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{{B}}Energy for living things{{/B}}

We have seen that all living things must take in and use energy to maintain their bodies, to grow, to obtain more energy, and to reproduce. Since the evolutionarily successful individual is one that leaves descendants in future generations, natural selection favours those individuals that can channel the most energy into producing offspring. The use of energy in other activities such as feeding, fighting, or growing is selectively advantageous only so far as these activities result in the organism's accumulating more energy to produce offspring.
Each individual has an energy income—all of the energy that it acquires during its lifetime. It also has an energy budget, its allotment of different amounts of energy to various activities. The most evolutionarily successful organisms are those which are most effective in conversion of energy to offspring. This does not mean that organisms use all their energy directly to produce offspring. For example, suppose that a tree converts some of its energy into growing a large root system, the energy thus spent cannot be used to produce offspring. Its large root system may enable the tree to obtain a great deal of water and minerals from the soil and so to produce more leaves, another diversion of energy away from the production of offspring. However, all of the leaves that the tree produces may then enable the tree to synthesize more food than it would have otherwise, and so allow it to make up for some of its previous energy expenditure by producing more offspring in the end. Thus organisms make energy investment which may ultimately yield energy gains that can be reinvested in the production of offspring. Sometimes these investments will turn out to be selectively disadvantageous because they postpone production of offspring. If the organism meets an early death, it will never get a chance to reproduce. So any item in an organism's energy budget must have the potential to produce an ultimate productive gain that is equal to the risks involved in diverting energy away from the immediate production of offspring.