Spandrels are byproducts of evolution. They arise due to the evolution of one trait, but this unintended trait may not have any function.
Take a look at this picture:
The above picture is of the arch of Constantine in Rome. This arch is a type of triumphal arch that is situated between the Colosseum and Palatine Hill. The purpose of this triumphal arch (or indeed any triumphal arch) is to serve as a gateway for the victorious Roman army when it came back home after winning a war.
This particular arch was erected to celebrate the victory of Constantine I’s victory over the tyrant ruler Maxentius in the Battle of Milvian Bridge in 312 AD. The arch shows a beautiful collage of the past Roman emperors who had ruled before Constantine. It is easy to imagine Constantine walking with his chest filled with pride as he walked through the arch, the emperors of yore congratulating him on his supreme victory.
I mentioned that the purpose of the arch was to serve as a gateway for a victorious Roman emperor after battle. The purpose was just to build an arch, and the decorations on the sides of the arch are just that, decorative. In other words, the triangular portion just above the arch could be called a byproduct of making an arch. It could have been left completely empty and it wouldn’t have made any difference at all to the structural integrity or shape of the arch.
But how could you allow Constantine to enter through a plain-faced arch? How embarrassing!
These parts of a building (most commonly seen in the construction of arches or domes) have a name—spandrels. As formally defined by the Oxford Dictionary, a spandrel is “the almost triangular space between one side of the outer curve of an arch, a wall, and the ceiling or framework.”
Spandrels are seen everywhere in architecture, but spandrels don’t only exist in man-made structures; they are found in nature as well!
In 1979, two evolutionary biologists (among various other titles), Stephen Jay Gould and Richard Lewontin, published a paper titled “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme”.
What are spandrels in biology?
Gould and Lewontin defined a biological spandrel as a byproduct of evolutionary adaptation. What this means is that spandrels are necessary ‘leftovers’ created because some other trait had evolved. This means that the spandrel isn’t an adaptation to anything in the environment. Instead, it is a secondary trait that arose from the development of another primary trait. The easiest spandrel to visualize is the human chin. The human chin is merely a byproduct of the growth of different parts of the jaw.
The development of the jaw was an adaptation to the kind of food humans used to consume during our good old prehistoric days. The chin is a secondary trait that developed because of the first jaw adaptation. This secondary trait isn’t an adaptation to any specific environmental conditions. The evolution of the chin, in isolation from the jaw’s evolution, would mean very little to the animal’s survival.
This proposal of a trait that served no adaptive purpose was a critique to the thought that every trait is an adaptation and has been selected for through natural selection.
The Panglossian Adaptationist
Dr. Pangloss, in Voltaire’s novel, Candide, is one of the most optimistic fictional characters ever created. His ‘all for the best attitude’ is on clear display in quotes like, “Things cannot be other than they are… Everything is made for the best purpose. Our noses were made to carry spectacles, so we have spectacles. Legs were clearly intended for breeches, and we wear them.” This quote, which Gould and Lewontin use in their ‘spandrels’ paper to critique what they thought was the absurdly optimistic adaptationist view of evolution. In their paper, they support a more pluralistic approach.
The debate on the merits of adaptationism didn’t start with Gould and Lewontin’s spandrels paper. It had been raging since the start of the evolutionary discussion itself. The first rift began in the 19th century, between two camps, Charles Darwin vs. Alfred Russel Wallace and August Weissmann.
Darwin held a pluralistic view of evolution. Pluralism in evolution refers to considering multiple factors that may have affected a trait. In popular science, natural selection is synonymous with evolution, but that isn’t the whole story. There are three other major mechanisms of (micro)evolution—genetic drift, mutations and gene flow. Pluralists argue that adaptationism often attributes a purpose to a trait because it must, not because the evidence leads to it.
Wallace and Weissmann took up the adaptationist mantle. Many other prominent minds helped to further shape and influence adaptationist and pluralist views. Adaptationism is a set of views that considers natural selection as the most important mechanism in the evolution of a trait.
Adaptationists argue that studying the evolution of traits solely through the lens of natural selection is the best approach. They believe that natural selection is the dominant force over all the other mechanisms, especially genetic drift. They argue that the environment weeding out the less fit individuals is the biggest pressure and driving force behind evolution.
Implications of spandrels
Gould and Lewontin didn’t only think that adaptationism showed an incomplete picture of the history of certain traits, but they argued that such a belief could lead to misleading and counterproductive results. Let’s use the example of the human chin. If one were to only use adaptationist thought, one would come to the conclusion that the chin evolved because humans found chins sexy.
Thus, evolution through the processes of natural and sexual selection maintained those humans who had chins. Subsequently, those humans who had sexier chins were more successful and therefore thrived. While chins can be a metric for attractiveness, this purpose of the chin came after its appearance in humans, argued Gould. This, is called the exaptation of function, according to Gould.
Spandrels have a wonderful ability to co-opt certain traits that appear to modern scientists as a primary trait. Consider architectural spandrels. They are used to display complex imagery and art that serves to elevate the aesthetic value of the building. Does that mean the architects created the spandrel solely to decorate? Obviously not, as that sounds illogical. Gould argues a similar case for biological spandrels. Think of evolution as an architect putting a non-adaptive trait to use.
There are many other examples. The hollow space in the shells of gastropods (snails) that arises as a consequence of the shell spiraling inwards. This space is used by some gastropods to store their eggs (a co-opted trait), but is left hollow in other individuals.
The hump of the great Irish deer is another spandrel that arose due to the elongation of the vertebrate so that the horned head of the deer could be supported. The hump throughout evolutionary time became patterned and co-opted for sexual selection.
Considering the above two examples, if one assumes that the hump of the deer was an adaptation, it would mean messing up the historical origin of the trait. Doing so might lead to errors in predicting when the trait appeared, or lead to ignoring other important interactions (environment, viral, other animals, etc.).
The examples we have considered thus far have touched upon physical traits, but what about behavioral traits? This is where the debate of whether something is a spandrel or not gets murky. Phenomena like war, language and art were considered by Gould as spandrels of a large human brain. Sure, they help humans get along and become the dominant species on the planet, but they might not have started out with that function as its inherent purpose.