  We investigate the selective forces that promote the emergence of modularity
in nature. We demonstrate the spontaneous emergence of modularity in a
population of individuals that evolve in a changing environment. We show that
the level of modularity correlates with the rapidity and severity of
environmental change. The modularity arises as a synergistic response to the
noise in the environment in the presence of horizontal gene transfer. We
suggest that the hierarchical structure observed in the natural world may be a
broken symmetry state, which generically results from evolution in a changing
environment.
