Plasticity affects a range of biological phenomena including behaviour, ecological interactions and morphology. An example of the last is that of plasticity in the head size of tiger snakes (Notechis scutatus)1.
Tiger snakes live in southern and central Australia and are venomous carnivores that prey on small vertebrates such as frogs, birds and mice. In addition to their ancestral populations on the Australian mainland, there are several populations on islands. Some of these were established thousands of years ago when sea levels were lower, enabling migration to take place, others much more recently (less than 100 years ago) through human activity. It happens that their prey animals tend to be larger on the islands than on the mainland, to an extent that makes them more difficult to capture and swallow. Once again, plasticity and subsequent assimilation are how these animals adapt to this problem.
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Generally, island snakes are bigger than their relatives on the mainland, and this includes having larger heads. Experiments show that head size in snakes from recently colonised island populations is plastic in that snakes fed on larger mice develop larger heads than those fed on smaller ones. However, that plasticity is not seen in snakes from islands colonised several thousands of years ago; these snakes have a large head phenotype regardless of the size of mice they are continually fed on in experiments. The long established island tiger snake populations therefore appear to have large heads by default, i.e. the formerly plastic head size range has become a fixed, hereditary large head.
Adaptations are the basis of evolution. A single fitness-improving change in a biological trait will not usually cause the animal’s species to change; new species arise when combinations of changed characteristics make an animal population significantly and permanently distinct from its ancestors. The island populations of those tiger snakes that migrated thousands of years ago, and which have a permanently large rather than plastic head size, have adapted but not changed to an extent that they are now new subspecies. If new environmental challenges appear, perhaps the resulting additional adaptations will cumulatively cause them to speciate. The few thousands of years since their first island occupation is a very short time on the evolutionary scale and, currently, they remain merely a variant within the species Notechis scutatus. We now go on to describe an example in which plasticity has played a distinctive role in the emergence of a new species. And this example teaches us about how plasticity actually guides the direction of evolution via something called the Baldwin effect.
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References
- Aubret, Fabien, and Richard Shine. ‘Genetic Assimilation and the Postcolonization Erosion of Phenotypic Plasticity in Island Tiger Snakes’. Current Biology: CB 19, no. 22 (1 December 2009): 1932–36. https://doi.org/10.1016/j.cub.2009.09.061. [↩]