a mate. An important trade-off in the dedication of resources to breeding has to do with predation risk: organisms that have to deal with an increased risk of predation often invest less in breeding. This is because it is not worth as much to invest a lot in breeding when the benefit of such investment is uncertain. These trade-offs, once identified, can then be put into models that estimate their effects on different life history strategies and answer questions about the selection pressures that exist on different life events. Over time, there has been a shift in how these models are constructed. Instead of focusing on one trait and looking at how it changed, scientists are looking at these trade-offs as part of a larger system, with complex inputs and outcomes. Constraints The idea of constraints is closely linked to the idea of trade-offs discussed above. Because organisms have a finite amount of energy, the process of trade-offs acts as a natural limit on the organism's adaptations and potential for fitness. This occurs in populations as well. These limits can be physical, developmental, or historical, and they are imposed by the existing traits of the organism. Optimal life-history strategies Populations can adapt and thereby achieve an "optimal" life history strategy that allows the highest level of fitness possible (fitness maximization). There are several methods from which to approach the study of optimality, including energetic and demographic. Achieving optimal fitness also encompasses multiple generations, because the optimal use of energy includes both the parents and the offspring. For example, "optimal investment in offspring is where the decrease in total number of offspring is equaled by the increase of the number who survive". Optimality is important for the study of life history theory because it serves as the basis for many of the models used, which work from the assumption that natural selection, as it works on a life history traits, is moving towards the most optimal group of traits and use of energy. This base assumption, that over the course of its life span an organism is aiming for optimal energy use, then allows scientists to test other predictions. However, actually gaining this optimal life history strategy cannot be guaranteed for any organism. Allocation of resources An organism's allocation of resources ties into several other important concepts, such as trade-offs and optimality. The best possible allocation of resources is what allows