  A central biological question is how natural organisms are so evolvable
(capable of quickly adapting to new environments). A key driver of evolvability
is the widespread modularity of biological networks--their organization as
functional, sparsely connected subunits--but there is no consensus regarding
why modularity itself evolved. While most hypotheses assume indirect selection
for evolvability, here we demonstrate that the ubiquitous, direct selection
pressure to reduce the cost of connections between network nodes causes the
emergence of modular networks. Experiments with selection pressures to maximize
network performance and minimize connection costs yield networks that are
significantly more modular and more evolvable than control experiments that
only select for performance. These results will catalyze research in numerous
disciplines, including neuroscience, genetics and harnessing evolution for
engineering purposes.
