A significant shift in the environmental conditions over a very short period of time caused most groups of dinosaurs to become extinct because of their inability to adapt fast enough. This allowed for a variety of niches to open providing current organisms new opportunities. Mammals were one group of organisms that took advantage of this environmental change, filling niches that were unoccupied and eventually developing new species that were specifically adapted to their particular environment. Which of the following is a term that most correctly describes this process?