Evolutionary convergence could be defined as the phenomenon of acquiring similar sets of characteristics in response to the adaptations to a common or similar environment by the ones of two or more totally unrelated or distantly related groups. Thisresults in, organisms with completely different body organisation come to process superficial resemblance.
It means that the organisms of different classes can acquire similar characteristics independently and separately to avail the similar environment.
The most common example of convergent evolution is the development of wings in insects, flying reptiles, birds and flying mammals. All of them, although belonging to different groups have one common feature, the development of wings for flight.
The aquatic vertebrates, cartilaginous and bony fishes, ichthyosaurs - type of extinct aquatic reptiles, and the dolphins and porpoises - aquatic mammals, are all superficially alike, having fusiform or streamlined body with a median dorsal fin, caudal fin and paired fins. In aquatic reptiles and mammals the limbs are modified into flippers. Although, for a layman all of them are fish, because of their superficial resemblance, but their anatomical details are markedly different.
The burrowing mammals - moles and gophers have their forelimbs modified for digging, but both belong to two different orders of class Mammalia. The former is an insectivorous and latter a rodent.
These species have evolved similar characters or traits that are independent of each other.
Convergent evolution is shown by analogous organs. Two species that are related to each other develops some how similar characters or features as a result of adaptation. It is also known as isomorphism.
Significance - The convergent evolution leads to formation of analogous similarities among different groups of organisms which indicates that evolution may lead to superficial resemblances. N.W. Pirie has pointed out that a kind of convergence might have played a major role during prebiotic evolution, before the chemical system accumulated characteristics of living organisms and became dominant or led to organic evolution.
In the end it could be concluded that in the evolution of life from simple to more complicated or complex forms both divergence and convergence have played an important role, but convergence is less frequent to that of divergence.
Anoles comprise one of the most diverse groups of vertebrate's genera with nearly 400species known so far. Many species of this type occurs in the island in the Caribbean and rest are found in central and northern South America.
Often beautiful anoles are captivating lizards with a rich behavioural repertoire and extensive variation among species. Most anoles are green, grey or brown, they are generally 35- 85 mm in body length and 1- 10 grams in mass, though some may be substantially larger.They have a generalised lizard body form with robust limbs and a moderately long tail.
Though there is significant variation among species. The two primary traits that characterise anoles with a very few exceptions, arepossession of expanded toe pads and an extensible colourful flap of skin, they develop which is attached to the throat.
Each island has an anole that lives among twigs. That has prehensile tails, short legs, lichen like pattern on their back, and slow, and creeping movements.Anoles lizards are one of the best examples of convergent evolution. Population of lizards on isolated islands diverge to occupy separate ecological niches, mostly in terms of location within the vegetation where they forage. Some living in the crown of trees, others low on the tree trunk near the ground others in the mid trunk area, some in twigs and some in the underlying shrubs.
Studies of anoles have clearly shown the basicconcept regarding the development of the principlesin ecology and evolution.
Two important patterns of diversity have been generated by substantial ecological and evolutionary research on anoles. First, on each of the islands of the Great Antilles- Cuba, Hispaniola, Jamaica, and Puerto Rico - sympatric species differ in habitat use, behaviour and morphology. For example species that use broad tree trunks near the ground tend to have long hind limbs, which they use to run quickly and jump great distances. In contrast, species that use narrow twigs high in the trees are known to have veryvery shortlegs and tend to creep very slowly to capture prey and escape detection by predators. Species that use the vegetation high in the tree have very large toe pads and are green in colour. In all, six types of habitat specialist, termed ecomorphs, have been discovered.
The second most important view comes from the study of anoles in the Greater Antilles is that, with a few exceptions, the same set of habitat specialists has evolved independently on each island. Phylogenetic analysis signifies that distinct species occupying the same habitat specialist category on the different islands and are not closely related.
These anoles communities served as a model system for the development of ecological theory in the year 1960s and 1970s and important early work on interspecific competition, niche variation, character displacement and other phenomena were performed on anoles.
Convergent evolution has been taken as an evidence of adaptation. A hypothesis of adaptation can be further tested by demonstrating that the convergent features are verybeneficial in the environment in which they occur. Work on anoles was important in developing the idea that ecological adaptation must be studied by examining measures of whole organism performance. These studies indicated the morphological and physiological variation among species leads to differences in capabilities that are appropriate to the different habitats that species occupy.
Cause of evolutionary diversification of anoles
The classic idea of adaptive radiation is that it results from inter specific competition driving initially similar species to diverge in resource use and adapt to new habitats.
These are some predictions that are strongly supported for anoles. A wide variety of research -including behavioural observations, comparisons across study websites and experimental manipulations - indicates that anole species very stronglyinteract and that interspecific competition for resources is likely the cause of their adaptive divergence.Changes in habitat use as a result of the presence of other in related species are well documented, and comparisons across populations demonstrate corresponding adaptive changes in morphology.