Tuesday, 16 September 2014

How assortative mating (of surplus offspring) could purge accumulating mutations each generation. Implications for human intelligence

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Sexual selection is usually a more proximate and often more rapid and powerful mechanism of natural selection than selection based on differential survival - because animals excluded from mating, or whose matings do not lead to viable offspring, have their genetic contribution cut short immediately - in the same generation.

Adverse sexual selection is reproductive death: genetic death.

Assortative mating is a mechanism of sexual selection in which similar females mate with similar males - similar in terms of some aspect of their phenotype such as 'beauty', status, or an ability such as intelligence - these aspects of phenotype being significantly correlated with genetic differences.

Thus the 'fittest' (those having highest reproductive potential and - through most of human history - the lowest deleterious mutation loads) will pair with the fittest, and vice versa

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When humans are producing a several-fold surplus of offspring, natural selection needs to have mechanisms by which as high a proportion as possible of the following generation are on average parented by those carrying the minimum load of deleterious mutations - and this requires that those carrying the greatest mutation load be mostly prevented from producing sexually mature offspring (I say 'mostly' because by chance some small proportion offspring of the population of those with highest mutation loads will - by chance - be low in mutations).

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In assortative mating, then, a minority of the population who are the most 'attractive' and fittest males and females will pair-off and have (on average) a large number of (mostly) fit children (without accumulation of mutations) - while the majority of the population who are least attractive, the least fit, and the heaviest mutation-load bearers will be left-over.

This population majority of left-over males and females may not have a sexual partner (reproductive death), or the females may have a share in mating with the minority of high fitness males, or else a male and a female pair who are both low attractiveness/ low fitness/ heavy mutation load will mate - but have a very low (but not zero) probability of raising offspring to viable sexual maturity.

(The effective of sexual selection is usually greatest on the males; with a higher proportion of males than females having zero matings, zero long term sexual partners, zero viable offspring. Thus, it is one function of the male sex to be the main way in which new mutations are purged from the population.) 

Therefore, assortative mating of this type - with 'the fittest' parenting almost all of the next generations offspring - has the potential, in combination with normal natural selection based on survival, to (on average) purge all newly occurred mutations from a population with each generation; so each new generation can - in effect - start afresh with a minimal mutational load.

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Reflecting on the above scenario in light of human society in the developed nations over the past century plus - it can be seen that the effects of the industrial revolution has been to put assortative mating into reverse; and not merely to fail to purge all new mutations from a population with each generation - but actually to amplify the proportion of new mutations in a population with each generation.

This happens by a combination of chosen sub-replacement fertility among the fittest (instead of, as has been usual in human history, the fittest parenting almost-all the next generation); with the least-fit who carry the heaviest mutational load parenting most of the next generation (instead of almost none of them). 

Furthermore, instead of a system which over-produces offspring who are then selected for optimum fitness; we now have a system which under-produces offspring - so there is no possibility of ordinary natural selection or sexual selection or any combination of the two being able to purge the new mutations from each new generation!

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In such a scenario it is easy to understand how a fitness-sensitive trait such as intelligence (as objectively measured by reaction time) has declined so much and so rapidly over the past century plus.

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The effect of failing to purge newly occurring mutations with each generation is that mutations accumulate and overwhelm that particular lineage with random genetic damage; each new generation will have more genetic damage and lower fitness than the previous generation; and after a number of generations, that particular lineage will lose fitness completely and become extinct. 
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Note: The above idea concerning the vital role of assortative mating in purging of mutation accumulation is substantially derived from my conversations with Michael A Woodley.

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Note added 18 September 2014

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' (http://en.wikipedia.org/wiki/Mutational_meltdown) .

The unusual twist with modern humans is that the populations have begun falling due to chosen sub-replacement fertility, and before mutation accumulation has reached a level sufficient to prevent fertility. This will probably have accelerated the severity of mutation accumulation, and increased the risk of mutational meltdown.
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7 comments:

Nicholas Fulford said...

Bravo, an excellently presented thesis and argument.

Counterpoint, an event - man made or natural - could introduce the conditions for a mass cull of human population. This could be a global war or a disease, or both. The least fit will die disproportionately in those conditions, and the aftermath will be a return to the normal selection methods without a high level of mutation accumulating. This would continue until conditions permitted the human population to once again fall into genetic decadence - unless some constraint is imposed that prevents the genetically decadent behaviour from emerging or the species becomes extinct due to an accumulation of mutations.

Adam G. said...

@Nicholas Fulford,
those kind of events don't disproportionately cull the least intelligent, though, at least not as much as sexual selection in Malthusian type conditions.

Bruce Charlton said...

@NF and Adam - The thing about mutation accumulation is that you do not return to the staring point - but a lower-fitness starting point.

But the argument does not simply apply to the whole human species, since the world population is segmented and conditions and timings have varied a lot - and also the different racial groupings probably had different starting points, repair mechanisms etc.

However, even after allowing for a wide margin of error, I think the phenomenon will not have left any group untouched.

For me, this is a case of what will be, will be and trying to take a proper and Christian attitude to whatever happens. This may not be qualitatively different from the kind of conditions the majority of humans have experienced through almost all of the world and history so it ought to be possible.

On the other hand, almost all of the modern environment goes against a proper attitude - being in the first place dishonest about the problem, then over optimistic about the outcomes, and often contaminated with selfishness and hatred that feed-off the fear.

I intend to write more about this matter - but I have a sense that back at the beginning of the industrial revolution in England there was a point when large numbers of low-fitness people who would have died before adulthood and failed to raise any children, became aware that this was now happening - and for this majority was a choice between either feeling grateful for what they had, or resentful for what they didn't.

The majority chose resentment, abandoned Christianity and gave their souls to the politics of resentment.

And among the high fitness people there was a different kind of choice, when they could have enjoyed marriage and families instead of its being a severe lifetime struggle - but they chose to switch their focus of interest to sex, status, and material accumulation - and the politics of greed and pride.

There was a choice between using prosperity for the highest things - for a more devout and evangelical Christianity, for community, for pure and disinterested arts and sciences - or for pleasure, novelty, distraction.

And the wrong choice was made there.

In and of itself, a large proportion of Mankind was granted a boon of material prosperity and disposable time - but en masse they chose to make it a curse. This need not have been, but on the whole it was.

The choice continues, opens up each day - but the conditions will be something like the opposite.

Bruce B. said...

The doomsday “preppers” will be the type that survives so I guess future generations will be selected for paranoia.

Bruce Charlton said...

@BB - "The doomsday “preppers” will be the type that survives". Beyond a few weeks, I doubt it. It's not a personality type suited to pre-industrial society. I think it's a question of groups large, localized, cohesive, organized and mutually-altruistic enough to survive the years of piracy - some types of religious groups.

deconstructingleftism said...

So what are the spiritual implications of all this? From the materialist standpoint most life, including much human life, is just refuse. The OT Hebrews placed a great value on reproducing. Christianity didn't originally, but modern Christianity places a high value on marriage and family life. Comfort for the genetic dead end is cold and hard to come by.

Bruce Charlton said...

@dl - I intend to write more on this. But through most of human history very few people have lived a full human lifespan - many die in the womb, many as children and so on; many are crippled, handicapped and dependent etc. Perhaps the spiritual implications are just 'business as usual'?...