The radiation of South American siskins was rapid, and was originally thought to have occurred around 3.5 million years ago due to a range expansion associated with the Great American Interchange and contingent upon the spread of mesothermal plants from the Rocky Mountains to the Andes.More recent work suggests the radiation occurred much later, within the last 1 million years, and speciation events may have been initiated by the climactic variations of the late Pleistocene.