Mechanisms of Speciation

Recently I read a report on Fijian Iguanas and found some interesting findings that I could relate to evolution, specifically to some of the mechanisms of speciation.

Fijian Iguanas (Brachylophus)

Brachys (βραχῦς) is Greek for “short” and lophos (λοφος) is Greek for “crest” or “plume.”

Outside of zoos, iguanas are only present in the New World (North, Central, and South America) and in Fiji and Tonga. The issue of the odd distribution of these iguanas is still being discussed by scientists today, and there are two main theories. First, that they migrated though a Melanesian land bridge that used to exist to connect Fiji to mainland Asia. This theory is supported by the fact that Iguana-like reptile remains were found in south-east Asia. The other theory, the one I find most interesting, is that Iguanas from South America migrated on vegetation mats 8000km over the Pacific Ocean to Fiji. This explanation is already considered sufficient to explain how several iguana species reached islands in the Caribbean and the Galapagos Islands, which are over 1000km from the mainland. South Equatorial surface currents today go through the Galapagos and past several south pacific islands, including Fiji, and if these existed millions of years ago it may be how the iguanas got to Fiji. The Galapagos Land Iguanas are also closely related to the Fijian Iguanas as they are both in the same subfamily: Iguaninae. If this theory is correct then the mechanism of speciation would be ecological isolation, as the vast distance between the two groups of iguanas would have provided the reproductive isolation needed for the two to evolve into separate species. The Fijian Iguanas also have an unusually long egg incubation period, over 9 month, evidence that eggs with long incubation periods may have survived the journey. This is an example of natural selection. Eggs with shorter incubation periods would have a lesser chance of surviving the trip. As a result, the genus Brachylophus has longer incubation periods because the opposite allele may have been wiped out during the trip across the pacific. From here the iguanas landed on separate islands, forming three distinct species, B. fasciatus (Fiji Banded Iguana), B. vitiensis (Fiji Crested Iguana), and B. bulabula. Bula is the Fijian word for hello.


Fiji Banded Iguana

banded f

(c) Female

banded m

(d) Male

The Fiji Banded Iguana, though in the same genus as the other two, is quite different, especially from the Fiji Crested Iguana, because of their different habitats. The Fiji Banded Iguana is found in the south-eastern islands in Fiji and lives in mainly wild forests, living in trees. Because of this, it has many adaptations that make it more suited to its habitat than the other iguanas. It is smaller and thinner than the crested iguana, making it more suited for jumping from tree to tree, but is usually about 5cm longer. Its tail is longer for balance, being over 2/3 of the lizards total length. Its green colour, with he paler bands that give it its name, are suited to its habitat, blending with the leafy background of the trees it lives with. This iguana eats leaves, flowers, fruits, and sometimes insects. They are able to flick their tongue out of their mouth, which is used during courtship by the male.

Fiji Crested Iguana

crested f

(e) Female

crested m

(f) Male

This iguana lives in the north-western part of Fiji, in dryer regions than the other iguanas. I generally lives in the forests near the beaches on uninhabited islands. This difference in habitat has caused the crested iguana to evolve different adaptations than the other iguanas. They feeding on trees and shrubs, particularly hibiscus flowers of the Vau tree. This iguana is heavier and shorter, not needing to climb as fast to hunt insects. It also has the ability to turn from green to black very quickly an has longer spines on its back (up to 1.2cm). The spines likely serve as protection, and the ability to change colour is likely because it faces a wider variety of habitats.

B. Bulabula

bulabula f

(a) Female

bulabula m

(b) Male

This species of iguana is native to the central islands of Fiji, living in wet habitats. It is very similar to the Fiji Banded Iguana, and was first classified as B. Fasciatus, but was found to be separate in a later study. It is still often referred to as the Fiji Banded Iguana, even though the females do not have bands. They used to live in the same habitat as the Fiji Banded Iguanas, and therefore have similar adaptations. The female bulabula does not have bands on it, which is likely either camouflage due to a difference in habitats from the Fiji Banded Iguana (this is unlikely as they share similar habitats), or because of a difference in mating preferences. Other than this, the Bulabula is almost exactly similar the the banded iguana. The Bulabula likely has a closer common ancestor to the banded iguana. It might even be that the crested iguana evolved from a different iguana that arrived in Fiji through rafting.


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