A study of venoms has drawn attention to an understudied aspect of natural selection, revealing how predators adapt to keep up with the species they're trying to kill.
For animals that possess venom it is an exceptionally powerful weapon, both to subdue prey and to deter predators, appearing at least 20 times across the animal kingdom. However, something lethal to one potential victim species may be almost harmless to others, so the armory needs to constantly adapt for changing targets, either the arrival of new prey or the appearance of resistance in familiar ones.
Consequently, venom evolves very fast, faster than the body features of the owner, making it an interesting case study for students of evolution.
At the Hebrew University of Jerusalem Dr Yehu Moran and Dr Kartick Sunagar studied 85 gene families producing 3,500 toxin sequences across both invertebrates and vertebrates. These clades of animals have been working on their venom for up to 600 million years, long before snakes decided legs were an encumbrance.
Natural selection can be divided into two processes. In positive selection, new proteins, and the genes that produce them, appear. Purifying or negative selection sees genetic variations that are no longer useful removed from the population. "Our research shows that while the venoms of ancient lineages evolve more slowly through purifying selection, the venoms in more recent lineages diversify rapidly under the influence of positive selection,” Moran said in a statement.
On the other hand, Moran and Sunagar found that the number of amino acids in a venom had no influence on how quickly it evolved – only age mattered.
The fact that most venom research has focused on snakes and cone shells, recent arrivals at the venomous beasts table, has created a skewed picture, Moran argues, making positive selection appear the dominant force. Meanwhile the centipedes, for example, have been engaging in negative selection for a long time, but very few of their toxins have been described.
When venom first emerges it requires a process of positive selection to become suited to the needs of the animals producing it. However, after a time there is no pressure to produce more deadly elements within the chemical brew, so evolution becomes about eliminating less effective toxins, since these still require a lot of energy to produce.
"The 'two-speed' mode of evolution of animal venoms involves an initial period of expansion, resulting in the rapid diversification of the venom arsenal, followed by longer periods of purifying selection that preserve the now potent toxin pharmacopeia,” Moran and Sunagar write. “However, species that have entered the stage of purification and fixation may re-enter the period of expansion if they experience a major shift in ecology and/or environment."
The finding could shed light on venom research's most high profile debate, in which Sunagar is also a major figure: Did reptiles evolve venom just once early in their evolution, or multiple times after diversifying?