Complexity by Subtraction

At the age of 17, I had never given any serious thought about “how life got here.” If you had asked me at the time I would have said I had no clue, which is ironic considering I now study and write about evolution for a living (well… trying to make a living).

Either way, as a young adult without any science background or knowledge, I was introduced to the world of science through the pseudoscientific concept of Intelligent Design. A family friend had introduced the idea to me and told me that it was a concept that could explain all of life. He sent me a book by biochemist Michael Behe titled Darwin’s Black Box. Of course, the book was way over my head but it seemed reasonable, logical … and scientific.

Throughout Darwin’s Black Box Behe argued that life could not have arisen via evolution because cells and organisms were “irreducibly complex.” He used examples of how the human eye is functionless in a “simpler” or “less complex” form, and therefore could not have been selected for gradually via natural selection. He also used the analogy of the mousetrap to explain that if you take out any one part of a mousetrap it will no longer be functional. He reasoned that that was because it required an intelligent designer, and that biological systems operated in the same way so their existence was proof of an intelligent designer.

I thought the ideas were certainly plausible. But more than anything it stimulated me to learn about the processes he was critiquing: biological evolution. After reading On the Origin of Species and several modern science books describing the processes by which evolution occurs, I instantly realized that Intelligent Design was a pseudoscientific attempt to legitimize creationism.

Several scholars have demonstrated that irreducible complexity is empirically false. There may be no “irreducibly complex” structures in nature. Eyes for example, while complex have (and do) evolve from simpler forms and structures. There are several examples of eyes in nature that can only detect a few photons of light. These are called “eyespots.” Eyespots are simple patches of photoreceptor proteins in unicellular organisms. The ability for eyespots to detect photons is minimal, however being able to distinguish between light and dark could be the difference from a meal or becoming a meal. Researchers have demonstrated that even very primitive eyes are adaptive and can evolve complexity gradually to suit various functions.

But the point of this article is not to provide another redundant analysis of why Intelligent Design is pseudoscience. This morning an article titled Complexity by Subtraction was published in the journal Evolutionary Biology that proposes an alternative evolutionary process to disprove irreducible complexity. In the article, evolutionary biologists Dan McShea and Wim Hordijk propose that it is also possible for complex structures to evolve from complex beginnings and then gradually become simpler. They call this idea “complexity by subtraction”. In this sense, the idea of complexity by subtraction counters the dominant mode of evolutionary thinking that posits that adaptation tends to select for ever more complex forms and structures. In their words:

Standard thinking says that the evolution of complex functionality is driven by selection, by the functional advantages of complex design. The standard thinking could be right, even in general. But alternatives have not been much discussed or investigated, and the possibility remains opent hat other routes may not only exist but may be the norm.

However, they do more than posit that adaptive processes may not always tend towards increasingly complex functionality. They used a computational model simulation to demonstrate that complexity by subtraction is theoretically possible. And they also used an example of skull complexity in nature to show that vertebrate skulls have actually decreased in complexity over time. Co-author Dan McShea stated:

The skulls of fossil fish consist of a large number of differently shaped bone types in the evolutionary transitions from fish to amphibian to reptile to mammal. In some cases skull bones were lost; in other cases adjacent bones were fused. Human skulls, for example, have fewer bones fish skulls.

The authors defined complexity in a biological structures as having “many different parts.” To me, both authors propose an interesting aspect of evolutionary theory that has yet to be explored fully. The notion that simplicitic functional structures could become adaptive is certainly counter to dominant thinking at the moment in evolutionary science. In the few examples I have heard of biological structures becoming less complex involve structures that were once functional, but are no longer. However, if functional biological structures can become simpler, this will really change our understanding of evolutionary theory. Although I am only aware of examples of complexity by subtraction from this paper, I do find both their computational simulation and examples of skull evolution from the paleontological record compelling. Of course there research raises more questions than answers and should spur interesting new research. For example:

  • Do functional structures tend towards simplicity or complexity?
  • Are their certain environmental situations that would lead towards one over the other?
  • Does complexity by subtraction impact evolution on the microevolutionary scale?
  • How does increased simplicity become adaptive?

This is all very interesting, but a report by Science Daily emphasized that this studies primary intellectual importance was to provide evolutionary scientists with “an alternative route” to debunking Michael Behe’s concept of irreducible complexity. To me, this seems like an unnecessary overstatement.

I am not sure whether McShea and Hordijk also believe this, but this concept is in no way an “alternative route” to debunking irreducible complexity (not that evolutionary scientists need a new concept to debunk irreducible complexity). The concept of complexity by subtraction simply explains how adaptation of a functional biological structure can become less complex over time; it does not explain how biological complexity itself evolves in nature. To explain increasing biological complexity an understanding of gradual adaptation is still all that is really necessary.

Either way, McShea and Hordijk have proposed and demonstrated an interesting evolutionary concept that complicates current theory. If I were to guess, I would suspect gradual adaptation for increasingly complex functional structures to be far more common than complexity by subtraction. Despite this, I am certainly excited to see what future research reveals about the role complexity by subtraction plays in the origin of species.

What do you think of complexity by subtraction?  Let Cadell know on Twitter!

Related Advanced Apes content:

The Adaptation Program

Universality of Preadaptation for the Human Condition

An Evolutionary Success!


About Cadell Last
I am a science educator, freelance science writer, and founder of The Advanced Apes based in Toronto, Ontario. In the past my academic research focused on the evolution, ecology, and behaviour of non-human primates (i.e., chimpanzees, gorillas, ring-tailed lemurs). Currently, my official blog, The Ratchet, can be found via The Advanced Apes and Svbtle. I enjoy exploring recent research in human evolutionary sciences, as well as biology, ecology, astronomy, physics, and computer science. My work has been featured in Scientific American, American Humanist, Richard Dawkins Foundation for Reason and Science, and Jane Goodall Institute of Canada. I am also exploring science popularization in new mediums in collaboration with PBS Digital Studios with an animated YouTube channel. You can contact me on Twitter (@cadelllast) or via email:

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