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Venom in snakes and some lizards is a form of saliva that has been modified into venom over its evolutionary history.〔(【引用サイトリンク】url=http://www.reptilepark.com.au/venom_snake.asp )〕 In snakes, venom has evolved to kill or subdue prey, as well as to perform other diet-related functions. Venom originated just once among all Toxicofera approximately 170 million years ago, and then diversified into the wide range of venoms seen today. The original toxicoferan venom was a very simple set of proteins that were assembled in a pair of glands. Subsequently, this set of proteins diversified in the various lineages of toxicoferans, including Serpentes, Anguimorpha, and Iguania. Several snake lineages have since lost the ability to produce venom, often due to a change in diet. The evolution of venom is thought to be responsible for the enormous expansion of snakes across the globe. The mechanism of evolution in most cases has been gene duplication followed by natural selection for adaptive traits. Some of the various adaptations produced by this process include venom more toxic to specific prey in several lineages, proteins that pre-digest prey, as well as a method to track down prey after a bite. These various adaptations of venom have also led to considerable debate about the definition of venom and venomous snakes. ==Evolutionary history== Until the use of gene sequencing to create phylogenetic trees became practical, phylogenies were created on the basis of morphology. Such traditional phylogenies suggested that venom originated in multiple branches among Squamata approximately 100 million years ago. More recent studies using nuclear gene sequences, have found the presence of similar venom proteins in several lizards within the Toxicoferans, demonstrating that venom originated only once within the entire lineage approximately 170 million years ago. This ancestral venom consisted of a very simple set of proteins, assembled in a pair of glands. The venoms of the different lineages then diversified and evolved independently, along with their means of injecting venom into prey. The origin of venom is thought to have provided the catalyst for the rapid diversification of snakes in the Cenozoic period, particularly to the Colubridae and their colonization of the Americas. Scholars suggest that the reason for this huge expansion was the shift from a mechanical to a biochemical method of subduing prey. The venom system has also subsequently atrophied, or has been completely lost, independently in a number of lineages. The American “rat snakes,” such as ''Pantherophis guttatus'', lost their venom following the evolution of constriction as a means of prey capture. The independent evolution of constriction in the fish-eating genus ''Acrochordus'' also saw the degradation of the venom system. Two independent lineages, one terrestrial and one marine, that shifted to an egg-based diet, also possess the remnants of an atrophied venom system. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「evolution of snake venom」の詳細全文を読む スポンサード リンク
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