The main backing for the Out of Africa theory is the Genetic Neutrality Theory.
The arguments of an African origin of modern humans and our dispersal across the globe is supported by the high genetic diversity found in modern African populations, with lower diversity elsewhere, and a gradient or cline in diversity that reflects less diversity as distance to the African homeland increases. Both of these factors are expected according to the Neutrality Theory.
Starting with a highly diverse population, if a small group from that population migrates (into Eurasia), it can only feasibly carry with it a sub-sample of the original diversity. This is known as a Founder Effect, the founders of a new population carry fewer genes than the population from which they split from.
This happened time and time again, as sub-sub-groups split from the main population and moved into Europe, Eastern, Northern, and Southern Asia, Melanesia, Australia, Polynesia, and across North America, and then, into South America.
The Neutral Theory states that each split reduces genetic diversity.
Genetic Heterozygosity
Heterozygosity is a measure of diversity. Each person receives genes from their parents, that code for different proteins and produce traits. Those who have two different varriants (alleles) of a specific gene, one inherited from each parent are heterozygous. If the alleles are identical, they are homozygous.
The image below shows two parents (both are heterozygous) each carrying two different variants A and a. The probability for passing them on to the next generation is simple there are four possible combinations, each has a 25% probability of occurring:
The chances are that two of the offspring will carry Aa alleles, and will therefore be heterozygous, while the other two will receive the same allele from each parent and be either AA or aa, carrying two identical copies. This makes them homozygous.
As we can see, a population that is 100% heterozygous as become 50% homozygous and 50% heterozygous. All the possible combinations of those homozygous and heterozygous genes are shown below:
As you can see 25% of each variant (AA, aa, Aa, and aA). So why would heterozygosity decrease? Suppose only aa homozygous couples mate, the chance of this happening is 1 in 16, or AA mate, again, 1 in 16. So 2:16 or, 1:8 chance of only homozygous mating and offspring. But... if these offspring meet and mate aa with AA, they would have a 100% heterozygous descent. This is true for large populations, but for smaller groups the founder effects and bottlenecks can reduce the allele diversity.
Genetic Bottlenecks
The argument of loss of heterozygosity, or its equivalent, increase in homozygosity is based on genetic bottlenecks, where a small sub-population splits and carries with it the homozygous variant, say only aa or only AA. Losing the possibility of reintroducing the lost allelle. This is a 1 in 16 chance.
Other causes of heterozygosity loss are natural catastrophes, war, and disease. But, why would such events affect the heterozygous individuals more than the homozygous. Wouldn't they be random, and therefore have an equal chance of impacting on hetero- and homozygous individuals?
Regarding the root population. There is the chance that the root from which a population split off from suffered some event that eliminated a large swath of it, while the migrating sub-population in another geographic location was not affected by it. Wouldn't that lower the heterozygosity of the basal group and make the sub-population appear as "enriched"?
Genetic Drift
Both Founder effect and Bottlenecks are part of process called Genetic Drift. As we saw, genetic drift takes place when random events, by chance modify which alleles passed on by parents to their offspring. They also include not only non-reproduction of certain individuals due to war, disease, natural catastrophes, but also loss of genetic variation due to people who don't reproduce because they die before mating, choose not to do so, etc. Genetic Drift isn't driven by evolution. The random changes may or may nor provide adaptations to a changing environment, so they may or not be acted upon by the forces of natural selection.
A sub-population may lose certain alleles, or others may become Fixed reaching a 100% frequency in the population due to chance events.
Mutations and Natural Selection
Random mutations take place, and modify the alleles, natural selection may also work, favoring the survival of individuals with alleles that provide adaptative benefits.
But, what about mutations, that happen by chance, that have a deleterious effect? Some mutations may have harmful consequences. The Neutral theory says that some deleterious mutations may rise to high frequencies in small populations due to fixation promoted by genetic drift. But, why wouldn't people carrying unfavorable genes be affected by natural selection, causing them and their descent to die out?
The Neutral Theory of Molecular Evolution
It was the creation of Motoo Kimura, who in 1968 proposed that at a molecular level, mutations are caused by random genetic drift. These mutations are neutral from a selective point of view. They aren't affected by natural selection.
Kimura has been criticized, for instance Kern and Hahn (2018), argue that modern, genome-scale data demonstrates far more evidence of adaptive evolution than the neutral theory allows, suggesting that natural selection (both positive and negative) shapes much of the genome.
As mutations take place by chance, the probability of them being neutral, deleterious, or beneficial would seem equivalent. So, why assume they are neutral? A beneficial mutation even if it is rare would confer an evolutionary advantage for those carrying it, and modify the population beyond what neutral models suggest.
Linked Selection. The loci (or addresses) that mark the location (locus) of a gene in our DNA isn't independent and isolated. Some genes or DNA sequences located close together on the same chromosome are inherited together, as a unit, during meiosis (linked chromosomes).
Selective Sweep is when an allele that improves the fitness of its carrier increases in frequency due to natural selection, is accompanied (hitchhiking) by other genes linked to it by physical proximity on the DNA strand are also increased in frequency even though they may be neutral. Finally, Background Selection is similar and has the opposite effect: deleterious alleles are removed by natural selection and neighboring neutral alleles are lost too, due to physical proximity to the harmful variants.
These examples show that "neutrality" is not necessarily true.
Molecular Clock
Kimura's theory states that neutral mutations took place at a constant speed, accumulating over time at the same pace. However, this is not true.
However mutations don't appear in a uniform manner in all loci along the genome, they arise unequally, and the probability of fixation depends on where they arise in the genome. This modifies how the clock ticks (Source). Furthermore, substitutions depend on population size, and generation overlap (Source).
Generation time is also an important factor: is it 20 or 30 years? 25? or 18? Over 10,000 generations this means a time scale that can vary from 180,000 to 300,000 years!
Back-Mutations and Recurrence Not Allowed
Kimura's theory, at least when applied in practice, has three axioms that are not true:
- Infinite sites, it assumes that each mutation takes place at a site that has never mutated before.
- No back-mutations, changes happen in one direction A → G. Which will never again flip back G → A
- No Recurrence, in practice there are multiple mutations that take place at the same site. The neutral theory does not accept it, there can't be multiple mutations at identical loci in different lineages.
A paper gives a great example of why and how a back-mutation can have positive effects (here showing how a base C = Cytosine mutates to T = Thymine and back):
"...simple back-mutation is expected to generate slightly advantageous mutations. For example, let us imagine that a site is fixed for C, and that a new T mutation occurs that is slightly deleterious with a disadvantage of −s. Let us imagine that this T mutation spreads through the population and becomes fixed. If a new C mutation then occurs at this site, it will be slightly advantageous with an advantage of +s, unless the relative fitnesses of the C and T alleles have changed. Such a change in fitness could occur because of a change in the environment or the fixation of mutations at other sites which have epistatic interactions with the alleles at a site of interest."
Americas: Great Dying
Regarding Amerindian diversity, we know that up to 90%, or more, of the Native Americans died during the century that followed European "discovery". Disease, war, famine, social disruption, force labor, etc. killed tens of millions of Amerindians. Lineages died out, massively. This is the unique and most massive genocide (albeit unplanned) in the history of humanity. How can we know the number of unique, diverse, divergent alleles that were wiped out during this event? In 1491, America probably presented a far more diverse genetic structure than it does now.
And this brings us to the other point: African "diversity".
African Diversity... is it real?
Finally, and this will be the subject of a future post, do modern Africans reflect the genetic makeup of ancient Africa 100,000 or 75,000 years ago? Is a modern Nigerian, Gambian, Angolan African representative of the ancient population from which the Out of Africa migrants split? Have other events taken place within Africa, isolated from the sub-population that migrated into Eurasia? Admixture with archaic hominins after the OOA event, admixture between many separate and formerly isolated hunter gatherer sub-populations could have led to a modern highly diverse African population, while the original OOA root was far less diverse.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2026 by Austin Whittall ©
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