  The causes of major and rapid transitions observed in biological
macroevolution as well as in the evolution of social systems are a subject of
much debate. Here we identify the proximate causes of crashes and recoveries
that arise dynamically in a model system in which populations of (molecular)
species co-evolve with their network of chemical interactions. Crashes are
events that involve the rapid extinction of many species and recoveries the
assimilation of new ones. These are analyzed and classified in terms of the
structural properties of the network. We find that in the absence of large
external perturbation, `innovation' is a major cause of large extinctions and
the prime cause of recoveries. Another major cause of crashes is the extinction
of a `keystone species'. Different classes of causes produce crashes of
different characteristic sizes.
