Discussion

The observed robustness, the success of the simulation, and the absence of setup time violations, together provide good evidence that the circuit operates as a well-behaved digital design. However, the simulation waveforms (Figure 5) show many spikes and glitches of various durations that are determined by analogue time delays through the circuit. Such dynamics are avoided in digital design as leading to unreliable or non-robust operation. Presumably the present design evolved to be insensitive to these, perhaps completely ignoring them.

Together, these findings give a fascinating result. The circuit has evolved to achieve robustness by finding a well behaved, clocked, digital mode of operation. However, it is not at all clear what digital design rules could have been formulated in advance, that would have guaranteed robust digital operation and yet permitted this evolved circuit.

A surprising hypothesis is suggested: Even within robust digital design, unconstrained evolution can produce circuits beyond the scope of conventional design rules. Previously it had been assumed that the domain of robust digital design was fully covered by conventional design rules, and that novel territory for evolutionary exploration must lie in other domains [13]. Given the undoubted utility of robust digital designs, it is an exciting possibility that evolution could explore novel regions of design space, containing circuits that may be better for some applications.

More work is needed to investigate this bold idea, and the results presented here do not yet conclusively prove it. Future work should also look at the characteristics of the evolutionary intermediates that aided the discovery of the final solution. Finally, it should be noted that in nature there are numerous strategies for robustness other than clocked digital design [10], and these may be of interest to evolutionary electronics when using analogue components.