Variation in venom composition is ubiquitous among snakes at all levels, from temporal variation within individuals to differences between major clades. Much recent research has tried to understand both the extrinsic drivers of this variation (e.g., natural selection, phylogeny) and the underlying genetic mechanisms.
A growing body of research has identified natural selection for optimization of venom to diet as an important driver of venom evolution, resulting in patterns of variation incongruent with taxonomic or phylogenetic affinities. The evolution of venom resistance in some prey species leads to the possibility of predator-prey arms races between snakes and their prey. However, while pairwise relationships between snakes and particular prey have received much attention, this is not the case for the role of venoms in complex food webs involving both snake prey and snake predators. Selection for a defensive function of venom has been postulated in some cases, but surveys of snakebitten patients do not reveal the pattern of early severe pain predicted by this hypothesis, including in species with cytotoxic venom activity.
At the genetic level, -omics technologies are providing unparalleled insights into the genetic mechanisms and processes underlying venom composition. Different genome-level and postgenomic processes control venom composition in different species: whereas genome-level presence or absence of toxin genes underlies most compositional variation in some rattlesnakes, post-genomic mechanisms play a much greater role in at least some Old World vipers, but generalisations are not yet justified by the available evidence.
Despite much progress, many knowledge gaps remain. More “genome to fang tip” studies of the genetic mechanisms underlying venom composition are required to detect general patterns. However, while –omics technologies promise to reveal the mechanisms of snake venom evolution, we remain critically short of rigorous natural history data on snakes and their interactions with prey and predators alike; hindering our understanding the role of venom in snake biology.