Recent results show that the Differential Evolution algorithm has significant difficulty on functions that are not linearly separable. On such functions, the algorithm must rely primarily on its differential mutation procedure which, unlike its recombination strategy, is rotationally invariant. We conjecture that this mutation strategy lacks sufficient selective pressure when appointing parent and donor vectors to have satisfactory exploitative power on non-separable functions. We find that imposing pressure in the form of rankbased differential mutation results in a significant improvement of exploitation on rotated benchmarks. Categories and Subject Descriptors I.2.8 [Artificial Intelligence]: Problem Solving, Control 							
						
							
					 															
					Andrew M. Sutton, Monte Lunacek, L. Darrell Whitle