  The evolution of structure in biology is driven by accretion and change.
Accretion brings together disparate parts to form bigger wholes. Change
provides opportunities for growth and innovation. Here we review patterns and
processes that are responsible for a 'double tale' of evolutionary accretion at
various levels of complexity, from proteins and nucleic acids to high-rise
building structures in cities. Parts are at first weakly linked and associate
variously. As they diversify, they compete with each other and are selected for
performance. The emerging interactions constrain their structure and
associations. This causes parts to self-organize into modules with tight
linkage. In a second phase, variants of the modules evolve and become new parts
for a new generative cycle of higher-level organization. Evolutionary genomics
and network biology support the 'double tale' of structural module creation and
validate an evolutionary principle of maximum abundance that drives the gain
and loss of modules.
