February 2023

Seeing differences in the biological world often leads to questions about nature and nurture. Did I outperform my sibling in basketball because I inherited my mom’s quick reaction time (nature) or because I practiced more (nurture)?  Is this golden delicious apple really old (nurture) or are they just a mealy apple type (nature)? Is our dog well behaved because she’s part golden retriever (nature) or because of all that puppy training (nurture)? Often the answers to such questions are not either/or. New research on venomous tiger snakes highlights just how intertwined nature and nurture can be – and how evolution has a hand in both!

Where's the evolution?

The Australian tiger snake, Notechis scutatus, is a native of the southern Australian mainland and nearby islands. One of these islands is a relatively new home for the snakes. Tiger snakes have been living on Carnac Island for only about 100 years. Scientists aren’t sure how they first got there, but a popular story involves them being abandoned on the island by a travelling snake performer. However they arrived, the snakes have since made the best of it and are thriving on Carnac Island.  On the mainland, tiger snakes mainly eat small frogs, lizards, and mammals, but the dry island has no frogs at all. Instead, these snakes feast on the chicks of the ground-nesting silver gull.

The gull chicks, however, make a mouthful for the snakes. The chicks are more than twice the size of the prey the snakes usually eat on the mainland. Correspondingly, adult snakes on Carnac Island have larger jaws than their mainland counterparts. Researchers observing this difference wondered, nature or nurture? Did the population of snakes living on Carnac Island experience natural selection – that is, did snakes with genetic variants (alleles) that produce longer jaws fare better on the island and leave behind more offspring – producing a modern population of snakes that is naturally long-jawed? Or is it something about life on the island (e.g., eating large prey as a juvenile) that causes snakes to develop larger jaws over the course of a lifetime? The first explanation (nature) is an example of evolutionary change and the second (nurture) is termed phenotypic plasticity by biologists.

Phenotypic plasticity is the degree to which an organism’s phenotype (its physical features) changes depending upon its current or past environment. Two organisms with the same DNA sequence (e.g., identical twins) may have different phenotypes (e.g., one may be taller) if they experience different environments; those differences represent phenotypic plasticity. All organisms exhibit some degree of phenotypic plasticity (e.g., an animal that receives more food will generally be heavier than a genetically identical animal that receives less food), but sometimes phenotypic plasticity can be extreme. For example, some fish become either male or female depending upon the temperatures they were exposed to as an egg. In the case of the Carnac Island snakes, researchers wondered if eating large chicks was causing snakes to develop longer jaws through phenotypic plasticity.

To help answer this question, scientists performed a “common garden” experiment. They took pregnant snakes from the mainland and pregnant snakes from the island (this species does not lay eggs but bears live young) and kept them in identical environments in the laboratory (the so-called common garden) until they gave birth. The baby snakes were then reared in this same environment and were given either large or small mice to eat, making four groups in the experiment:

• island snakes eating large prey
• island snakes eating small prey
• mainland snakes eating large prey
• mainland snakes eating small prey

Importantly, the snakes that got the small mice were given more mice so that the total amount of food each group got remained the same. By keeping the environments for all the snakes the same, aside from the size of their food, the researchers reasoned they’d be able to figure out what differences between the island and mainland snakes were caused by internal genetic differences (nature) and which were caused by environmental differences in the size of available prey (nurture).

The results of this experiment are shown in the graph. The baby island snakes started off with slightly longer jaws than the baby mainland snakes – a difference that is likely attributable purely to nature, that is, more gene versions coding for longer jaws in the island snake population. But the big difference between the two groups only showed up over time. Island snakes fed large prey developed much longer jaws as they grew than did any of the mainland snakes, and they grow longer jaws than the island snakes fed small prey. The scientists concluded that island snakes have a lot of phenotypic plasticity in jaw length and their jaws respond to the size of the food they are eating, while mainland snakes have little phenotypic plasticity in this feature and they grow the same length jaw regardless of their diet. The latest research built on this foundation by CT-scanning the skulls of the snakes to figure out exactly which bones were responsible for the increased jaw length. Most of the difference came down to just two bones, one in the lower jaw and one in the upper.

How did island snakes end up with more phenotypic plasticity? Probably in the same way they wound up with those slightly longer jaws at birth: evolution by natural selection. The amount of phenotypic plasticity that an organism has is shaped by its genes. For example, some people gain muscle quickly when they lift weights and others gain more slowly – even if they lift exactly the same weights for the same amount of time. This comes down, at least in part, to genetic differences. Similarly, when tiger snakes first arrived on Carnac Island, some of them likely had genes for more plastic jaw development and some had genes encoding less plastic jaw development. On the mainland, plasticity didn’t much affect survival and reproduction because most of the prey was small, but on the island, plasticity could have been a game changer. Snakes with more plasticity were able to grow bigger jaws, get more food, and reproduce more, spreading the genes for plastic jaw growth through the population. It’s easy to get caught up in the idea that it’s either nature or nurture, evolution or phenotypic plasticity – but in this case, the Carnac snakes’ long jaws seem to have come about through the evolution OF phenotypic plasticity.

Over time, island snakes that had some of those jaw differences hard-wired from birth probably had an advantage because they could eat larger prey right away – and this is how that small difference in jaw length that scientists observed at the beginning of their common garden experiment arose. Over many thousands of generations of natural selection, we might expect further evolutionary change in this direction in the island population, producing snakes with longer jaws at birth and less phenotypic plasticity. Evolutionary biologists suspect that this is a common mode of evolution: consistently useful traits that develop over the life of an organism through phenotypic plasticity are likely to become, over evolutionary timescales, fixed traits that are present throughout the individual’s life.

The evolutionary change that scientists have so far documented in the Carnac tiger snakes might sound small – just three millimeters of extra jaw growth in 100 years of evolution. But over thousands and millions of years, such small changes can accumulate into brand new adaptations and distinct species. And even without projecting forward in time, three millimeters can seem like a lot depending on your perspective… say, for a gull chick facing down an extra three millimeters of gaping mouth on a venomous snake!