As I write this I am glancing at my clock and counting down the hours until dinner time. It’s a Saturday but I’m not going out. This can only mean one thing: Doctor Who is on! There is something strangely appealing about the time travelling escapades of such a quintessentially British hero which I find impossible to resist. Throughout his travels the Doctor has had a myriad of wonderful and bizarre encounters ranging from dinosaurs in space to all-consuming god like stars. As a biologist, I find that a lot of what I study is just as wondrous. So imagine my delight when the this episode of our intrepid space farer – entitled “Listen” – and his adventures begun with a monologue concerning the theory of evolution. However my joy was not long lived – although I relished in it whilst it lasted, as an age old misconception reared its ugly head, far more frightening than any Dalek or monster.
The beast known as Adaptionism.
What is Adaptionism? In its most fundamental form it is the view that every trait an organism possesses is a result of natural selection and that any other evolutionary process is negligible. I would of course admit that natural selection does play a huge role in evolution – I don’t think anyone could deny this – however, there are other more subtle forces at work. These other forces can be assigned to two groups: random forces and constraints. Random forces range from genetic drift to chance mass extinctions such as the Cretaceous-Palaeogene extinction ignited by the Doctor Who-worthy meteorite that landed in the Yucatán Peninsula in Central America. However in this case I want to look at the former, the random meandering of genes in and out of extinction. In 1968 Motoo Kimura made a radical proposal – that the overwhelming majority of mutations are neutral. This proposal was known as the “neutral theory of molecular evolution” and was built upon further in 1973 by Tomoko Ohta into the “nearly neutral theory of molecular evolution.” As you might imagine, this later addition stated that most mutations are neutral or nearly neutral.
What does neutral mean? Well in this case it means that the mutations under investigation do not alter the organism in any perceivable way, or by such a small amount that genetic drift drowns out their effect – meaning that they are invisible to natural selection! This means that in these cases, evolution will proceed in a random manner dictated by chance. Indeed slightly maladaptive mutations may also slip through via genetic drift if a population is small enough. As it is in these populations that the adaptive signal of a mutation is drowned out.
What of the second group? Constraints are best visualised as developmental channels that force evolution down certain pathways. Allow me to clarify. Evolution is not like an engineer who may start from scratch for each new model of car. It alters parts that are already present, many of which might prevent other features from being added. Want a new exhaust? I’m afraid not, your lowered suspension doesn’t give you enough clearance for one. This places fascinating limits on what evolution can achieve and explains a great many things. It may explain why we humans our stuck with our cancer causing metabolism and less seriously, it explains why cats haven’t developed can openers for claws.
A further constraint is best explained by the late Steven Jay Gould. He often liked to employ metaphors in describing evolutionary processes, most famous amongst them being the church spandrel. Spandrels are the protrusions that extend above a circular arch and are an architectural necessity of constructing the arch; Gould argues that there are many features of organisms today that are no more than spandrels. These spandrels are required for other, likely adaptive traits to develop but are not themselves adaptive and indeed they may even be maladaptive. What does this all mean for evolution? It means that species will be stuck on local adaptive peaks. That traits will become adapted to a great degree, but will never be able to reach perfection due to their constraints both mechanistic and historical.
Now, I know what you are thinking, how does this relate to Doctor Who? Well, in the monologue to which I previously referred, the good Doctor is discussing a variety of species. Predators such as lions and species with elaborate defences such as puffer fish, he comments on the apparent perfection of these organisms to their particular niche. He poses the question, if there is the perfect offence, and the perfect defence then is there a perfect hider? But wait just a darn second! If organisms are constrained then might they not be stuck on a local adaptive peak? There is no such thing as a perfect hider (although some praying mantises come pretty close!), so unfortunately for the Doctor, he’s set himself up to chase a creature which doesn’t exist.
It seems easy to forgive sci-fi programmes such as Doctor Who when they take liberties with science. Indeed it often makes for better television and I have never taken arms when physics gets the same treatment. However, evolution is different and for good reason. It is perhaps the most fundamentally misunderstood topic in the whole of science, not just by students, but also by educators as well (click here for an example).
Perhaps I am being over-dramatic, but Doctor Who does lend itself to melodrama, and I think the Doctor would blush at his mistake. Like the laws of time, there are also laws he would certainly appreciate the chance to learn more, especially when the subject matter is something we think we know so well.
“The most erroneous stories are those we think we know best.”
Steven Jay Gould.