Thursday, 30 March 2017

Mouse Utopia and Human Extinction? - A reader-friendly summary

In the late 1960s, US scientist John B Calhoun created a ‘Mouse Utopia’ – an artificial environment which provided what he regarded as the perfect breeding conditions. To everyone’s amazement, and without any signs of disease or hardship; after a few months of rapid population growth, the mouse colony ceased to reproduce at all; and soon became extinct – every single mouse dying within three years.
Could Mouse Utopia have lessons for humans? Well, that depends on the explanation for extinction – but if Michael A Woodley’s hypothesis that the mice died of ‘mutational meltdown’ is correct, then humanity’s days could be numbered.
Woodley’s hypothesis, which seems to be the only one to fit the facts (unlike Calhoun’s own explanation of ‘overcrowding’), is that it was exactly the utopian perfection of conditions which killed all the mice.
The idea is that mice depend upon a very high death rate (mostly from predation) to filter-out new and harmful genetic mutations which spontaneously arise each generation. When nearly all of each mouse generation survives and breeds, then the harmful mutations rapidly build-up to produce genetically unfit mice who lack desire to breed, and who neglect their young.
Exactly the same explanation could apply to Western humans since the industrial revolution; because the average premature death rate, both pre-natally and in childhood, has declined from about two-thirds to only one percent. If we assume that most of the uterine and childhood deaths occurred among those least-fit children with the most mutational damage – this must mean that Western Man has accumulated something like eight generations worth of damaged genes.
The  irony is that probably greatest triumph of the industrial revolution - the saving of so many lives of children - was perhaps therefore also the cause of a future 'Mouse Utopia' scenario...
But how bad are human mutations? Geneticist Michael Lynch estimates that there may be dozens of slightly-harmful mutations per human generation, among which approximately one to three mutations will be significantly harmful. So, lacking the harsh and cruel filter of high child mortality rates, each modern human generation would be expected to accumulate a couple of extra harmful genes.
What effect might these accumulating mutations have? Well, in a word they will reduce ‘fitness’ – Lynch suggests each harmful mutation might reduce reproduction by one percent - which means they will in multiple ways reduce reproductive potential for a given environment (multiple ways according to the chance of which gene is mutated).
So, it seems that the average modern human surely must be less genetically fit than his ancestor of two hundred years ago. Then, why isn’t this more obvious?
One reason may be that the most sensitive measure of reduced fitness is psychological, rather than physical. We are therefore first likely to see biologically-damaging changes in social, sexual and offspring-rearing behaviours; before we see any clear increased rate of diseases (although an increased rate in genetic disease would eventually be observable).
However, on the other side of the coin; reduced fitness would only be directly measurable if the environment had stayed exactly the same as in 1800; whereas we know that the modern environment is much less harsh. There is less starvation and less infectious disease; there have been breakthroughs in medicine and surgery, the expansion of the welfare state – and so on. It seems likely that a softer and more comfortable modern environment is mostly concealing a decline in modern fitness – and life expectancy has been rising for many decades.
But declining fitness is evident in the most fundamental biological measure – which is reproduction. It was declining reproduction that caused the extinction of Mouse Utopia; and the same pattern can be seen in the prosperous developed nations today.
For example, the age-adjusted native population size has been declining in Britain for several decades, since deaths exceeded births in the 1980s. The average number of children per women is now much less than two, especially among the wealthiest and most highly educated (ie. those best able to care for children). Median average age (excluding recent migrants) is probably into the late forties, which means that a large majority of native Brits are beyond reproductive age. 
In brief, the British population is headed for extinction – and for psychological rather than physical reasons; and the same applies across all developed nations. Woodley’s ‘mutational meltdown’ scenario would be expected to kick-in as population decline combines with mutation-cased fitness decline to make a down-spiral of ever fewer, ever more damaged people.  
What are the prospects? In a way the human situation may be worse-off than Calhoun’s mice; because we are in the position of having to maintain our own ‘utopia’.  But mutation accumulation will progressively erode our ability to sustain a modern civilisation – as average health, adaptiveness and abilities decline with each generation. Sooner or later, the older mutated population will become unsupportable by the even-more-mutated younger generation.
On the positive side, it seems likely that something important is missing from the simplified biological model underlying the above calculations; and there are positive, cohesive forces at work which may tend to compensate for mutation accumulation. I personally believe that there are as-yet undiscovered factors which tend to work in opposition to the degenerating effects of natural selection.  
The question is whether any such positive factors will be strong enough and rapid enough to let us escape from the collapse of ‘Mouse Utopia’ which otherwise seems inevitable.  
More on Mouse Utopia can be found at Bruce Charlton’s web pages: