Mutations that result in higher global population estimates are saved, while those that generate lower population estimates are discarded. They searched across sets of parameters in World3 and detected unexpected relationships that otherwise would have gone unnoticed. With only those two parameters perturbed, the global-population prediction soared. Such a result, so different from the model-maker's prediction, suggests that World3 may generate big errors in some circumstances and therefore might need refinement. Of course, the fact that you can break a model doesn't mean that that model is bad.
All models have to be responsive to their inputs.
Useful models lie in a delicate area between being too responsive to inputs and being too unresponsive. Miller's use of biological metaphors is no accident. As an undergraduate at the University of Colorado and a graduate student at the University of Michigan, he was deeply intrigued by the common historical and theoretical roots of economics and biology. Malthus solemnly predicted that the growing human population would inevitably overwhelm the ability of Earth to support it. Malthus said human-population crashes are inevitable.