Stress can be viewed as an environmental probe that targets energy carriers, and hence can determine the energetic efficiency or fitness to survive. Thus, variability and evolutionary potential are interpreted in terms of the ecological scenario of predominantly stressful environments in the wild that restrict energy availability. This can explain how the observed variability of direct fitness traits is high at extremes of abiotic stresses, giving U-shaped curves for variability that incorporate more benign regions between the extremes. Some consistency with interpretations based upon conventional quantitative genetic techniques occurs, incorporating this ecological/energetic approach. However, investigations into the quantification of stress levels are required for more comprehensive assessments. Even so, evolutionary potential can in principle be investigated in terms of energetic consequences of the functional biology of organisms in their challenging habitats. This approach appears predictive for variability patterns of direct fitness traits as well as for developmentally more complex morphometric traits and for relationships among fitness traits in natural populations. That is, energetic costs are basic in determining evolutionary potential across variably stressful environments.