Note: the following is based on a speculative conversation with Michael A Woodley - and it was Michael who invented the tern Giga-death to describe the post-industrial world of collapsing population. This post continues from:
The Industrial Revolution had the effect of allowing many billions of people who would have died to stay alive - this meant that genetic mutations which would have been eliminated by death during childhood instead accumulated.
The Industrial Revolution has kept the mutants alive - including (very probably) you, and me, and billions more like us!
Indeed, for reasons which are somewhat uncertain, most people in most of the world chose not to have replacement numbers of children (the only exceptions being among the most devout members of highly traditional religions) and a combination of contraception and abortion mostly allowed them to achieve this choice.
Therefore, on the one hand mutations have been accumulating, generation upon generation, with (approx) one or two deleterious mutations being added to each lineage with each generation; on the other hand, people who exhibited traits caused by deleterious mutations - such as lowered intelligence and impaired long-termist conscientiousness, or higher impulsivity, aggression and criminality - were positively selected, were genetically favoured - simply because their pathologies meant they were either unable or unwilling to use fertility-regulating technologies.
In other words, accumulating mutations which damaged functionality actually amplify reproductive success under present conditions and for the past several generations.
Or, after the Industrial Revolution, humans have been bred for increased levels of genetic disease: we are now a mutant-enriched species.
Thus the proportion of mutants rose from two causes:
1. Incremental accumulation of spontaneous mutation, generation upon generation.
2. A positive feedback effects whereby damage from mutations (unless lethal or nearly so) would amplify reproductive success.
So there are more mutants in the population from accumulation, plus these mutants are more successful at reproducing and thereby spreading the mutations.
This trend is amplified further by recent and current patterns of mass population migration from areas with higher established mutations loads which have been further amplified by the effects of the industrial revolution, and various degrees of population replacement.
At some point, the proportion of mutants - who are on average significantly damaged in functionality - will become so great that the Industrial Revolution will fall-apart, collapse; the 6-7 billion excess population will be unsupportable; there will be a Giga-death (i.e. billions of deaths) scale of mortality over some period - and eventually the world will again become agrarian.
However, countries such as Europe will not return to the medieval level of agriculture but to somewhere below the mediaeval level in terms of social complexity and productivity.
We could expect something more like the Dark Ages or the Ancient British (Iron Age) agriculture at best - or maybe even back to the productivity of Neolithic (New Stone Age) agriculture if metalworking is lost.
In other words, productivity after the Industrial Revolution will be below the levels of productivity before the Industrial Revolution - because the post-Industrial world is full of damaged mutants.
This is important to recognize. A population of mutants whose intelligence has been dragged-down to a certain level will be much less functional than a population where selection has kept it in equilibrium at that level - the mutants will be carrying multiple pathologies in addition to their impaired intelligence.
But arriving at this situation will take a while.
Before then there will be the Giga-death phase: the die-off of the billions of excess population which have been sustained by an Industrial society of frequent productivity innovation, but otherwise not.
So, there will be lots of mutants of many different types, living among a contracting, collapsing, population - as the billions in excess of the earth's agrarian carrying capacity begin to die-off.
This world of mass dying will provide a new kind of selective environment - some mutants may reproduce vary rapidly under these strange (and temporary) conditions by evolving to exploit unusual resources which are (temporarily) abundant in a Giga-death world..
And if the dying-off lasts a few generations, some weird mutant 'scavengers' may come to dominate in some places.