  Complex evolving systems such as the biosphere, ecosystems and societies
exhibit sudden collapses, for reasons that are only partially understood. Here
we study this phenomenon using a mathematical model of a system that evolves
under Darwinian selection and exhibits the spontaneous growth, stasis and
collapse of its structure. We find that the typical lifetime of the system
increases sharply with the diversity of its components or species. We also find
that the prime reason for crashes is a naturally occurring internal fragility
of the system. This fragility is captured in the network organizational
character and is related to a reduced multiplicity of pathways between its
components. This work suggests new parameters for understanding the robustness
of evolving molecular networks, ecosystems, societies, and markets.
