Evolution of snake venom no relation Burmese python
Venom in snakes and some lizards is a form of saliva that has been modified into venom over its evolutionary history. In snakes, venom has evolved to kill or subdue prey, as well as to perform other diet-related functions. While snakes occasionally use their venom in self defense, this is not believed to have had a strong effect on venom evolution. The evolution of venom is thought to be responsible for the enormous expansion of snakes across the globe.The evolutionary history of snake venom is a matter of debate. Historically, snake venom was believed to have evolved once, at the base of the Caenophidia, or derived snakes. Molecular studies published beginning in 2006 suggested that venom originated just once among a putative clade of reptiles, called Toxicofera, approximately 170 million years ago. Under this hypothesis, 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 also lost the ability to produce venom. The Toxicoferan hypothesis was challenged by studies in the mid-2010s, including a 2015 study which found that venom proteins had homologs in many other tissues in the Burmese python.  The study therefore suggested that venom had evolved independently in different reptile lineages, including once in the Caenophid snakes. Venom containing most extant toxin families is believed to have been present in the last common ancestor of the Caenophidia: these toxins subsequently underwent tremendous diversification, accompanied by changes in the morphology of venom glands and delivery systems.Snake venom evolution is thought to be driven by an evolutionary arms race between venom proteins and prey physiology. The common mechanism of evolution is thought to be gene duplication followed by natural selection for adaptive traits. The adaptations produced by this process include venom more toxic to specific prey in several lineages, proteins that pre-digest prey, and 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. Changes in the diet of a lineage have been linked to atrophication of the venom.Selection pressure
Long-standing hypotheses of snake venom evolution have assumed that most snakes inject far more venom into their prey than is required to kill them; thus, venom composition would not be subject to natural selection. This is known as the "overkill" hypothesis. However, recent studies of the molecular history of snake venom have contradicted this, instead finding evidence of rapid adaptive evolution in many different clades, including the carpet vipers, Echis, the ground rattlesnakes, Sistrurus, and the Malayan pit viper, as well as generally in the diversification of PLA2 proteins. There is phylogenetic evidence of positive selection and rapid rates of gene gain and loss in venom genes of Sistrurus taxa feeding on different prey. As of 2019, evidence existed both of "overkill" occurring in some lineages, and rapid adaptive evolution, and an evolutionary arms race with prey physiology, in many others.The genes that code for venom proteins in some snake genera have a proportion of synonymous mutations that is lower than would be expected if venom were evolving through neutral evolutionary processes; the non-synonymous mutation rate, however, was found higher in many cases, indicating directional selection.  In addition, snake venom is metabolically costly for a snake to produce, which scientists have suggested as further evidence that a selection pressure exists on snake venom (in this case, to minimize the volume of venom required). The use of model organisms, rather than snakes' natural prey, to study prey toxicity, has been suggested as a reason why the "overkill" hypothesis may have been overemphasized. However, the pitviper genus Agkistrodon has been found to be an exception to this; the composition of venom in Agkistrodon has been found to be related to the position of the species within the phylogeny, suggesting that those venoms have evolved mostly through neutral processes (mutation and genetic drift), and that there may be significant variation in the selection pressure upon various snake venoms.Several studies have found evidence that venom and resistance to venom in prey species have evolved in a coevolutionary arms race. For example, wood rats of the genus Neotoma have a high degree of resistance to the venom of rattlesnakes, suggesting that the rats have evolved in response to the snake venom, thus renewing selection pressure upon the snakes. Resistance to venoms of sympatric predatory snake species has been found in eels, ground squirrels, rock squirrels, and Eastern gray squirrels. All these studies suggested a co-evolutionary arms race between prey and predator, indicating another potential selection pressure on snake venom to increase or innovate toxicity. The selection pressure on snake venom is thought to be selecting for functional diversity within the proteins in venom, both within a given species, and across species. In addition to prey physiology, evidence exists that snake venom has evolved in response to the physiology of predators.Besides diet, there are other possible pressures on snake venom composition. A 2019 study found that larger body mass and smaller ecological habitats were correlated with increased venom yield. Another study found that weather and temperature had stronger correlations with snake venom than diets or types of prey. While venomous snakes use their venoms in defence (hence the problem of snakebite in humans), it is not well known to what extent natural selection for defence has driven venom evolution. The venoms of the Texas coral snake, Micrurus tener, and other species of Micrurus have been found to contain toxins with specific pain-inducing activity, suggesting a defensive function. However, a questionnaire survey of snakebite patients bitten by a wide variety of venomous species showed that pain after most snakebites is of slow onset, arguing against widespread selection for defence. The spitting of venom displayed by some species of spitting cobra is solely a defensive adaptation. A 2021 study showed that the venoms of all three lineages of spitting cobra convergently evolved higher levels of sensory neuron activation (i.e., cause more pain) than the venoms of non-spitting cobras, through the synergistic action of cytotoxins and Phospholipase A2 toxins, indicating selection for a defensive function.In contrast to venom composition and toxicity to specific lineages, venom yield, or the quantity of venom produced by an individual snake, has not been found to vary with the body-mass of prey animals, and instead to vary with the body-mass of snakes producing it. Yield increases with snake body-mass in a consistent with the hypothesis that snakes invest a constant proportion of metabolic output into producing venom, which is metabolically costly.
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