Wednesday, 19 February 2014

The modern world is selecting for pure fertility

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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

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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.

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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.

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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.)

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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. 

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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'. 

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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.

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