Download Book Review Werner Callebaut and Diego Rasskin

Book Review
Werner Callebaut and Diego Rasskin-Gutman (Eds.) (2005). Modularity: Understanding
the Development and Evolution of Natural Complex Systems. (Vienna Series in
Theoretical Biology). MIT Press, Cambridge (Mass.). 464 pp.; (Hb) ISBN 0-262-033267, $ 55.00.
Modularity is an old concept in the biological sciences. As early as the 18th century,
comparative anatomists such as Cuvier and Saint-Hilaire discussed structural modules
representing the parts of an organism, usually at the adult stage (see Winther (2001) for a
recent review). Embryologists recognized developmental modules as parts of the embryo
that change over developmental time. In the 1930s Joseph Needham postulated that
development consists of distinct processes that while operating in coordination, can be
dissociated into separate elements (Needham 1933). He proposed that they can evolve
separately from each other, thereby laying the foundation for the present-day study of the
evolution of development. The advent of protein crystallography in the second half of the
20th century uncovered the fundamental modular structure of proteins. They are
composed by domains, which can be defined as evolutionary modules (Murzin et al.,
1995) or defined from a purely structural perspective (Orengo et al., 1997). These protein
modules are the building blocks of proteins, which can be reused and combined in
different ways in evolution (Levitt and Chothia, 1976; Chothia, 1992; Vogel et al., 2005).
Many other areas of biology have been touched by modular thinking. The most
recent is cell biology (Hartwell et al., 1999), where the molecular networks that underlie
cellular processes were recognized as modular. For example, protein-protein interaction
(Rives and Galitski, 2003; Spirin and Mirny, 2003; Pereira-Leal et al., 2004) generegulatory networks (Ihmels et al., 2002; Segal et al., 2003) and metabolic networks
(Ravasz et al., 2002) have all been shown to display a modular organization, in which the
modules correspond to discrete functional units. The most popular area are clearly the
neurosciences, with a long standing controversy over distributed computing (Hopfield
1982) versus compartmentalization of processing and representation (Fodor 1983), a
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controversy fueled by the advances in neuroimaging technologies (see for example
(Downing et al. (2001)).
Modular systems are not limited to biology. In fact, in most areas of human
pursuit we find the mark of modular thinking. Its success lies on allowing a complex
system or task to be broken down into smaller, simpler functions. The individual modules
can be modified or operated on independently. Thus, changes in one part of a system
should not affect other parts. This makes it a popular programming paradigm in computer
science, particular for collaborative projects. Modularity is very common in industrial
processes where the modularization of design allows cost reduction while accelerating
product development process; increasing the range of product variations a company can
offer the market and enabling efficient technological upgrading of products. Modularity
also found its way into organizational theory modularity (Langlois and Robertson 1992;
Langlois 2002; Hoetker 2006). The above are just a few of the many possible examples
of modular thinking in different scientific disciplines and human enterprises.
In face of this array of disciplines where modular thinking has left its mark, it is
important to take stock and determine on one hand whether the modules discussed in the
different fields are in fact similar entities, and on the other hand what each discipline’s
notions of modularity can contribute to other disciplines. This is the framework in which
this book makes its contribution. It follows from the fifth Altenberg Workshop in
Theoretical Biology, “Modularity: Understanding the Development and Evolution of
Complex Natural Systems”, held In Austria in October 2000. This workshop brought
together researchers from many different fields with the aim of addressing three very
ambitious goals, which I quote from the book’s preface:
1) Survey the variety of disciplinary contexts in which “modular thinking” … plays a role
2) Clarify… what modules are, why and how they originate and change….
3) Bring about useful knowledge transfer between diverse fields….
As a foreword, the book contains a paper by the late Herbert A. Simon, discussing
near decomposable (ND) systems. ND systems are composed of subsystems in such a
way that the interactions among subsystems is of a different order of magnitude from that
of the interactions within subsystems. Drawing examples from different areas of research,
Simon illustrates the practical consequences of a hierarchical ND architecture in the study
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of complex systems –
the short-run behavior of each subsystem is essentially
independent from the other subsystems, each subsystem can be studied without explicitly
considering in detail the interactions at lower hierarchical levels, and interactions at
higher hierarchical levels can be considered constant in the time frame relevant for the
analysis of the subsystem. Although this concept was initially developed in the context
of mathematical economics, Simon’s foreword shows near decomposability (modularity)
as a physical property of many complex systems.
The book is organized into four parts. In part I (“The Ubiquity of Modularity”)
Werner Callebaut introduces the reader to the many issues that will be discussed
throughout the book, providing a global logic and integration between contributions. In
particular, a small discussion on types of modularity (developmental, morphological,
evolutionary, etc.) makes essential reading to understand the contributions later in the
book.
Part II of the book (“Evo-Devo: The making of a modular world”) is purely
biological and its many contributions are mostly focused on two main issues - the origin
of modules and the interplay between modularity and selection. I will highlight three
contributions that I, with my molecular and cellular background, found particularly
interesting. The first is an overview of ideas and models of the evolutionary origin of
modules by Wagner, Mezey and Calabretta. The popularization of the notion of
modularity in recent years brought about a quite confusing body of literature on the
subject, in particular on the origins of this property at different scales of biological
organization. This first discussion of the different models, what distinguish them and
what does not is a welcome contribution and will prove most useful to researchers in the
field. Also, their proposal of a plurality of coexisting mechanisms becomes ever more
evident as we consider the other contributions in this book. The second contribution I will
highlight is that by Meyers and Fontana on modularity in RNA structure. Their
discussion is routed on physical properties, advocating a role for physical constraint in
the origins of modularity. Finally, Schlosser discusses modules as units of selection,
explicitly bringing the evolutionary theoretical dimension to the notion of modularity.
In Part III of the book (“Evo-Patterns: Working toward a grammar of forms”), as
the title suggests, the emphasis is on form, on modules as physical observable entities, as
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recurring patterns. In the contributions of McShea and Anderson, Rasskin-Gutman, Eble
and Thomas the objective nature of morphological modules in biological organization is
discussed, with emphasis on its implications to our understanding of the evolutionary
process. In the first contribution we are asked to consider evolutionary transitions as gain
and loss of parts (modules), either by need (function is no longer needed) or by constraint
(function is no longer acceptable). Parts/modules can be considered at many levels of
organization, and McShea and Anderson discuss the hierarchical nature of modularity, a
recurring notion in all contexts of modularity. Rasskin-Gutman discusses morphospaces
as a conceptual framework where the multiple morphological building blocks come
together to define possible organic forms, identifying the need to understand the rules that
dictate which forms are and will be possible. Eble discusses empirical aspects of
morphological modularity, including the problem of identifying morphological modules
as organizational units within organisms, as variational units across organisms, as well as
causal actors. He revisits the discussions on the origin on modularity, evolvability and
selection at the macroevolutionary level. Thomas considers morphological modularity in
skeletal space to address the convergence, invention and discovery of form. Part III is
aptly closed with two contributions that depart from the biological focus of the previous
chapters, and considers modularity in art (Jablan) and as a universal property (Buscalioni
and collaborators). Jablan discusses the notion of impossible forms such as the Thierry
figure, built from possible modules, in this case the Koffka cube. This discussion is
reminiscent of that by Rasskin-Gutman on morphospace, specifically with respect to the
idea that there are many accessible combinations of modules, some of which are
impossible, some are possible but not observed, etc. The challenge lies in deriving a
grammar that allows us to understand which particular combinations of forms have a
biological meaning. Jablan’s contribution here illustrates how crossing disciplinary
boundaries can help us in achieving this goal. Buscalioni and coworkers consider patterns
in a variety of contexts, namely architecture and the arts to highlight the tripartite nature
of modularity, encompassing the whole modular structure, the individual modules and the
interactions between modules. It is interesting to observe how at the molecular and
cellular level the focus is shifting from attempting to understand the whole organism by
identifying the parts, epitomized by the “molecular biology” paradigm, to “systems
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biology” with its focus in the understanding of the whole through the understanding of
the interactions between the parts.
Part IV of the book (“Modularity of Mind and Culture”) takes us into a different
realm. The first three contributions deal with modularity of mind. In the first Calabretta
and Parisi discuss the contention between connectionism and modularism. They describe
two artificial life simulations where modularity emerges in evolving populations of
neural networks, suggesting that the two perspectives can coexist. The next contribution,
by Gobet, includes an overview of the uses of modularity in psychology. Velichovsky
argues against modularity of cognitive organization in its current “horizontal” meaning of
separation of processing modules, and argues instead for the idea of “vertical
modularity”, defending the use of Berstein’s motor control theory as an alternative,
aiming to reconstruct the hierarchical evolutionary mechanisms of mind and brain.
Translations from Bernstein’s Russian originals are provided, as appendixes to
Velichovsky’s contribution (the table of contents and part of the introduction of “On the
construction of movements”). The contribution by Marengo and coworkers takes us into
a different field, and introduces the notion of modularity in economics. Division of labor
and market selection represents modules and their interconnections/coordination
respectively. They discuss the role of near decomposability on module coordination, thus
illustrating how modular thinking can impact in economics. Finally Oller addresses the
issue of thought and language. His notion of natural logic is grounded on the assumption
that there is a limited range of logical possibilities for structures that can occur in any
language, and that these represent necessary modules of function, similar to the notion of
possible forms in Rasskin-Gutman’s morphospace discussed in part III.
This book is clearly not comprehensive in its survey of disciplines touched by
“modular thinking”. But it did not aim or need to be. Modularity plays a role in so many
different contexts that it would be impossible to address them all in a single volume. The
inclusion of contributions from fields outside of the natural sciences is a bold choice by
the editors. Although it may be questioned in respect to fitting under the heading “natural
complex systems” of the book’s title, it illustrates well the enormous gap that divides the
different research agendas making use of “modular thinking”, as well as the many
commonalities between different forms of modularity. We find that notions like
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hierarchical modularity, plurality of generative mechanisms, possible “modules” among
others recur in different contexts, which suggests that a general theory of modularity,
following in the footsteps of Simon’s near decomposability and encompassing inorganic
matter, life, mind and culture may not be unrealistic.
This is not a book for the uninitiated. Most chapters go deep into specific research
agendas and will make tough reading for anyone outside the field. The editors made an
effort to introduce each part, summarizing the contributions of the different authors and
attempting to contextualize them. However, bridging the different chapters is a challenge
mostly left for the reader. This said, while I read this book and in particular contributions
outside my own specific field, I found inspiration for new questions and experiments. So
maybe the most ambitious goal of the workshop, to catalyze knowledge transfer between
fields, may not be as ambitious as I deemed it to be. To have such an introduction to the
many faces of modularity is wonderful, and this book is a welcome addition to the limited
but hopefully growing body of literature on the subject. My feeling is that the book will
become indispensable reading for anyone interested in modularity.
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José B. Pereira-Leal
Instituto Gulbenkian de Ciência
Rua da Quinta Grande 6
Apartado 14
P-2781-901 Oeiras
Portugal
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