Darwin's theory of evolution by natural selection provides a general explanation for the diversity of life. If organisms differ in traits that are heritable, then differences in reproductive success among individuals will result in changes in the relative prevalence of traits in subsequent generations; over time, this process of natural selection leads to adaptation, an improved fit between the features of organisms and the demands of the environment.

Adaptations may often be manifested in the biochemical makeup of organisms. This is because an organism's biochemistry has functional and economic consequences that influence reproductive success. From a functional perspective, an organism's makeup affects its ability to meet environmental challenges because compositionally distinct biomolecules, cellular structures, and tissues have different chemical properties that determine their biological roles. From an economic perspective, an organism's makeup reflects its demand for structural resources, and environmental scarcity of particular substances may constrain the evolution of reliance on those substances. The composition favored by natural selection should reflect the optimal compromise between these functional and economical considerations.

Ecological stoichiometry, the study of the balance of energy and chemical elements in living systems, provides a general structure for examining how the composition of life has evolved. Clearly, organisms are not bags of independently functioning atoms, and a focus on elements will often miss the consequences of how elements are arranged in biochemicals. However, this focus has significant advantages because it provides a currency that facilitates comparisons across diverse taxonomic groups and levels of biological organization.

Consider the following examples that illustrate some of the ways in which elemental composition can have adaptive significance.

The water flea, Daphnia, is a common herbaceous crustacean found in lakes, ponds, and quiet streams. There is now ample evidence that the rate at which a Daphnia can grow is functionally related to the concentration of phosphorus (P) in its tissue when P is scarce in the environment. The connection between body P levels and growth rate exists because growth rate depends on the concentration of RNA in Daphnia tissue, and RNA is an abundant cellular component that contains more P than other major biomolecules. This linkage among whole-body P concentration, RNA levels, and growth rate is the central prediction ofthe growth rate hypothesis (GRH), which is described more thoroughly in Organismal Ecophysiology.

Aquatic mollusks depend on calcium (Ca)-rich shells for protection against fish and crustacean predators. The protective benefit of a shell for mollusks depends on its thickness, form, and Ca content, which affects hardness and other physical properties that help shells maintain structural integrity in the face of attack. Because Ca influences shell function, it is not surprising that Ca availability is thought to have played an important role in the evolution of molluskan shell morphology. Ca availability can also induce short-term changes in shell investment, which has been demonstrated in the freshwater snail Lymnaea stagnalis. The relationship between Ca availability and shell investment suggests that the evolutionary significance of predation pressure will be mediated by the types of resources available in the environment.

Aphids are insect herbivores that tap into a plant's phloem and feed on sap. Solutes in sap tend to be rich in carbon (C) but contain few nitrogen (N)-rich amino acids, P-containing molecules, or other important minerals and vitamins. Sap thus creates a significant dietary challenge for an insect because its elemental composition (high C:N and C:P ratios) differs substantially from the balance of these elements in insect tissue. Adaptations to this imbalance include phenotypes that promote mutualistic interactions with bacteria and ants. For example, aphids such as the greenbug aphid, Schizaphus graminum, house intracellular bacteria which supply essential amino acids to their hosts after receiving C and nonlimiting amino acids. Aphids have also evolved the ability to produce a carbon-rich exudate called 'honeydew', which some species present to ants in exchange for protection from aphid predators and parasites. Thus, the compositional disparity between aphids and their food may have been a key factor promoting the evolution of mutualistic interactions.

These examples illustrate how elemental composition can play a role in a wide range of phenomena that ultimately affect an organism's reproductive success. It follows that conditions favoring particularly strategies for maximizing reproductive success will also have resulted in evolutionary changes in elemental composition. If so, organisms should differ in elemental composition, and these differences should be associated with biochemical differences that affect functional capabilities. Let us review some of the available evidence for evaluating these predictions.

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