Speciation is the formation of new and distinct species in the course of evolution. The Fijian banded iguana and the Fijian crested iguana likely started out as the same species but different environmental circumstances had them evolve to be two seperate species.
The Fijian Banded Iguana is considered a national treasure by the government of Fiji. It is found on the more southern Fijian islands and is an arboreal species. Male banded iguanas have blue or white stripes on a bright green base while females are more uniformly green with occasional faint banding or spotting. Both sexes have yellow underbellies and yellow rimmed nostrils. Like other iguanas they are able to change colour to blend in with their surroundings. They can grow up to 60 cm long (about half of this would be tail). This species of iguana is omnivorous, eating leaves, flowers and insects.
The Fijian crested Iguana is a critically endangered species of iguana and is found on the north-western islands of Fiji. It was once known to be on 14 of the Fijian islands but is currently only found on 3. The crested Iguana is a brilliant green colour with 3 white stripes that are sometimes edged with black. These iguanas can grow up to 75 cm long. They have distinctive crests lining the length of their backs. Each spine on the crest can grow up to 1.5 cm long. They have long, strong claws which makes them very good at climbing. The Fijian crested iguana is herbivorous eating leaves, shoots, flowers and roots.
Although very similar, these two species of iguanas have several traits that have evolved differently through ecological isolation and natural selection over millions of years. Although it is unknown how the two species got to be on different islands, it is most likely that they both started on the same one and the island slowly seperated into two different ones. This would explain the different traits they have that are specialized for the environment on the seperate islands. One of these different traits is the defense mechanism. In the crested iguana, the defense mechanism is to change colour to camoflauge itself from predators, much like a chameleon. This would have developed as a result of having visual hunting predators. The banded iguana does not change colour. Its defense mechanism is to go up into the trees and hide. This defense would be very effective if the predators present were primarily ground dwelling, as feral goats and mongoose are. In terms of how natural selection ties in, in both scenarios the iguanas with the most valuable traits survive and live longer than the iguanas that do not have those traits. This means the iguanas that live longer reproduce the next generation, and the next generation will have those favorable traits. An interesting point to add, is the fact that the Fijian crested iguana is critically endangered. This may have something to do with the defense mechanism it adapted versus the defense mechanism the banded iguana adapted. The process of going up into the trees would be more effective in preventing being eaten by a goat than just trying to blend into the background would.
Another trait of the banded iguana is the ability to eat insects. After the probable island split, there may have been less vegetation, and therefore less source of food. The ability to eat insects would have evolved slowly out of necessity. The iguanas that could eat insects would live longer and reproduce, passing this gene onto the next generation. This is another example of natural selection.