We would be mistaken to assume that evolutionary mechanisms can produce any trait we might imagine. Evolution is constrained in many ways. Lack of appropriate genetic variation, for example, prevents the development of many potentially favorable traits. If the allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be highly favored by natural selection. Most possible combinations of genes and genotypes have never existed in any population and so have never been tested under natural selection.

In addition, constraints are imposed on organisms by the dictates of physics and chemistry. The size of cells, for example, is constrained by the stringencies of surface area-to-volume ratios (see Figure 5.2). The ways in which proteins can fold are limited by the bonding capacities of their constituent molecules (see Key Concept 3.2). And the energy transfers that fuel life must operate within the laws of thermodynamics (see Key Concept 8.1). Keep in mind that evolution works within the boundaries of these universal constraints as well as the constraints described in this section.