Large extinctions in an evolutionary model: the role of innovation and keystone species.

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 coevolve 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.