Degeneracy and complexity in biological systems

Degeneracy, the ability of elements that are structurally different to perf orm the same function or yield the same output, is a well known characteris tic of the genetic code and immune systems. Here, we point out that degener acy is a ubiquitous biological property and argue that it is a feature of c omplexity at genetic, cellular, system, and population levels. Furthermore, it is both necessary for, and an inevitable outcome of, natural selection.