It has come to my attention that people are sometimes confused about the theory* of evolution. This might take several forms, from mixing up facts or misunderstanding processes to an outright denial of the whole thing. This bothers me. No one should have to go through life without a basic understanding of the most revolutionary theory of science ever conceived.
I’ve therefore written this blog post in order to try to explain the theory of evolution in simple, non-sciency, everyday terms.
But before I begin, I have to mention the 154 year old book On The Origin Of Species By Means Of Natural Selection by Charles Darwin. It’s arguably the most important book ever written and it has influenced not only biologists and other scientists but popular culture and society as a whole. It has made Charles Darwin the only scientist to ever become immortalised by having an ism named after him (by contrast, there is no Newtonism or Einsteinism). It’s also a very well-written book, easy to read (even though it can sometimes feel like it is mostly about pigeons) and I recommend that you download your free copy right now.
All done and ready to go? Good. OK, here we go.
The theory of evolution is based on three components: heredity, variation and selection. I’ll go through these one by one before summarising the modern theory of evolutionary.
The first part of the theory of evolution is the concept of heredity, that character traits are inherited from the parents to the offspring. We are all familiar with this phenomenon and it’s been a well-known fact for millennia. What hasn’t been known is the nature of heredity, how it actually works. Until quite recently, we believed it worked like mixing paint, so that the offspring became a mix of its parents’ traits.
Thanks to an 19th century Austrian monk called Gregor Mendel, we now know this is not the case, and that traits are in fact passed on as discrete units – genes. We’ve also figured out that genes are made up of a special kind of molecule called DNA and that they are collected in their thousands into chromosomes.
The second part of the theory of evolution is the observation that offspring seem to vary. Within a litter of animals or a collection of seedlings, there will be slight (or sometimes not so slight) variations in appearance, strength, endurance and so on. We see this in our own children, in our pets and in our garden plants, but it’s equally true for wild animals, plants, fungi and all other life-forms that reproduce sexually. In addition to visible traits, there are also hidden or more subtle variations in traits like capability to process certain types of food, resistance to pathogens and so on.
The reason for this variation is due to how the genes of the parents are combined. With sexual reproduction (as opposed to cloning), the chromosomes are recombined within the offspring to form a random variation of the traits the parent possess. It works sort of like a tumbler, where the chromosomes from the parents are mashed together, with different bits of the offspring’s chromosomes coming from either the mother or the father.
There is also another form of variation which occurs at a slow but more or less regular rate: mutations. A mutation is when a gene is changed so that it results in a different function. There is a range of different causes for mutations like certain chemicals, radiation or even infections. Most of the time, the mutation takes place in a somatic cell (i.e. a regular cell that makes up the body of the organism**, like a skin cell or a muscle cell) but sometimes it happens in an egg or a sperm and then the mutation becomes hereditary and the offspring could potentially display a completely different character trait than that of either parent organism.
The final part of the puzzle is the concept of natural selection. This is the key to understanding how evolution works, and is the most often misunderstood part of the theory of evolution. In essence it’s a variation of the artificial selection we’ve been undertaking for centuries on our livestock, pets and plants. We can easily see how our careful selection of desired traits has resulted in the multitude of breeds and stocks we have in agriculture today. Natural selection works in a similar way in that traits that are to a benefit for the organism will help it keep healthy and produce more offspring. Over generations, that particular trait will become more and more common in a population until the population is different enough from other populations to no longer reproduce with them and it becomes a new species.
Now, all of this takes time. For any kind of visible change to take place 100s if not 1000s of generations will have to pass. In larger organisms this would take many thousand or even million years, but in smaller ones – like bacteria – the changes can be seen within weeks or even days.
Evolution in a nutshell
And that’s the theory of evolution in a nutshell. It’s heavily based on Darwin’s idea of natural selection, but with the deeper understanding of heredity and variation that modern genetics has given us and can be summarised in one paragraph:
The theory of evolution states that all life is related and that traits within each organism are hereditary, albeit with some variation. This variation is played out in competition where the most fit for their environment produces more offspring and that variant becomes more numerous as a result.
It’s a very neat theory that clearly explains how life have managed to become so diverse and how it all links back to one single organism (called LUCA – Last Universal Common Ancestor), way back in the mist of time some 3.8 billion years ago, when life first appeared on the planet. In the 150 odd years since the theory was first conceived, it has withstood countless attempts to disprove it but only grown stronger as more and more facts pile on in its favour.
It’s all pretty amazing and in my opinion truly awe-inspiring in its breath-taking simplicity.
* A quick note on the use of the word ‘theory’: rather than representing a vague idea or hunch as is often the case in civilian use, a theory is the highest order of empirical knowledge in the world of science; once a set of hypotheses have been combined, they can then be elevated, through extensive testing and multiple attempts to disprove it, to the final stage of being a scientific theory. For instance, we have come up with the theory of gravity in order to explain the everyday phenomenon we experience of not being flung into outer space. It doesn’t mean that we don’t know if gravity exists or not, just that we have constructed a theory for how it works. (Coincidentally, the theory of gravity is on much shakier grounds than the theory of evolution.)
** Mutations that occur in regular somatic cells sometimes cause cancer. In fact, we all get multiple new cancerous cells every day, but our immune system is very good at tracking them down and destroying them. It’s only when that protective system fails that we develop cancer.