Structural Biochemistry/Natural Selection of Aluminum and Silicon
Darwin’s theory of natural selection can be described as a competition between species in a certain area containing a variety of consistent effects. However, when looking at a simplistic, molecular level, the theory of natural selection could apply as well within the boundaries of a cell. At the molecular level, consistent effects that influence selection of certain molecules or atoms include reaction thermodynamics and reaction kinetics. Reaction thermodynamics are defined by equilibrium constants, which are explained even further through physical properties of reaction products such as solubility and stability. However, reaction kinetics is defined on how exactly equilibrium of a system or reaction is approached. In biochemistry, reaction kinetics helps to explain why certain biological pathways are selected for more than others. It is certain consistent effects that select for these certain pathways such as concentrations of reactants and products and amount of competitors as well as physical constraints within a cell such as membranes and fluid flow acting as transport systems. These are what are known as kinetic barriers, and these are what help to describe the selections and non-selections for atoms such as aluminum and silicon.
Selection of Aluminum 
Aluminum is the most plentiful metal on the planet and the third most abundant element in the Earth’s crust. However, in biochemistry, aluminum has a very limited, small role. This can be explained through two reasons: its selection out of biological systems due to its physical and biochemical properties or its nonexistent participation in the selection of essential elements in biochemistry. Both reasons have been explained through the properties of aluminum. For example, the lack of selection of aluminum in biological systems can be explained through its slow ligand exchange rate, which makes aluminum a very poor metal co-factor, a non-protein chemical compound bound to a protein in order for it to function, for enzymes. However, it is aluminum’s nonexistent participation in the selection of essential elements in biochemistry that explains this phenomenon the best.
Aluminum has proven to be a very reactive element, but yet, it has ultimately been selected out of biochemical pathways. Aluminum is the main instigator for the occurrence of acid rain due to its high affinity to bind to oxygen-based functional groups. An example of its high affinity to bind to oxygen-based functional groups is that it out-competes Mg (II) by a large factor for the complex molecule, ATP. Al (III) is a redox inactive cation, but it is a very strong pro-oxidant that helps catalyze certain reactions such as iron driven redox reactions. Aluminum has also been shown to be a good immunogen due to its role as an immunological agent to modify other agents. Certain physical and biochemical properties have shown that aluminum contains the potential to appear in certain biochemical pathways such as its substitution for Mg (II) in metal-nucleotide complexes. Although many of these observations prove that aluminum is a relatively abundant, reactive atom, one observation has supported the theory of the evolution in the absence of biologically available aluminum. Christopher Exley performed a study on the acute toxicity of aluminum in the Atlantic salmon, and he discovered the aqueous form of silicon, silicic acid, protected these fish from the toxicity of aluminum. Based on this evidence, the non-selection of aluminum can be explained through silicic acid’s ability to reduce the biological activity of aluminum, making it inactive in nature.
Selection of Silicon 
Silicon is the second most abundant element in the lithosphere and is known to be an essential element. However, silicon has been studied and many scientists have determined that silicon contains no biochemical activity or function. One reason for this can be described for its lack of bioorganic diversity. For example, no Si-C bonds or Si-O-C bonds are known to exist in nature, and because of this non-selection, silicon has not been known to be present in organisms. The fact that silicon bonds require much more energy to break also explains why organisms contain carbon within their biochemical pathways instead of silicon. Silicic acid is the most common form of silicon known to exist in nature. Silicic acid is weak acid as it loses its first proton when the pH reaches 10. Because the pH of systems usually range from 7 to 8, the fact that silicic acid cannot lose a proton until the pH reaches 10 indicates that silicic acid is rather inert and not reactive. In addition, only three significant reactions involve the element silicon. These reactions are the autocondensation to yield amorphous hydrated silica, its reaction with aluminum hydroxide to yield hydroxyaluminosilicates, and its reaction with excess molybdate to yield Keggin-like molybdosilicic acid complex. Aside from these reactions, no known bioorganic reactions have been known to take place with silicon in them. As a result of this, the selection for silicon in biochemical pathways have been absent and selected against.
Exley, C., 2009, Darwin, natural selection and the biological essentiality of aluminum and silicon, Vol. 34 No. 12, 20 September 2009.