an organism to achieve an optimal life history strategy and obtain the maximum level of fitness, and making the best possible choices about how to allocate energy to various trade-offs contributes to this. Models of resource allocation have been developed and used to study problems such as parental involvement, the length of the learning period for children, and other developmental issues. The allocation of resources also plays a role in variation, because the different resource allocations by different species create the variety of life history strategies. Capital and income breeding The division of capital and income breeding focuses on how organisms use resources to finance breeding, and how they time it. In capital breeders, resources collected before breeding are used to pay for it, and they breed once they reach a body-condition threshold, which decreases as the season progresses. Income breeders, on the other hand, breed using resources that are generated concurrently with breeding, and time that using the rate of change in body-condition relative to multiple fixed thresholds. This distinction, though, is not necessarily a dichotomy; instead, it is a spectrum, with pure capital breeding lying on one end, and pure income breeding on the other. Capital breeding is more often seen in organisms that deal with strong seasonality. This is because when offspring value is low, yet food is abundant, building stores to breed from allows these organisms to achieve higher rates of reproduction than they otherwise would have. In less seasonal environments, income breeding is likely to be favoured because waiting to breed would not have fitness benefits. Phenotypic plasticity Phenotypic plasticity focuses on the concept that the same genotype can produce different phenotypes in response to different environments. It affects the levels of genetic variability by serving as a source of variation and integration of fitness traits. Determinants Many factors can determine the evolution of an organism's life history, especially the unpredictability of the environment. A very unpredictable environment—one in which resources, hazards, and competitors may fluctuate rapidly—selects for organisms that produce more offspring earlier in their lives, because it is never certain whether they will survive to reproduce again. Mortality rate may be the best indicator of a species' life history: organisms with high mortality rates—the usual result of an unpredictable environment—typically mature earlier than those species with low mortality rates, and give birth to more offspring at a time. A highly unpredictable environment can