptation rate (i.e., the "cost of complexity") in a nonlinear fashion, resulting in the highest adaptation rate for organisms of intermediate complexity rather than low complexity
• the observed scaling exponent falls in a narrow range that maximizes the optimal complexity
• the genome-wide observations of overall low pleiotropy, high modularity, and larger per-trait effects from genes of higher pleiotropy necessitate major revisions of theoretical models of pleiotropy
• pleiotropy has not only allowed but also promoted the evolution of complexity