Wednesday, 19 February 2014

The modern world is selecting for pure fertility


Consider world population, and how long it has taken to add each billion to the world population - plus projections for the next couple of billion people over the next thirty or so years.

1804 1 billion
+123 years

1927 2 billion
+33 years

1960 3 billion
+25 years

1975 4 billion
+12 years

1987 5 billion
+12 years

1999 6 billion
+12 years

2011 7 billion
+14 years

2025 8 billion
+18 years

2043 9 billion


Since there were first 2 billion people in 1927, in the lifetime of those yet living - the human species has added another 5 billion!

What does this imply about natural selection?

In broad terms these numbers confirm that what is going on is that world population growth is driven by what I call (with not much exaggeration) pure fertility - which is to say that those parts of the world population grow which have the highest fertility - which have the most babies - and 'nothing else' matters.


In other words, the genes of those humans who had the most babies are those which have been proportionately amplified in the gene pool of the human species. Roughly, all babies born have added to the gene pool.

This applies whatever the cause of having the most babies - religion, living in a low technology society, desire to care for for many children, fecklessness, impulsiveness or inability to plan, hyper-promiscuity, even mass rape... all the genes associated with all causes of high fertility have been amplified - and amplified a great deal as a proportion of living humans.


Likewise, in this post 1927 context of adding a billion people or two people every  generation, those groups who were of low fertility, who did not even reach replacement fertility (having less, perhaps much less, than two children per woman),  have had their contribution to the actually existing world population completely swamped - it would have declined a lot anyway, but the sub-replacement fertility has further diminished the contribution - and this contribution will be further diminished by the consequent high average age of the remaining population.

(In a population average age 45, the average women is infertile, indeed the great majority of women are infertile.)


In the past, human population was (almost) never kept down by low fertility, because fertility was always at above replacement levels - far above replacement levels.

What kept the world population down to a billion and less up to 1804 was that there was enough resources to support that many people - any children who were born above this level would die - fertility was, in this sense irrelevant.

Repeat - pre-1804 fertility was irrelevant. 

But what the modern world excels at is keeping children alive up to reproductive age - since 1927 every child born (more or less, approximately) - wherever that child is born and whoever are its parents - has been (one way or another) raised to adulthood and reproductive capacity, and this continues. 


So, over the space of about 100 years we have gone from a pre-1800 world where fertility was irrelevant to the natural selection of humans - that is, selection for pure mortality; to a post 1927 world where human evolution is almost entirely being driven by birth rates - a world of pure fertility.  

The average modern human, especially the average modern young human, is essentially a product of very rapid short term selection for pure fertility.

This is a new and unique situation in its scale - genetically its effect would probably be similar to the situation after a genetic bottleneck when the population has gone very low - e.g. due to most of the population having been wiped-out and a new population being founded from the survivors (e.g after a severe plague or natural disaster); or the 'founder effect' of a small population inhabiting a new environment rich in resources and without predators or diseases - when the constraint on population growth is, briefly, 'pure fertility'. 


Or the situation resembles the population growth of parasites who enter a new naive host environment that is helpless to restrain growth of the microbial invaders - when selection is for virulence rather than resistance.

In a nutshell and by analogy - a (pre 1804) human population being selected by pure mortality was being selected for resistance to the environment; but a (post 1927) human population being selected by pure fertility is being selected such as to enhance its virulence.



Thursday said...

And it is quite the hodge podge of traits that is being selected for: religion and fecklessness in the West certainly.

Ingemar said...

I expect the Dark Enlightenment types to latch on to this.

I expect the Leftists to wake up to this as well--being sons of Satan, will they drop the facade of caring for the poor and make genocide popular again, or will they let the coming catastrophe play out and claim, "we were planning this all along!"

Bruce Charlton said...

Another way to look at this - from the perspective of The West - is that the share of Western genes in the human species has gone from about half (1920) to about ten percent now (if mass migration to The West is taken into consideration, and added to the estimate here)

So there has already been a great deal of natural selection since 1920 - and a lot more of the same inevitably to come due to age structure differences.

Steve Moxon said...

Actually, pronounced fertility decline in 'the demographic transition' is now globally ubiquitous: ALL societies are experiencing it with the exception of still extant forager (hunter-gatherer) communities.
I'm shortly to publish on the adaptive nature of the DT. It's a population self-regulation mechanism. It's recently been found in mammals that (stranger-presence) 'crowding' stress of males is epigenetically transmitted specifically to female offspring, triggering fertility decline and aberrant reproduction-related behaviours across several generations. This has no apparent function other than plausibly population self-regulation (to which there is no theoretical objection given advances in modelling 'mutualism' without invoking 'multi-level' selection).
Out-breeding depression is a complementary additional (not mutually exclusive) secondary biological aetiology -- genetic relatedness as it becomes more distant produces incompatibilities between genes from the different parents through their widely separated co-evolutionary contexts, leading to correspondingly lower fertility.

Bruce Charlton said...

@SM - You can call it fertility decline if you wish - meanwhile we keep getting extra billions of people! - and a billion was the population of the whole world in 1804.

A billion is an awful lot of people, and we are expecting two billion more in our (expected) lifetimes.

Funny kind of fertility decline...

My point is that it is actual numbers that matter in the short and medium term (of decades) - and peak actual population - not percentage changes.

Frankly I find this kind of 'don't worry' attitude to the *stunning* already and future increases (and transformations) in the species population to be weirdly bizarre verging on hyper-grossly-irresponsible!

George said...

We currently live harvest to harvest as well, with the benefit of innovations such as chemical fertilizers (oil based) and GMOs. There is no gigantic surplus harvester voluntary reduced productivity to prevent mass starvation if something went terribly wrong (climate change negatively effecting arable land?)

John D said...

Bruce, if you say that increases in the global population are grossly irresponsible, are you then implying that something could have been done by good, responsible Christians to forestall this?

I'm curious what that might look like. Certainly faithful Catholics have no solutions, and given your stance on eugenics I'm at a loss as to what you would be in favor of (or not be opposed to).

Bruce Charlton said...

@JD - "Bruce, if you say that increases in the global population are grossly irresponsible" - But I don't say that! I say that what is weirdly irresponsible is the prevailing attitude that world population is not a problem because the rate of increase is declining.

It is the absolute number of people that matters, and the nature/ make-up/ composition of these people - and these numbers (plus background knowledge of human differences) show that the human rate has changed, evolved, very substantially in the past century.

Anonymous said...

Moxon's got the right of it, moribundness is setting in to just about all populations except the very religious. Birth rates are crashing and we've been fairly stable at 7 billion for quite a while now, and there is some inferential evidence (utilities consumption, lack of infrastructure to accurately estimate births, etc) that this number is overstated.

Your own chart shows the milestone distance lengthening once again.

People are embracing a horrid sterility worldwide, and it remains to be seen how long it will take for the extremely tiny number of fecund folks to replace them and/or convert them to fecundity.

Bruce Charlton said...

TUW - "we've been fairly stable at 7 billion for quite a while now"

That is not true - or at least everything I have read contradicts it.

An increase of 70 million a year in world population is the population of Great Britain extra per year (the most densely populated nation in Europe - and it feels like it).

When the population exceeds production then it is the number of actual people that matters, not the rate of increase.

I am quite honesty mystified that people can't understand this - maybe it is a 'clever-silly' phenomenon, to disregard the obvious (just look at the numbers!) and focus on second order issues.

Bruce Charlton said...

To continue the above point, if I have not been clear - on the one hand we have billions of people accumulating on the planet - on the other hand the thing that people are always talking about is that the population rate of growth is declining; will reach a plateau in (?) another 100 years or so; that there is some kind of automatic process by which populations undergo a demographic transition to lower fertility.

On the one hand fifty-hundred million extra people per year - all of whom demand somehow to be fed, housed, etc - mostly by coercive extraction of resources from a dwindling productive population - yet on other hand - don't worry, it won't go one for more than another few generations!

Of course it won't go on! The question is *how* it won't go on - and what will be left afterwards.

Also the DT is an historical observation, not an explanation - certainly not a prediction.

Fertility declines for reasons, and grows for reasons. One reason it will stop declining and start growing is natural selection - the people who don't have children will become genetically extinct - those who do (for *whatever* reason) have above replacement fertility will hand on those traits to their offspring, which will therefore have higher fertility than current generations (i.e. present and future populations have been and are being *bred* to be resistant to whatever caused the demographic transition.

Misanthropist said...

Surely a major reason why population is not yet declining despite falling fertility is that in much of the world the generations of people that have experienced lower fertility are not yet old enough to be dying in large numbers. Only once those people are old enough to start dying in large numbers would one expect to see death rates outnumber birth rates and natural population decline result. Although I think some countries have already started to experience population decline, such as Russia, Japan, Italy, Germany.

I think it is safe to say that the world is getting close to peak population and population will start to decline in the not too distant future. That does not necessarily mean that overpopulation will not be a concern, as it is quite possible that such a correction will come too late to mitigate such problems.

And while it may be true that the planet cannot sustain a larger population, it is equally true that population decline (or more importantly, both aging and declining populations) will impose severe economic difficulties that are every bit as challenging.

Bruce Charlton said...

@M - Populations in countries with median age in the forties are indeed dying off, but the growth comes from places with median age in the mid teens to thirties - and these will be fertile for some decades to come.

Ageing is having and will have greater effects on the economy - but also differences in intelligence and personality (which are highly heritable) since the intelligence and personality profile of the most fertile are both associated (on average) with much lower productivity (especially in a modern economy).

SFG said...

Just wondering--what's your objection to eugenics? Our usual objection to the Nazis (and some DE types have none!) is their practice of mass murder, but that's really an objection to mass murder (which is wrong according to most moral traditions), not eugenics.

Bruce Charlton said...

@SFG -

takarada said...

I think people are not talking about this because it leads to eugenics as the only possible solution which we know is a bad thing because Hitler thought it was good thing.

Somebody on reddit said this:
I wonder if this could explain the Fermi paradox.

A species/civilization selects for intelligence and develops technology to the level where it eliminates infant mortality and chronic resource shortage. At
that point selection switches to fertility, resulting in an unsustainable population boom, and subsequent overpopulation and collapse. There is never enough time to develop technology further and become an interstellar civilization.

Essentially civilizations oscillate on the boundary between selecting for intelligence and selecting for fertility until they exhaust their planetary resources required to get out of the gravity well. At that point it's game over as far as interstellar civilization goes.

Bruce Charlton said...

@t - Eugenics is not the only answer - see my previous comment.

takarada said...

@Bruce Charlton
Singapore has tried to get university graduates to have more children with financial incentives. Its not really been successful.

I have had otherwise intelligent people tell me that fertility cannot be selected for. Hilarious.

Another favorite is that random genetic drift is driving evolution now.

Bruce Charlton said...

@t "Singapore has tried to get university graduates to have more children with financial incentives. Its not really been successful."

Only devout belief in a religion (and not all religions) can motivate modern Man to choose to have children.