Genetic Algorithm - dynamic fitness function

Hi,

I need to generate a dynamic fitness function for the genetic algorithm.

I just need to minimize the makespan (sum of all variables), but the number of variables is dynamic. It can be 5 now and 20 during the next run, so that I can not formulate the fitness function using a scalar formulation.

Please do not answer by referring to this article http://www.mathworks.com/help/gads/computing-objective-functions.html?searchHighlight=Computing+Objective+Functions as it handles only objectives with a fixed number of variables.

Thank you very much for your help