The repeated evolution of complex adaptations – crop mimicry by weeds, for example, or CO2-concentrating C4 photosynthesis – shows the power of natural selection to solve difficult problems that limited fitness in past environments. The sophistication of natural selection’s innovations contrasts with the relatively simple changes (e.g., increasing the expression of existing genes) readily achievable by today’s biotechnology. Mutants with greater expression of these genes arose repeatedly over the course of evolution, so their present rarity indicates rejection by natural selection. Similarly, medical interventions that simply up- or down-regulate existing physiological mechanisms presumably recreate phenotypes also rejected by past natural selection. Some tradeoffs that constrained past natural selection still apply, such as those resulting from conservation of matter. But tradeoffs between present human goals and individual fitness in past environments may represent fairly easy opportunities to achieve our goals by reversing some effects of past selection. This point is illustrated with three examples, based on tradeoffs between (i) individual-plant fitness versus whole-crop performance, (ii) the fitness of symbionts (rhizobia) versus that of their legume hosts, and (iii) human fertility versus longevity in the context of environmental cues, such as consumption of ‘famine foods’, that predict trends in population size.