EVOLUTION CAUSES AMPHIBIANS TO BE LIMBLESS
Almost all aquatic species, mostly marine evolved some 40-50 million years ago from terrestrial tetrapod ancestors during some past ten to twelve million years. All living whales lack hind limbs and have forelimbs modified as flippers. Their limbed ancestors such as the Eocene Ambulocetus natans use paraxial locomotion powered by limbs. When this species are inside the water, locomotion is facilitated by a combination of tail undulation and paddling, which use the pelvic limbs. But now modern cetaceans use oscillatory locomotion. Aquatic mammals rely much less on vision and olfactory senses than terrestrial mammals. A major selective pressure during the transition to the aquatic environment was adaptation to underwater hearing. Some species such as whales, dolphins and others use acoustics for communication, orientation and detecting, recognizing and localizing companions, competitors, mates, predators and prey. The external ears have been lost and thus reducing hydrodynamic resistance during swimming.
Different approaches from paleontology, morphology, development, genetics and molecular biology has tried seek an integrated explanation for whether highly specialized phenotypes such as vertebrates that are not closely related, could result from similar genetic or developmental mechanisms. The origin and evolution of whales was addressed concerning the series of morphological changes that facilitates the progressive reduction and ultimate loss of hind limbs. Genetic evaluation was done on limb loss in snakes and to legs-less lizards, to see whether similar mechanisms which have driven the loss of hind limbs in whales could have been used. Also to see to which degree has such mechanisms diverged in the evolution of similar evolutionary trends shown by limb loss.
Limb loss and concurrent morphological and physiological changes is associated with the transition from land to water are discussed within the context of the current whale phylogeny. This is mostly emphasized concerning the fore and hind limb development, how the fore limbs are transformed into flippers and how the hind limbs are regressed leaving no elements or vestigial skeletal elements. Hind limbs started regressing after the ancestor of whales entered the aquatic environment. Elongation of the body during transition from land to water causes the loss of hind limbs.
In most snakes limbliness was associated with adoption of new lifestyle and was driven by developmental changes associated with elongation of the body. Adaptation to burrowing between the parallel reflect structural and functional changes associated with the switch of locomotion to axial. However, selective pressures acting on a wide range of developmental processes and adult traits other than the limbs could have driven the loss of hind limbs in whales. All limbs less tetrapod are descended from limbed ancestors. Most possess limb buds at some stage during their development. Limbless ness represents arrest of limb bud development rather than absence of limb initiation or regression of a fully formed limb.
Andrews, R. C. 1921. [A remarkable case of external hind limb in a hump-back whale] [Cited on 2006 05 10] Available from:
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