A snakes life challenges Darwin's theories on Natural Selection and Evolution

If you are into fossils, dinosaurs, global extinctions, primordial soups and the emergence of slimy the lobe-finned fishes from the murky depths up onto dry land during the Devonian period, then you most probably believe that Charles Darwin hit a home run with his theories on evolution and natural selection: that is, if a particular individual of a particular species just happens to be born with a fantastic, new and novel trait that improves its chances of survival (a mutation at birth), then that individual will likely have a wonderful life and make lots of babies that will also possess that same trait. These babies will then grow up, and make lots more strong, fit and healthy babies; and so on it goes until this new fancy trait becomes INKED into the genotype or the genetic blueprint of the species. Pity all of the remaining individuals of the species not born with the trait as, according to Darwin and most other evolutionary biologists of the era,  they will wither on the vine. In short, they will not produce very many happy babies, each of whom will drag itself through a miserable existence and most likely die before they get to experience the joy of child birth and the uplifting experience that is parenting. As a consequence their genetic blueprint (their origin, their ancestry) will simply fade into obscurity! Too bad, so sad but such is the power of natural selection.

When it comes to evolution and survival of the fittest, Charles Darwin was right, and everyone else is wrong, wrong, wrong – maybe that is true, maybe it is not.

We all agree that over geological time (we are talking centuries or millions of years, not days and weeks here) an organism’s natural environment is constantly changing. Darwin’s theory of evolution and natural selection fed from this constant change and a species urgent need to stay on top of that change. With change comes challenge: organisms must adapt, evolve and overcome in order to continue their species. If they don’t they die.

But what if you possessed a whole bunch of really neat traits that were not particularly useful now, but were to become very useful in the near future? Moreover, what if when a change came you did not need to adapt, evolve and overcome because you happen to possess the perfect body plan: you did not need to evolve because you, and those within your species, genus, family or even order already possess the traits needed to survive a new world order?

All of a sudden, we see that there may be more than one explanation as to how organisms have changed over time: more than one explanation for the “origin of the species”. Published on 24 November 1859, this work was considered to be the foundation of evolutionary biology.

Charles Darwin is not wrong, not by any stretch. But he was not necessarily all right either. Whatever the case, his ideas dominated the minds of evolutionary theorists for decade upon decade!! There is another explanation for how the organisms we see around us every day came to be and, believe it or not, the best examples come from the least loved and, who most naively consider, the most under-evolved fauna group in the world – the snakes.

A very simple comparison of two separate populations of the Western Tiger Snake, Notechis scutatus, can be used to poke holes in Darwin’s theory of evolution and natural selection. How? Well, quite simply, two populations of the same snake species survives in two radically different environments: one lives in a freshwater lake in suburban Perth, Western Australia, and the other lives on a near-shore desert island less than 5km from Perth’s coastline. These two populations of tiger snake are genetically identical. They have not evolved, adapted, or capitalized on any radical mutations at birth – they just happen to be extremely tolerant to a broad range of ecological challenges that would quickly kill off other less hardy organisms. They have the perfect body and they know how to use it. For example, Tiger Snakes at Herdsman Lake don’t really demonstrate any urgency to drink fresh water. Why would they? They are surrounded by the stuff all year round and the prey they eat (frogs) are basically saturated sponges with legs! On Carrnac it is a different story. For the majority of the year there is no fresh water available to drink and there is not enough water generated from prey consumption and digestion to keep snakes going. As a consequence, Carnac snakes go bananas over the water that becomes available during summer thunderstorms. That makes perfect sense, but would you believe they display this reponse whether they are thirsty or not? Basically, they react opportunistically to available water whether they need it or not. If it rains they emerge from their water conserving burrows, where they have been dodging the heat and aridity for months at a time, like a swarm of bees out of a hive to drink as much as they can as quick as they can. What’s even more interesting is their ability to determine salty water from fresh, with snakes often drinking off their own bodies to avoid taking in salty water off the ground – a phenomenon only ever recorded by science in one other species of snake. In contrast, summer thunderstorms mean nothing to Herdsman Lake snakes, whether they are thirsty or not. Is the evolution of this behavior an outcome of natural selection – no it is not. On the flip side, the theory of natural selection has it that if a trait is not utilized it will eventually be selected out. On Carnac Island, the tiger snakes feed on Silver Gull chicks. Whilst doing so they fall victim to vicious, though fairly well justified, aerial attacks from the adult gulls. The gulls bombard the snakes about the head with a stabbing, tearing beak and it is most common for the poor snake to emerge from its culinary pursuits with full belly, but unfortunately no eyes. A large proportion of the snakes on Carnac are completely blind, with their eyes fused over with scar tissue. 

Head damaged caused by adult Silver Gulls

However, this does not disable the snakes at all. Blind individuals continue to feed and mate for the term of their natural life. They maintain body condition and daily activity schedules that are not dissimilar to their visually enabled conspecifics. Gazing distant toward the evolutionary horizon (pardon the ocular orientation of my pun), and as per Darwin’s theory, one must infer that eventually the Carnac tiger snake population will evolve to be born with no eyes and a fortified head shield to protect it during meal time. Natural selection and evolution – who needs it? Just make do with what you have got.

 Less than 1km across the water Garden Island is located. There are tiger snakes on Garden Island too, but as they look and behave identical to the rest of the mainland population they are of little interest to you and I – no disrespect meant. A far more interesting story persists within the islands population of giant South West Carpet Pythons. Insular gigantism is a common phenomenon, particularly within reptiles. Basically, if you have the ideal basic body plan, as reptiles do, and you can manage to swallow huge prey, then you will get big: very big. But on Garden Island the story is just that little bit more interesting. Neonate (newborn), juvenile, sub-adult and many mature adult pythons only ever grow to about 1.5m in length and weigh not much more than one kilogram. This is typical of the size and weight of South West Carpet Pythons throughout WA. However, many adults (typically males) can grow to 5 m in length and weigh several kilograms. If that was not basis enough for a good story, then ponder this: there are very few individuals on the island that fall into the size and weight range around the middle of this growth spectrum. Thus this population is extremely unique in having a bimodal size distribution. If you were to graph the population’s size you would have two peaks showing high numbers of snakes around the 1-2 m mark and the 4-5 m mark, separated by a trough with few individuals between 2 – 4 m. Why? Or more importantly how is that interesting? From birth to young adult, both male and female snakes feed on a range of small prey, including mice, rats, lizards and nestling chicks. At maturity, the females choose to invest all their energy into breeding which limits their capacity to grow much larger than they need to in order to bare young. As in many animal groups, the female always seems to be the gender making all the sacrifices to continue the species! The lazy males just continue to eat and accrue resources that they invest into little more than growth and the pursuit of females. Sound familiar? When you are an ectotherm (taking energy from heat in the environment) you can do this. When you are an endotherm (constantly burning body resources to maintain body warmth) this is not so easy. At this point I digress, and again question why biologists consider reptiles and amphibians to be an ‘evolutionary backwater’. Back on task, where this story becomes remarkable is right at the point, in size and weight, that a python is fortunate enough to take down its first juvenile Tammar Wallaby, of which there are thousands on the island. Once they are big enough to take one, they quickly grow bigger and are far more capable of taking larger Tammar Wallabys more frequently. Their growth rate explodes to the point where they attain a size only limited by their ability to stay alive. This predator prey interaction dictates that the population exists as two sub populations: the giant gluttonous snakes that feed on five star, Michilin Plate winning wallaby and the smaller individuals that have to make do with their three star diet of house mouse and lizard. Natural Selection causing insular gigantism – no, sorry. And they call mammals ‘Higher Vertebrates’. Not in this instance.

In my lifetime as a biologist, edging into nearly 40 years spent pouncing on my quarry, I have only ever seen three death adders in the wild. The name was derived from the term Deaf Adder, arising from the observations that one could virtually stand atop these cryptic creatures and, like a sniper, they would not give away their position. Things are not so for this species on the islands of the Archipelago of Recherche. They are everywhere and everywhere they are they are obvious. I first must affirm my ability as a zoologist and ask myself if am simply missing them when I go searching in the mesic woodlands and forests of mainland Western Australia. Surely I am not so inept that I can’t find a snake that is as fat and as long as my own arm? At the risk of self-validating my skills, I am going to assert that they are simply thin on the ground on the mainland but most abundant on these islands. Why? Their traditional mainland habitat does not have a high carrying capacity (ability to support) for the prey that adders eat, and if you are going to sit in one spot it can be sometime between opportunities to snatch prey. The islands, however, present a different balance of energy in the food chain. Lots of food for adders, means a reduction in the need for total crypsis to capture unsuspecting prey. Moreover, more food also means more adders and lack of predators means that individuals lack the intrinsic inertia to employ predatory avoidance strategies, such as need to stay hidden. Natural Selection – no. Shear laziness in an island paradise changing the population dynamics of these island adders – yes, more likely.

When you start to consider the Dugites on Rottnest Island, the Mulga Snakes on Barrow Island, the famous Chappel Island tiger snakes, not to mention the Rough-scaled Python, and the Arafura File Snake (I could go on), Darwin’s theory starts to look a little like a wedge of Swiss cheese.