An alternative view of engineering is that the field deals with solving constraints or understanding constraints imposed by the characteristics of the parts or organizational structures. Therefore, understanding or developing a theory of constructive engineering principles of life will also yield insights into limits and constraints of biological systems.

The previous chapter discussed how the interior of the cell is highly organized. In fact, much of nature is highly organized, and the organization, or regularity, often seems to emerge without any external direction. A single fertilized egg develops into a mature multicellular organism with all of its many organs, limbs, and blood vessels in the right places. An ecosystem damaged by fire gradually returns to its original mix of species, reorganizing the interdependent community. This chapter will explore the common organizational characteristics and constructive principles of biological systems that lead to complex behaviors, products, and processes.

CORE CONCEPTS

Some core concepts that link different kinds of complex systems are modules, nodes, networks, emergent behavior, topology (or architecture), and robustness. Table 6-1 provides definitions of these terms and gives examples from several different kinds of systems.

A brief caveat is in order. In this chapter, the terms “modularity,” “emergence,” and “robustness” will be used to describe characteristics of biological systems that arise at different scales and are in need of further conceptual development. However, the terms have been used in other ways in different domains. However described and however generalizable they may be, the phenomena of modular organization, complex ensemble behavior that might be called emergent behavior, and robustness in biological processes exist and can be described and measured. Whether the best approach will be classical, using existing tools, or whether an entirely new set of formalisms will be required, the problem remains that effective conceptual and theoretical treatment of those topics is not yet available. A satisfactory description or computation of those phenomena is a critical challenge for the future of biology.

Characteristics of Modules

In every biological organization certain divisible parts are recognizable whose repetition and elaboration seem to generate the whole. These parts are often recognized as physically distinct units—the canonical example being the individual organism. In some cases, such units had a conceptual existence before their physical manifestation was known. An example is “the gene” as described before the development of the chromosomal theory



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