Degeneracy vs Redundancy
I'm actually writing a paper on a related subject (more about that at some point) and came across an article that made the point about the difference between perfect and partial redundancy much better than I did. One of the authors is Gerald Edelman, who won the Nobel Prize for Medicine and Physiology in 1972 for his classic on the structure of antibodies. They argue that there's a difference between degeneracy in biology and redundancy in engineering:
Now that's what I'm talking about!
The contrast between degeneracy and redundancy at the structural level is sharpened by comparing design and selection in engineering and evolution, respectively. In engineering systems, logic prevails, and, for fail-safe operation, redundancy is built into design. This is not the case for biological systems. Indeed, not the least of Darwin's achievements was to lay the argument by design to rest. But, for obvious economic reasons, design is by far the major component of most technical efforts in modern society. In general, an engineer assumes that interacting components should be as simple as possible, that there are no "unnecessary" or unplanned interactions, that there is an explicit assignment of function or causal efficacy to each part of a working mechanism, and that error correction is met by feedback, modeling, or other paradigms of control theory. Protection can be afforded by planned redundancy, but adventitious compensation for error is neither expected nor usual. Irrelevancy is avoided from the outset.
By contrast, in evolutionary systems, where there is no design, the term "irrelevant" has no a priori meaning. It is possible for any change in a part to contribute to overall function, mutations can prompt compensation, stochastic interactions with the environment can lead to strong selection, often there is no fixed assignment of exclusive responsibility for a given function, and, unlike the engineering case, interactions become increasingly complex [...]