15 The Cost of Modularity

15.1 Introduction

Biological organisms are complex systems, as are most modern technical artifacts.
However, most of the entities of both classes are much less complex than the number
of their components would allow for. This is in part due to their modular organization—the
fact that they are not maximally integrated systems. Modularity means that the parts
of the system are grouped in such a way that strong interactions occur within each
group or module, but parts belonging to different modules interact only weakly (Simon
1969; Lewontin 1978; cf. also Alexander 1964). Since the maximum possible degree of
complexity depends on the number of components of a system and on the number
of interactions between these components (e.g., Simon 1969: 184), a limitation of interac-
tions in a system that consists of partly independent subsystems reduces complexity.
Exactly such limited interaction among subsystems occurs in systems organized in a
modular way.
Modularity may be best known from industrially produced technical artifacts. The whole
industry that produces integrated electronic circuit elements is built on the idea of grouping
several parts together and using such integrated modules as components of larger systems.
Similar design principles hold for washing machines, cars, and most obviously, stereo sets,
which even require the user to plug together physically separated modules. Modularization
is economically advantageous as it facilitates designing, constructing, and maintaining
artifacts (Ulrich and Eppinger 2003; Pahl et al. 2007). But modular organization is not
restricted to the realm of artifacts, in which economic principles rule. Biological evolution
brought about modular systems long before technology did. Anatomists and physiologists
have been aware of the modularity of organisms for a long time. Near-decomposability,
the analytic equivalent of modularity,^1 does not apply only on the macroscopic level, where
the organs of higher metazoans form clearly delineated structures that perform a limited
set of functions. Metabolic and gene regulatory networks, cellular signaling systems, and
developmental pathways are other instances of nearly decomposable systems (see section
15.2).^2

Ulrich Krohs