Current information on the rate of mutation and the fraction of sites in the genome that are subject to selection suggests that each human has received, on average, at least two new harmful mutations from its parents. These mutations were subsequently removed by natural selection through reduced survival or fertility. It has been argued that the mutation load, the proportional reduction in population mean fitness relative to the fitness of an idealized mutation-free individual, allows a theoretical prediction of the proportion of individuals in the population that fail to reproduce as a consequence of these harmful mutations. Application of this theory to humans implies that at least 88% of individuals should fail to reproduce and that each female would need to have more than 16 offspring to maintain population size. This prediction is clearly at odds with the low reproductive excess of human populations. Here, we derive expressions for the fraction of individuals that fail to reproduce as a consequence of recurrent deleterious mutation (ϕ) for a model in which selection occurs via differences in relative fitness, such as would occur through competition between individuals. We show that ϕis much smaller than the value predicted by comparing fitness to that of a mutation-free genotype. Under the relative fitness model, we show that ϕ depends jointly on U and the selective effects of new deleterious mutations and that a species could tolerate 10’s or even 100’s of new deleterious mutations per genome each generation.
Yann Lesecque,
Peter D. Keightley,
Adam Eyre-Walker.
A Resolution of the Mutation Load Paradox in Humans. Genetics 2012; 191: 1321-1330.
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I am not suggesting that the above paper is the last word - far from it. Its conclusions require modification in light of some important features the authors have neglected.
However, the basic point is that - according to a well established genetic calculation, it would be expected that 88 % of humans would fail to reproduce. The authors regard this as a long-standing unsolved paradox, and try to suggest an answer. But it may not be a paradox - it may simply be what happened in human populations most of the time and in most places through history (in equilibrium, on average) up to about 1800.
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Even if this number is too big, even if it is much too big, the point is that in order to prevent the accumulation of damaging mutations generation upon generation, in order to prevent the population being overwhelmed and destroyed by genetic damage; a lot of humans would need to fail to reproduce...
Which, given that - in pre-contraception and -abortion eras - a lot of humans are born (i.e. fertility is high), then there must be *very" high child mortality rates.
To put this in terms of eugenics, a large majority of people would not be allowed to reproduce at all, or else a large majority of children would have to die (or be killed) merely to stop dysgenics from mutation accumulation - this would have to happen just for things to stay the same.
To actually improve the functional-adaptedness of the population - in other word to practice eu-genics (by differentially breeding from the better- adapted) would have to come on top of this.
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To put it simplistically - to perform actual eu-genics as a matter of state policy would require something like the following:
1. Slaughter c. 88% of children or sterilize c. 88% of adults, to stay the same - and then...
2. Of the remaining c. 12%, breed only from the best adapted minority - to improve the population.
Knowing this, are you still in favour of eugenics?
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