Black-headed siskin

This species is part of a rapid recent adaptive radiation of Spinus finches in Central and South America, which produced at least eight other recognized species: S. atratus, S. crassirostris, S. spinescens, S. yarrellii, S. magellanicus, S. olivaceus, S. xanthogastrus, and S. barbatus. The black-headed siskin was the earliest of these species to diverge.[2]

It was originally proposed that the radiation occurred around 3.5 million years ago, when an ancestral form, perhaps similar to the modern-day black-headed siskin, was able to adaptively radiate into South America. In this version of events, the dispersal into South America happened before the isthmus of Panama was complete (around 2.7 million years ago), but when mesothermal montane plant species were able to disperse across the narrowing strait. The novel plant communities in South America would have created many new niches into which the ancestral species adaptively radiated.[2]

More recent work suggests the species radiation happened much later, within the last 1 million years, and posits the radiation happened due to climate change in the late Pleistocene. The changing climate could have fragmented various habitats in the Andes, initiating allopatric speciation.[3]

Spinus notatus passing the Isthmus of Panama when it was closed[2]