Why do Motifs Occur in Engineering Systems?

Recent years have witnessed new research interest in the study of complex systems architectures, in domains like biological systems, social networks etc. These developments have opened up possibility of investigating architectures of complex engineering systems on similar lines. Architecture of a system can be abstracted as a graph, wherein the nodes/vertices correspond to components and edges correspond to interconnections between them. Graphs representing system architecture have revealed motifs or patterns. Motifs are recurring patterns of 3-noded (or 4, 5 etc.) sub-graphs of the graph. Complex biological and social networks have shown the presence of some triad motifs far in excess (or short) of their expected values in random networks. Some of these over(under) represented motifs have explained the basic functionality of systems, e.g. in sensory transcription networks of biology overrepresented motifs are shown to perform signal processing tasks. This suggests purposeful, selective retention of these motifs in the studied biological systems. Engineering systems also display over(under) represented motifs. Unlike biological and social networks, engineering systems are designed by humans and offer opportunity for investigation based on known design rules. We show that over(under) represented motifs in engineering systems are not purposefully retained/avoided to perform functions but are a natural consequence of design by decomposition. We also show that biological and social networks also display signs of synthesis by decomposition. This opens up interesting opportunity to investigate these systems through their observed decomposition.