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In addition to the problem of mutation accumulation by relaxation of selection, when a population has begun shrinking, as is the case for the native populations of all Western and developed nations, there is an increasing danger of extinction due to 'mutational meltdown' - when deleterious mutations accumulate so rapidly that they overwhelm a population before it can evolve an escape.
(http://en.wikipedia.org/wiki/Mutational_meltdown) .
Mutational meltdown was first described as a threat for small populations of asexual organisms; later this was widened to sexual organisms and then to large populations - so mutational meltdown has gone from being a specific case to probably a universal possibility.
The unusual twist with modern humans is that populations have begun falling due to chosen sub-replacement fertility, and before mutation accumulation has reached a level sufficient biologically to suppress fertility. In other words psychological factors have anticipated biological factors - and presumably both psychological and biological population decline will combine to increase the degree of reduced fitness resulting from mutation accumulation.
This will probably have increased the risk of mutational meltdown, and of extinction.
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I would assume that the catastrophe-causing mutations that you are worried about are ones that primarily cause behavioral and cognitive problems. So you’re not expecting to see catastrophe resulting from large fractions of the population with, for example, muscle-wasting diseases or cancers or weak bodies. Is this a fair summary of your concern?
ReplyDelete@BB - No! The initial concern - from WD Hamilton - is of medical problems. Hamilton called this the Planetary Hospital.
ReplyDeleteHowever, the brain is a very large mutational target, and about half of mutations affect brain functioning in some way. Intelligence is a fitness proxy - so it is expected that intelligence would decline (and reaction times get slower) as mutations accumulate.
Bruce C.,
ReplyDeleteLet me see if I understand the concept.
Suppose that an individual of a species has a very high mutational load. Given an extremely favorable environment, the individual is still capable of surviving and perhaps even mating. If the individual had a high quality mate, its children would be no worse off than it, and perhaps better. Its line would continue. But if the individual had a mate that possessed an equal level of mutational load, the children would likely be too damaged to breed or even to survive, even in an extremely favorable environment.
Now let us suppose that the environment has been so favorable that lots and lots of mutational load has accumulated in most of the population. Fertility and population will drop. At the same time, the remaining population will not be much improved genetically because the few individuals with high quality genomes will usually end up mating with more damaged individuals. The offspring will continue to be damaged, and because the favorable environment still persists, mutations will continue to accumulate among the survivors, and so on and so on. Eventually, even if the few remaining high quality individuals could be sorted out and mate only among themselves, they are too few to sustain a population on their own because they don't have the genetic diversity and have a small enough breeding pool that they are susceptible to any new mutations. Result: extinction.
I don't see this happening with humanity, partly because of our vast numbers and different niches we occupy, partly because of assortative mating, but mostly because the favorable environment we're is maintained by cognitive and behavioral processes that are probably the most vulnerable to mutational load, so we'll get back to Darwinian conditions long before the mutational load gets to a species-ending point.
And indeed, there is a growing awareness that global intelligence is declining. Zero Hedge had a chart showing this phenomenon.
ReplyDelete@Adam - I think that seems right, although in real life there are some other factors impinging in addition.
ReplyDelete"I don't see this happening with humanity, partly because of our vast numbers and different niches we occupy,"
I tend to agree - although we could see 6 billion people die and still have a population similar to historical levels.
However, if historical conditions return, then fewer then historical numbers of humans will survive, since they will be mutation damaged and less fit. If humans are thinly spread into small groups, they are more likely to go extinct (from random variation alone, population fluctuations may fluctuate to zero).
" partly because of assortative mating,"
However, assortative mating has been working towards extinction of low mutation humans for the past 200 years - because the fittest/ least mutation load people strongly sub-replacement numbers of children.
"but mostly because the favorable environment we're is maintained by cognitive and behavioral processes that are probably the most vulnerable to mutational load, so we'll get back to Darwinian conditions long before the mutational load gets to a species-ending point."
You may well be correct - but the reason I mentioned MM is that when mutation load increases quickly in a dwindling population, sometimes the population goes extinct before selection can find a way out from the downward spiral.
Much hinges on how quickly mutation load increases in the human species under selective conditions which actually amplify mutation accumulation. A paper is in preparation estimating this...
It usually takes a couple of events hitting at or near the same time to put the nail in the coffin. If, for example, a very deadly pathogen, (let's for purposes of illustration say an airborne Ebola-like virus since that virus is in the news so much right now), combined with a high mutational load that has accumulated over 200 years; it could wipe out a vast swath of the human species - perhaps beyond recovery.
ReplyDeleteAdd to that the degree of specialization that exists due to our being a social species; and very few exist who have the skill, temperament, and health to live completely off the grid.
Natural selection would hit our species very hard due to all these factors. Failing and in many cases toxically failing infrastructure, from entropy and insufficient technicians to bring these systems down in a controlled fashion would just add to the pressures on our species.
By easing the pressure of natural selection we may have critically endangered the health of our species through the build up of a mutational load. The most likely communities to survive would be isolated largely self-sufficient communities that have a low level of technological reliance and lower life expectancies due to a lack of medical treatments for diseases that have been continuously culling the herd and preventing reproduction by the least fit.
Ironically, the most developed countries with the highest life expectancies are the one's which would experience the highest death rates, and collapse most completely.
@NF "Ironically, the most developed countries with the highest life expectancies are the one's which would experience the highest death rates, and collapse most completely"
ReplyDeleteI think that may (unfortunately) be true, in the sense that these are the populations with (on theoretical grounds) greatest mutation accumulation, and also where the native population has been declining in numbers (if the effects of increased life expectancy are removed) for maybe a couple of generations.
In the developed world, we live lives as if on life-support machines - with so much cocooning and such a high level of medical intervention. machines and devices and the rest of it - that it is hard to calculate who would survive removal/ collapse of the cossettings of modernity (I *certainly* would not).
I'd like to know what you think of this: Mutation Accumulation Hits Wealthy the Hardest
ReplyDelete@August - I think that must be correct - at least in a relative sense.
ReplyDeleteThe 'demographic transition' (to lower fertility) began in the upper classes and worked its way down over a couple of generations - so the effect of mutation accumulation would probably have hit the upper classes first.
However these were the classes with the lower baseline mutational rates, so probably the net effect was a narrowing of the class differences in mutational load.
Never underestimate evolution. It uses a shotgun approach to hit a bullseye. And remember the amount of relative inbreeding amongst the upper classes. It could build up bad mutations very quickly.
ReplyDelete