A 3-foot-long iguana sat sedately beside a path on Little Cayman in the Caribbean Sea.
I had seen iguanas on other islands, but this one looked unique. It was a Sister Isles rock iguana, Cyclura nubila caymanensis, which occurs only on the Sister Islands, or Little Cayman and Cayman Brac, two small islands separated by six miles of open ocean.
Iguanas are large, herbivorous reptiles with large feet, a prominent fold of skin hanging from the throat and a series of spines beginning at the nape of the neck and extending on the dorsal midline the full length of the back. This specimen had a grey background with dark grey or black vertical bars extending from the back to the edge of the belly, which was a warm reddish brown. The head was adorned with large, rounded scales that looked like beads, and the tail was heavy and as long or longer than the body.
A closely related iguana, colloquially the blue iguana but formally Cyclura nubila lewisi, is limited to Grand Cayman, a slightly larger island 60 miles to the south. It grows to 5 feet long, and the males turn bright blue during mating season. Yet another form, the Cuban rock iguana, Cyclura nubila nubila, is found only on Cuba, about 100 miles north of the Sister Isles. It has a blue head, smaller head scales and shorter toes.
The iguanas on Grand Cayman, the Sister Isles and Cuba are the same species, but they have reliably differentiated characteristics that justify recognition as subspecies. Genetic data revealed that iguanas from Cuba colonized Grand Cayman several million years ago and the Sister Isles more recently.
If we were to pan further out so that Jamaica, the Bahamas, the Turks and Caicos, Hispaniola and Puerto Rico come into view, we would see the geographic ranges of seven more species of rock iguanas. One species is limited to Andros and Exumas in the Bahamas, with separate subspecies on each island. Jamaica, Acklins Island in the Bahamas and the Turks and Caicos each has its own species. One species is limited to Hispaniola and nearby Mona Island, with recognized subspecies on each. Finally, a species is limited to tiny Anegada Island, east of Puerto Rico.
Oceanic islands have long been known as showcases of adaptive radiation, and because iguanas are usually common and accustomed to humans, they are one of the most conspicuous examples of speciation on oceanic islands. Isolation imposed by salt water reduces or eliminates migration among islands, and the cessation of gene flows allows populations to adapt to their local habitats and to become differentiated from one another.
The Galapagos Islands, 600 miles west of Ecuador, are one of the most isolated archipelagos on Earth and also a living showcase of evolution, for the Galapagos has the highest proportion of endemic species, which are species that occur in only one geographic location. The probability that a species could colonize the islands is very low, but the lucky immigrant species have radiated into clusters of endemic species.
Genetic data suggest that that the geographically widespread green iguana arrived in the Galapagos first and evolved into the only marine iguana found anywhere and several new species of land iguanas, all endemics. The most common land iguana has a dark back and tail, with highlights of green around the mouth, orange on the chest and forelegs and a blue circle around the eye. But a yellow species (this species is controversial, not recognized by all of the experts) is also present in a few localities, and a rare pink species was found in 1986 and is now recognized as a full species.
The Galapagos Islands are also home to seven endemic species of lava lizards and six endemic species of geckos. Each of these endemic clusters evolved from a single species that colonized the Galapagos millions of years ago.
Jeff Mitton (firstname.lastname@example.org) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.