IntroductionEvolution is the process that organisms change over time due to natural selection. This can be caused by adaptive radiation; a geographical separation from the original ancestor causes the species to adapt to the new habitat. The change of habitat causes a different allele being more successful for survival, leading to an overall shift in natural selection and to the species. Anolis lizards have been said to be one of the best genera to exhibit adaptive radiation in recent science firstly this is because the genus has shown extensive speciation this has led to a large amount of research being done on the genus and because it has shown this adaptation to be rampant in different niches.History and current distribution of Anolis lizardsAnoles lizard’s common ancestor is thought to have originated from South and Central America. The genus is supposed to have evolved 40 million years ago, evidence for this is has been seen in fossils (amber specimens) found in the Dominican Republic dated to Oligocene (epoch of the Paleogene period) or Miocene (epoch of the Neogene period). The invasion of Anoles lizards to the Caribbean occurred twice, with the second colonizing the islands (Losos and Thorpe 2004). Today this colonization has been divided into the Anolis of the Greater Antilles and the Anolis of the Lesser Antilles. The Greater Antilles include the islands: Cuba, Hispaniola, Jamaica, and Puerto Rico, while the Lesser Antilles includes an older arc of islands and a younger arc (Losos and Thorpe 2004). The greatest amount of research has been seen in the Greater Antilles due to 111 species co-existing there, this is where the largest number of species inhabit diverse niches (as many as 11 species living sympatrically have been observed) (Jackman and Larson et al 1999).Diversity of Anolis lizardsTo this day in the Caribbean, an excellent example of Anolis ecomorph can be viewed. 150 species of the Anolis genus has been thought to have evolved from as few as 2 initial ancestors (Losos and Schneider 2009). The main species groups being: Trunk-Ground Anolis- these anoles live on broad trunks and have small flat bodies, Grass-Bush Anolis – found on narrow vegetation near the ground and have small flat bodies. Both Anolis species groups have long legs and small toe pads. The other two species groups are the Crown Giants the largest of the Anoles groups and Twig Anoles both found in the trees (Figure 1). They also have similar body shapes with larger bodies but smaller limbs but large toe pads (Hhmi.org 2017). This difference in toe pad size and leg length exhibits how the different species groups have adapted to survive in the different habitats. Longer leg length allows the ground Anolis to be better sprinters increasing their chances of escaping predators. Those species with larger toe pads are able to adhere more to branches allowing them to live higher in the tree branches, a further adaptation (Hhmi.org 2017). It has been seen that the characteristics in each of these groups have developed on each island of the Greater Antilles, further research and screening of the lizards’ genome has revealed that the independent species on each island are not closely related to each other even though the exhibit similar features (Givnish and Systma 1997). This is compelling evidence for adaptive radiation causing these features in the Anolis lizards.Figure 1 (A. cybotes, Hispaniola, top left and A. lineatopus, Jamaica, top right, with dewlap extended), twigs (A. valencienni, Jamaica, bottom left) and tree canopies (A. chlorocyanus, Hispaniola, bottom right). Photo credits: top right, bottom left: L. Mahler; bottom right: M. Losos. Sourced from Losos and Schneider 2009) Specific morphologies and adaptive variationsAnolis lizards have 2 specific morphological characteristics: one being the subdigital expandable toe pads. These are composed of laterally expanded scales which are termed lamella which is covered in millions of hair-like structures called setae. This allows for the toe pads to adhere to surfaces such as tree branches (Losos and Thorpe 2004). The second characteristics being a gular throat fan called a dewlap, this is possessed by all males and most females. The dewlap is presented in different behavioural characteristics such as territorial rivals, predators and finding potential mates. The use, colour, and shape of the dewlap are species-specific (Losos and Thorpe 2004). These morphological features allow for the species to diversify. The main reason for the speciation of Anolis is inter-specific competition (Losos and Schneider 2009), this competition causes several members of the population to move to a different niche in search of resources. In the new niche, there would be diverse conditions meaning that a distinct set of characteristics would now be more successful. The separate population then adapts to the new habitat this usually occurs through the dewlap, for example, change of colour to make the dewlap more visible in the habitat or a change to the head movement (Losos and Schneider 2009), this maximizes the intra-specific communication enhancing the chances of the species reproducing and succeeding. The change in successful characteristics then leads to adaptation to the new habitat such as the change in leg length and the size of toe pad to allow better survival. A change to the dewlap results in the lizards becoming reproductively isolated from each other, limiting intraspecies reproduction between the old and new species. Leading to complete species separation, till the two don’t recognize each other as the same species (Hhmi.org 2017).Conclusion
The evidence presented exhibits that adaptive radiation is a major factor in the speciation and evolution of Anolis lizards. This can be seen by the wide variety of species under the Anolis genus (400). Also, by how on separate islands in the Caribbean the Anolis species have developed similar features which have improved their survival in various niches, even though none of these similarly featured lizards is closely related to each other.