I will discuss the emergence of modularity in examples from the natural world.
Dynamical systems typically evolve in a changing environment, and I will show
that the level of modularity correlates with the rapidity and severity of
environmental change.  Emergence of modularity is driven by noise in the
environment and is facilitated by horizontal gene transfer.  This mechanism is
evident in a number of systems, from viruses and bacteria to development and
physiology. Bacterial metabolic networks show increasing modularity as the
physical environment or horizontal gene transfer rate increases, and
experimental protein interaction data shows that protein networks have become
increasingly modular over the last four billion years. More recently,
modularity provides early warnings in the evolution of influenza flu strains
and in heart rate anomalies in physiology.  I will describe a principle of
least action that governs the emergence of modularity in certain limits.

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