By Navneet A Vasistha
If you’ve been to Amsterdam or Venice or seen photographs of these beautiful cities, you would have marvelled at the many canals and the bridges connecting them. Have you ever wondered what prevents the water in these canals from mixing with one another? Of course, it’s because of the embankments that separate them. Had these canals been endless, the water in them would have flown indefinitely never to mix with each other.
A similar phenomenon exists in population genetics called as ‘canalization’. Ask any geneticist and they will tell you that the field of genetics exists because of the enormous diversity that exists among organisms of a species. However, beyond superficial differences such as skin colour, petal colour, height etc., individuals of a species are remarkably conserved in their appearance. In humans, for instance, the position of eyes and nose is conserved to a relative position on their faces, while the length of arms and legs remain proportional to the body size. Even substructures of the brain follow scaling rules showing that we are products of strong evolutionary conservation.
This might seem confusing at first. The genome of any two human individuals varies by roughly 0.5-1 per cent. In the human genome, which is nearly three billion DNA ‘alphabets’ long, this tiny variation accounts for almost 15 million differences.
The interaction between genes and the environment can produce noticeable features in an organism, called ‘phenotypes’. Thus, the interaction between the environment and an incredibly large genetic pool of human individuals should produce many such phenotypes. Why then are we only superficially different from each other? Barring some unfortunate developmental abnormalities, how is the body plan of individuals within a species so similar? This is due to ‘canalization’. Plainly speaking, canalization is when organisms resist variation occurring due to environmental or genetic changes. But before we delve into what this means, let’s step back to understand how this phenomenon was initially discovered.
In the early 1900s, Thomas Hunt Morgan, a geneticist, began using the fruit fly (Drosophila melanogaster) in his laboratory at Columbia University, to study the basis of heredity. The fruit fly has a short developmental cycle of 10-12 days. During this short period, eggs laid by the female hatch into larvae. Larvae, on completing their development, transform into pupae where they remain suspended in cocoon-like structures before emerging as adult flies. Using the fruit fly as a model organism, Morgan provided the first description of the gene’s physical location. This work also won him the Nobel Prize in Physiology or Medicine award in 1933.
Catalysed by a visit to Morgan’s lab in 1935, Conrad Hal Waddington, a developmental biologist, began a series of experiments to understand the link between genetics and the environment. In his experiments, Waddington took two batches of the fruit fly’s pupae from a normal population. The first batch was allowed to develop without any disturbance while the second batch was exposed to a heat-shock (an increase in temperature for a short period of time). Pupae from both batches were then left to develop into adults. He noticed that in the latter batch, 40 per cent of the adult flies were born with an unusual gap in the posterior crossveins of their wings. This suggested that the heat-shock given to these flies as pupae had resulted in a change in phenotype —from one having the crossvein to one without it. It also indicated that the heat-shock was not always sufficient to induce the defect as 60 per cent of the flies in that batch had developed a normal wing morphology. When Waddington selected and mated the unusual flies together (called cross-veinless), he noted that the resulting generations retained the phenotype even in the absence of the heat-shock.
Based on these observations, Waddington suggested, “the capacity to respond to an external stimulus…must be under genetic control”. In other words, the heat-shock had changed the genetic code and thus could be transmitted down from generation to generation.
In the first batch that served as a control group, a small number of flies spontaneously developed the cross-veinless phenotype. This showed that the same change could occur by other means – not just by heat shock. Again, when Waddington selectively mated these mutant flies together, the number of cross-veinless flies increased with each successive generation. Just as in the previous case, here too, the change was genetic in nature. He further added, “developmental reactions as they occur in organisms… are in general canalised [emphasis added]. That is to say, they are adjusted so as to bring about one definite end result regardless of minor variations in conditions.” Thus flies that showed a cross-veinless phenotype either due to a heat-shock or a spontaneous genetic mutation, had broken away from their canalised state of normal wings.
This simple experiment offers deep insights into a variety of phenomena. Take for example the rising problem of drug resistance seen in bacteria. Bacterial populations in the wild contain both antibiotic sensitive bacteria as well as their resistant forms. After sustained exposure to antibiotics, the resistant bacteria continue to survive while the sensitive ones are all selectively eliminated. The new population, thus, consists entirely of resistant forms. This resistance is then retained even in the absence of any antibiotics in the environment. In this example, both the sensitive and resistant forms represent canalised states. The antibiotic represents the external stimulus (equivalent to the heat-shock) that causes the bacteria to shift from one to another. Without the presence of antibiotics in the environment, the bacterial population would’ve largely remained sensitive.
As canalization restricts variability in phenotypes, it is compared to an electrical circuit’s capacitance. Just as capacitors can store electric charge, genes or their products can resist change in an organism’s appearance by ‘storing’ the capacity for variation. Genetic or environmental changes that disrupt such genetic capacitors reveal new phenotypes that were previously hidden. These new phenotypes can then be subjected to evolutionary selection and eventually lead to the generation of new species. If not for such a mechanism, species would evolve at a faster rate without adaptation to their environment. This would lead to many species perishing as a result and increase their susceptibility to disorders in the ones that survive.
The yellow and red bars represent separate canalised states. The organism (green) switches when it encounters a strong environmental change but resists milder ones.
In addition to genetics, canalization has also been used to describe conserved behavioural phenomena. An example comes from studies on Mallard and Wood ducklings. As a rule, ducklings respond only to maternal calls of their own species. This holds true even for ducklings hatched in incubators with no prior exposure to their mothers. An acoustic analysis of maternal calls shows that each species of ducks uses a unique call ‘signature’. Mallard ducklings respond to a call with a repetition of four notes per second while Wood ducklings respond to a call of 1200 Hz. The ability of the young ducklings to respond stems from the similarity between their own rudimentary call and that of their mother. When Mallard ducklings were prevented from hearing their own vocalisations they were seen to be incapable of responding to their mother’s call. Such ducklings could be made to respond to maternal calls of other species (chicken or Wood duck) if reared in the presence of those calls. Thus, vocalisations of the ducklings and their reciprocation canalises them to be responsive to the calls of their mother.
Similar parallels have also been drawn in social psychology, where canalization of an individual’s behaviour is determined by the social structures, values and goals around him/her. Thus kids born in a family of doctors (or engineers or artists) might be canalised to select a similar profession. Similarly, societal motivation or peer pressure might also direct a child’s behaviour. Children can also develop based on personal interests in which case they ‘self-canalize’ their behaviour.
A related term ‘robustness’, is often used in engineering, economics, ecology and statistics to denote systems that can tolerate disturbances in its functioning. For instance, a robust computer program is one that can cope with errors during execution of the software codes. Or a robust ecological niche is one that can retain species diversity despite local changes that might be driven by human activity.
Despite the acceptance of canalization and its contribution to the theoretical basis of population genetics, the physical basis of canalisation has not yet been emphatically demonstrated. While single genes have been described to have canalising roles, it is also likely that large genetic networks (with several genes controlling a phenotype) will lead to stronger canalization. In a similar way, a robust computer network will not be disrupted unless a large fraction of component computers are affected.
It is remarkable how such simple experiments led Waddington to describe a phenomenon with such diverse applications. Even though he lived in an age where many discoveries regarding the nature of the gene had not been made, the concepts and ideas that he developed have become well beyond the realm of biology. The idea that evolution proceeds not just by creating diversity but also by preserving the best and most stable solutions is an invaluable addition to fully understanding the great web of life.
This article was crafted with the help of Shruti Muralidhar and Abhishek Chari – members of IndSciComm.