Adaptation and Evolution

There appears to be some confusion about what is meant by the term ‘adaptation’ in the concept adaptation of species. It’s hardly surprising, because when we talk about genetic ‘adaptation’ of species the word is used in a very particular – and potentially misleading – way.

The dictionary definition of the verb to adapt is:

To make (a person or thing) suitable or fit for a purpose, or conformable to specified conditions, standards, or requirements; (now esp.) to make suitable for a new purpose or to a different context or environment.

The very word ‘make’ in this context implies a conscious or deliberate act on the part of some intelligent actor determined to bring about some convenient alteration in order to render the object in question more suitable for a given environment.

In the context of evolutionary biology, this implicit concept of deliberative action is totally wrong, and is misleading in its apparent bespeaking of some kind of a quasi-anthropomorphic ‘response’ to anterior stimuli as a means of explaining evolution.

All animals, and human beings, are able to ‘adapt’ (in the common use of the word) to changed circumstances during their lifetime – some better than others. But in terms of evolutionary biology, adaptation is always as a result of the accidental effects of random mutations that happen to increase the survivability of the species. This usually takes place slowly over many generations, even aeons. There are notable exceptions – such as the case of the Peppered Moth in the course of the Industrial Revolution – but they are also instances of accidental evolution. I’ll come back to that later…

The key point is this, succinctly put by Ruth Hershberg in her paper Mutation – The Engine of Evolution: Studying Mutation and Its Role in the Evolution of Bacteria (Cold Spring Harbor Perspectives in Biology 2015 Sep; 7(9): a018077):

“Genetic variation is a prerequisite to evolutionary change. In the absence of such variation, no subsequent change can be achieved. Genetic variation is ultimately all generated by mutation.”

As the statement above implies, the mutation precedes the evolution, the new evolutionary example of the species becoming more prevalent simply because it happens to be better suited to survive in that environment. Over time, even aeons, there will be many other mutations – but if they are not better suited then they will not reproduce and will therefore not ‘take’.

As such the whole process is mindless, random and chaotic – not truly ‘adaptive’ in the normal sense of the word, but rather accidentally fortuitous.

Now we can return to the fascinating example of the Peppered Moth. They key thing is that in the Midlands and North of England in the early 19th Century there were two varieties of peppered moth – the more common light-coloured version, and the less common black version.

Here is the light-coloured version:

And here is the dark:

The light-bodied moths were able to blend in with the prevailing light-coloured lichens and tree bark, and the less common black moth was more likely to be eaten by birds. As a result of the common light-coloured lichens and English trees, therefore, the light-coloured moths were much more effective at hiding from predators, and the frequency of the dark version was about 0.01%.

However, during the early decades of the Industrial Revolution in England, the countryside between London and Manchester became blanketed with soot from the new coal-burning factories. Many of the light-bodied lichens died from sulphur dioxide emissions, and the trees became darkened. This led to an increase in bird predation of light-coloured moths, as they no longer blended in as well in their polluted ecosystem: indeed, their bodies now dramatically contrasted with the colour of the bark.

Dark-coloured moths, on the other hand, were camouflaged very well by the blackened trees. The population of dark-coloured moth rapidly increased. By the mid-19th century, the number of dark-coloured moths had risen noticeably, and by 1895, the percentage of dark-coloured moths in Manchester was reported at 98%, a dramatic change from the original frequency.

See here for a short BBC story on the peppered moth: Famous peppered moth's dark secret revealed - BBC News.

Thus, it appeared, superficially, as if the species had actively and purposefully ‘adapted’ by changing its colour from light to dark, to suit the changing environment better. In fact, however, all that happened was that (in evolutionary terms) overnight the darker variety became less visible to predators, and the lighter ones became more visible.

All of this occurred in a phenomenally compressed period of time due to two factors:

• the rapid effects of industrialisation on the countryside

• the previous existence of two differently coloured varieties of moth.

Normally, one would expect such an adaptation to come about much more slowly, because environmental changes (not caused by mankind) usually take place over a much longer period of time. Furthermore, any adaptation would also be awaiting a series of chance mutations one of which one would eventually bestow a survivable advantage to the species.

But the principle and the process are fundamentally exactly the same.

Here we are now in the middle of a viral pandemic, and we watch fearfully for mutations to Covid-19 that may make it ‘worse’. In relation to viruses, there is good news, and bad news. The bad news is that viruses mutate at a much more frequent rate than complex organisms, such as animals. The other bad news is that one of these mutations may turn out to be more transmittable or more deadly than the current strain – or both.

However, the good news is that the end objective of the ‘selfish’ gene (as Dawkins puts it) is not to kill hosts, that is a by-product, it is survivability. I put the word ‘selfish’ in inverted commas to stress that this is another example of easily misunderstood language, the virus has no active intelligence – as before, it is ‘selfish’ as regards mutations solely in respect of the fact that one strain may survive more effectively than another. It is still a random process.

In order to maximise survivability, however, the virus strain that will prevail over another is one that does not kill its host, but rather one that affects more and more hosts – people – than another. This is does by being virulent, but not deadly.

There may be all sorts of mutations of Covid-19 that are – in the short term – worse for us humans; as indicated, they may turn out to be more transmittable or more deadly than the current strain – or both.

However, in the longer term, the strain that will outlast and replace the others is the one that is highly transmittable (and maximises the reproductive population of the virus) but does not kill. Killing a host puts an immediate end to its ability to feed and shelter the virus, and to its ability to communicate the DNA of the virus to other hosts, and so on.

Flu is a good example of this. The strain responsible for the 1918 pandemic, which killed more people than the First World War, was so successful at killing its hosts that it burnt itself out, and since then we have had much less deadly, but much more transmittable versions that mutate every year and leave us guessing about which flu vaccine to stockpile.

We can only hope that Covid-19 gets to that point as quickly as possible.