You may have occasionally heard the idea that humans have stopped evolving which, in short, is not true. Such a belief is likely due to a misunderstanding of the science which I won’t go into in much detail of it here but rather try and explain some interesting processes people are actually referring to. I will, however, give a minimal overview of (human) evolution but bear in mind that evolution is a highly complex topic with many subtleties not remotely covered here. So with that in mind, here is Dr Blur’s overly simplistic definition of the day:
Evolution is a process whereby a population genetically changes over time. An individual cannot ‘evolve’, it is only through genetic changes that occur in your sperm and eggs and the subsequent mixing of parental genes during reproduction that makes the offspring genetically unique. This needs to be occurring in a population in which the frequency of various mutations (known as alleles) changes – this is evolution. This can be directional or not. A directional example might be for evolving taller in which various mutations that encode taller people spread throughout the population over successive generations (i.e. goes from novel, less frequent to most frequent). Now we have taller people, yay.
As mentioned, evolution need not be directional and this is the topic I would like to discuss in the context of contemporary and future human evolution. When people talk of humans no longer evolving, they are likely (know it or not) talking about this non-directional evolution. This is particularly prominent in humans due to what is known as ‘cultural buffering’. This is a concept that I think is clearest illustrated by the example of clothing. Clothing was a human ‘cultural’ (behavioural/non-genetic) invention some many thousands of years ago that enabled the colonisation of colder climes (think inuits and the sami) but it also played another important role: as a ‘buffer’. Clothes might have enabled skinnier people to survive in colder environments, assuming having more fat was advantageous. In essence these otherwise less adapted people are buffered against the harsh ravages of nature and can survive to reproduce when before the 'cultural buffer' (clothes in this case) they would not have.
Therefore a technical model for cultural buffering is that is reduces the negative selective pressures on deleterious mutations.
This reduces the effect of so called ‘purifying selection’ which, despite sounding like something out of Nazi Germany, is an integral part of natural selection which sculpts and refines species to their habitats and challenges. Interestingly, such a relaxation of selection (e.g. you don’t need to keep that layer of fat any more) can also lead to directional evolution. Imagine that storing that extra fat or any other trait is energetically expensive, then mutations that used to be deleterious (e.g. not being able to put on fat) now become advantageous as they don’t waste energy so now you can use that energy for your brain development for example. This actually also relies on another factor, a limited supply of energy in the first place! Without this constraint (which is largely the case in the calorie rich western world), there actually is no benefit either way and so whether you put on the fat or not means nothing: the clothing keeps you warm on one hand and you can easily afford to make that fat layer AND develop your brain on the other. The general trend, however, will always be to accumulate errors (warning: data corrupted) as with buffering there is less pressure on removing these genetic corruptions.
So, now to some modern examples to explore this idea a bit more and what better example than the marvel of modern medicine! Sadly only a marvel for those with access to it of course. Modern medicine has extended life expectancies and allowed the survival to reproduction of many people who otherwise might not have. Some interesting examples include type 1 diabetes (the genetic one, not the one heavily linked to an excessive sugar diet and consequent insulin resistance) and cystic fibrosis (CF). People born with these conditions in the past would rarely live more than a few years and would certainly not survive to reproduction but now can lead relatively normal (but by no means easy) lives into adulthood and potentially parenthood. Being genetic, these conditions can be passed on to offspring with a CF parent having a ¼ chance of the child also having CF while type 1 diabetes is more complex. Of course many people that now survive when they might not have before go on to contribute significantly to society and as such I certainly don’t see this as a bad thing – the genetic lottery can be cruel and unforgiving. In many ways you can view cultural buffering as the great leveller neutralising the effects of this lottery.
So, we have not stopped evolving, we have simply become more insulated from the world around us. Mutations still occur between generations, some 100 new ones per baby, it is only that increasingly less of these mutations might be under any particular selection. When previously some of these could have proved detrimental, they are now tolerated. Cultural buffering is a very human feature; a product of our large brains, curiosity and language and will only become a greater definer of who and what we are. While I may have implied cultural buffering allowing ‘bad genes’ to spread is not necessarily a bad thing, although social Darwinists might disagree and in fact many misinterpret Darwin’s theory for their own benefit. Who’s to say a mutation is bad anyway if we can find a way to tolerate it and allow a human a healthy life. As far as evolution is concerned, a deleterious mutation ceases to be deleterious if it is no longer selected against. Another aspect of human evolution, and evolution in general, I have not covered is sexual selection which might be something for another time.
I explore the future of this subject here.
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