Adaptive radiation
An example of a more modest innovation is the evolution of a fourth cusp in the mammalian tooth. This trait permits a vast increase in the range of foodstuffs which can be fed on. Evolution of this character has thus increased the number of ecological niches available to mammals. The trait arose a number of times in different groups during the Cenozoic, and in each instance was immediately followed by an adaptive radiation. With birds, the evolution of flight opened new possibilities, and at least two huge adaptive radiations occurred (one before and one after the K/T extinction event). Even more striking was the evolution of insect flight, which led to huge radiation in the Mesozoic era. Then, these insect groups developed ways of feeding on flowering plants. They now outnumber all other forms of animal life by quite a margin.