Tuesday, 22 August 2017

Tiger Snakes Won the Venom Wars

Australian Eastern tiger snake

Source: Wikipedia
Why tiger snakes are on a winner - UQ News - The University of Queensland, Australia.

Venomous snakes make interesting subjects for study in that they represent an example of a complex evolutionary arms race which can be traced in their venom. This is often a complex mixture of related but subtly different chemicals, often coded for by genes which are themselves subtly different versions of one another.

The arms race is due to snakes usually preying on a range of prey species, each of which has selection pressure to evolve resistance to the snakes venom. But, if the snake merely responded to one prey's resistance by changing it's venom, this might be less effective against another of its prey species, so there is evolutionary pressure on the snake to retain it's 'older' venom but supplement it with a newer version as well. This occurs by gene duplication, then mutation of the 'spare' copy.

Additionally, it is almost axiomatic that as snakes evolved and diversified into different species complete with new prey species, their venom evolved too.

But scientists based at the Venom Evolution Lab, School of Biological Sciences, University of Queensland, St Lucia, QLD, Australia, have found that tiger snakes and their close relatives all have very similar venom - so similar in fact that antivenom to tiger snake venom is also effective against a wide range of Australian snake bites.

Sadly, the paper in Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology is behind a paywall and the publishers want £23.20 just to reprint the abstract. However, a University of Queensland press release contains the following quote by lead author, Dr Bryan G. Fry:

A long-held belief is that snake venom varies with diet - that is, as the snakes evolve into new species and specialise on new prey, the venom changes along with it. Our research has shown that tiger snakes and their close relatives have toxins that are almost identical, despite this group of snakes being almost 10 million years old.

We worked out the reason was that the toxins target a part of the blood clotting cascade that is almost identical across all animals. So we have a new addition to the theory of venom evolution; that when the target itself is under extreme negative selection pressure against change, then the toxins themselves are under similar such pressure. This is a novel twist to the chemical arms race which most snake venoms evolve under. But it is one with direct human benefit since this is why tiger snake antivenom is so effective against treating the effects on the blood by quite a few other Australian snakes. No other antivenom is so widely useful.

Dr Bryan G. Fry, Lead author.
Venom Evolution Lab, School of Biological Sciences,
University of Queensland, St Lucia, QLD, Australia
Quoted in Why tiger snakes are on a winner, QU Press release

Tiger snake, Western Australia

Source: Wikipedia
The explanation for this high degree of conservation which appears to run counter to the normal evolutionary course of events is because the common ancestor of it and its related species hit on a mechanism which their prey are unable to change. They cannot respond to the evolutionary pressure by changing the mechanism the tiger snake venom attacks. The mechanism is a precise stage in the blood clotting cascade which is highly conserved in animals because any change would prevent blood clotting.

So, hitting on this highly conserved mechanism means the tiger snake and its relatives have effectively won this aspect of the evolutionary arms race and with their prey unable to adapt, there is no pressure for them to either. Something that might have appeared to be a problem for the Theory of Evolution turns out to have an evolutionary explanation.

For creationists, apart from the small problem of the Bible saying that snakes eat dust, there is some explaining to be done here. Firstly, why have these evolutionary arms races between venomous snakes and their prey species in the first place because they make absolutely no sense from an intelligent design perspective. Why design solutions which are then themselves seen as problems requiring solutions? But then why, having apparently hit on the tiger snake 'solution' to the 'problem' of prey species being redesigned to resist snake venom, why not apply that design to other snakes?

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