Mark Blows has made key contributions to the development of a general framework for understanding the evolution of complex traits. He has overturned conventional wisdom concerning the nature of the two fundamental components of evolutionary change; genetic variation and selection. He has changed entrenched views on the availability of genetic variation, showing how the multivariate distribution of genetic variation generates genetic constraints among multiple traits. The strength of selection on multiple traits was shown by Blows to have been consistently underestimated, and he has gone on to show how selection changes levels of genetic variation, and how genetic constraints bias evolutionary trajectories.