Charles Darwin made the role of islands in evolution famous with his observations in the Galapagos. He recorded giant tortoises grazing on the slopes of volcanoes 600 miles from the coast of South America. Cormorants that had lost the ability to fly were present on the islands. And then those famous birds Darwin believed to be blackbirds, gross-bills, and finches all turned out to be ground finches that superficially looked like other species of birds.

Darwin presented the birds he collected on his voyage around the world to the Zoological Society of London in 1837. The Society gave the specimens to John Gould for identification. Gould met with Darwin after he had examined the specimens and discussed the birds from the Galápagos Islands. Darwin had thought the birds were blackbirds, gross-bills, and finches. But Gould said – no, they were not. Instead, he explained to Darwin that they were all a series of ground finches so peculiar’ as to form ‘an entirely new group, containing twelve species.’ This story made the newspapers.

The importance of islands in Darwin’s theory of evolution by natural selection is difficult to overestimate. Islands isolate land-dwelling animals from other populations over time. That isolation leads to inbreeding on each isle, causing species to evolve and become genetically distinct. Given time and genetic differences, island and mainland populations that once shared an ancestor become new species.

Darwin had not bothered to label his finches by island. Fortunately, others on the expedition had taken more care. Darwin now sought specimens collected by captain Robert FitzRoy and crew members. From them, he showed that the species were unique to islands, an essential step in the inception of his theory of evolution by natural selection. However, the absence of labels with island information meant Darwin had missed an important point – geography influences biology.

Gould’s conclusions about the other birds changed the paradigm that species were immutable. Darwin had labeled the four mockingbirds by island, and he speculated that if they were varieties, the stability of species would be undermined. It would suggest that species could change. Gould told him that three of the mockingbirds were distinct species, and they had relatives on the mainland, close relatives, but not identical.

Thomas Bell, a herpetologist at the British Museum of Natural History, confirmed that the giant tortoises were native to the Galapagos, not an alien species brought from some other island that was being stored on the islands by sailors. Combined with the finch and mockingbird findings, it was likely that each island had its indigenous tortoise. But it was too late to prove the point: Darwin had missed the opportunity or eaten his way through it. Only a baby tortoise had survived, which lacked the adult’s distinguishing features. Darwin had a problem explaining dozens of new but related species on each island in the Galapagos archipelago.

The idea of sky islands is a mid-20^th-century concept. In a detailed ecological treatise on birds in the pine-oak woodlands of southern Arizona, Joe Marshall (1957) never used the terms “island” or “archipelago.” Still, he recognized the similarity between the patches of pine-oak forests on mountaintops. However, it was not until 1967 that the moniker “sky islands” was coined by Weldon Heald in his book The Chiricahua Mountains.

Using the Madrean Sky Islands of Arizona as an example, Warshall (1994) noted biotic communities are stacked on mountainsides and add a vertical part to plant and animal migrations. The valleys act as barriers when their ecology is hostile to the species trying to colonize or move across it. Still, they act as bridges or corridors when valley ecology supports the species. The mountains are cradles of evolution at mid and high elevations; like oceanic islands, they produce endemic species. Warshall listed 22 other sky island complexes around the world. Not all mountains are sky islands, only the ones with distinctly different plant communities at the upper elevations compared to those at lower elevations.

The largest Madrean sky island ranges in Arizona are the Baboquivari Mountains, Chiricahua Mountains, Huachuca Mountains, Pinaleño Mountains, Santa Catalina Mountains, Rincon Mountains, Santa Rita Mountains, and Whetstone Mountains. The sky island ranges in nearby states include the Animas Mountains in New Mexico and the Guadalupe Mountains, Davis Mountains, and Chisos Mountains in west Texas.