Assuming stress levels to which free-living populations are normally exposed, an association between rapid development time, a long life, success in mating and size of sexual ornaments can be predicted. Fitness at one stage of the life cycle should therefore correlate with fitness at other stages under this environmental model. Assuming that stress targets energy carriers, high-energy efficiency underlain by stress-resistance genotypes that are likely to be heterozygous is the basis of this prediction. Stress-resistance genotypes therefore have a role in promoting the energy efficiency required for organisms to accommodate a stressed world. Selection for energy efficiency to utilize heterogenous resources implies that the process of speciation should normally occur rapidly and be rarely observed. It follows that the ecological species concept is primary to other species concepts. The intensity of selection for stress resistance goes from an extreme in the highly disturbed and stressful environments of living fossils to relatively stable abiotic habitats, where specialist diversifications and adaptive radiations are likely. Between these extremes, a punctuated pattern of evolutionary change may occur in perturbed environments during a transient phase of increased resources. In abiotically benign tropical habitats where energy constraints are low, specialization of resource utilization by learning appears possible.