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Research Update:
Bottlenecks and
Assortive Mating
in Humans


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Michael Ferguson
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Research Update: Paleosociology Project
Terms in the article highlighted in red are defined in a glossary below.


Evolutionary Bottlenecks and Assortive Mating in Humans


The human genome appears to be littered with the fingerprints of evolutionary bottlenecks. The whole controversy started about 15 years ago when Molecular Biologists identified one woman who lived in Africa between 140,000 and 290,000 years ago who was the mother of us all. She has been christened the Mitochondrial Eve. This was soon followed up by the identification of a Y-chromosomal Adam who lived at approximately the same time and in the same location.

Over time, the estimated dates have become slightly more recent and the location of 'Adam and Eve' has been disputed. However, the general conclusion that everyone alive today has one man who is their exclusively patrilineal ancestor and one woman who is their exclusively matrilineal ancestor has survived.

There has been much confusion among both scientists and laymen about what Adam and Eve mean. It is a highly complex issue of Population Genetics full of subtle complications. They don't imply the Biblical scenario of Adam and Eve. They were not a couple. In fact, they undoubtedly did not live at the same time. What this evidence implies is that all humans alive today are descended exclusively from a very small population (under 10,000) that eventually populated the whole world. This population carried Eve's mitochondria and Adam's Y-chromosome exclusively due to a well understood principle in Population Genetics, the founder effect. The controversy rages over why this population as it grew didn't interbreed to any significant degree with any other populations.

It has been very difficult to generate a plausible explanation for why this population didn't interbreed with the indigenes. The first explanation that got press was that they simply moved into a new area, took over and slaughtered all the locals. It has been suggested that a plague, asteroidal impact or climatological disaster may have wiped out all other humans. It has also been suggested that somehow this population had become genetically incompatible with the rest of humanity. In other words, every mechanism put forth to explain the genetic evidence was a bit far fetched.

Recently, several other evolutionary bottlenecks have been identified. Apparently, all modern day Europeans are descended from just a couple of hundred people or less [note:link no longer valid]. Oxford Geneticist Bryan Sykes has identified 33 clusters of mitochondrial DNA in modern populations, each suggesting yet another evolutionary bottleneck. It is beginning to appear that large modern populations having their genetic roots in small original populations is a recurring theme with us humans. This renders the more improbable explanations mentioned above untenable. An asteroid might wipe out nearly all of humanity once -- maybe. But, over and over again? No.

Research Update

We have been working on a potential explanation that is the mathematical result of assortive mating patterns. Simply put, the populations may have interbred, but contemporary populations are only descended from individuals who didn't. It is important to recognize that Molecular Biologists have not proven that the populations didn't interbreed, only that no genetic evidence for it has been found in modern populations.

We begin by making the observation that nearly all cultures will build a consensus with respect to what constitutes an ideal mate and preferentially mate toward it. What traits are important and how they should be conformed, varies from culture to culture, however, we know of no culture where the conversation doesn't take place.

One could speculate that one of the evolutionary causes of complex culture building in humans is to allow local populations to discuss what traits are locally adaptive and to selectively breed toward it. What is locally adaptive is highly affected by the structure of the culture itself, so the process is recursive. In other words, one of the most important survival traits in humans is the ability to function effectively within one's own culture.

The effect of this hypothesis is to create a situation where those best adapted to the local culture and environment are most likely to mate within their own community. They are perceived to be the highest quality mates and will be in demand. They will have the luxury to choose a mate who closely conforms to the local ideal. The probability is overwhelming that the chosen mate will be a member of the local community since the local community has been selectively breeding for those traits.

This implies that the probability of intercommunity breeding is highly skewed toward the individuals who deviate most significantly from their local ideal. Since the offspring of such a union is likely to conform even less to the local ideal than the parent originally from the community in which the offspring resides, it is highly likely that these offspring will be the source of any future intercommunity breeding. Furthermore, this population of intercommunity breeding is not well adapted to either community.

An easy way of thinking about this hypothesis is as follows. Two communities in close proximity are actually comprised of three, semi-isolated breeding populations. Those who breed almost exclusively within community A. Those who breed almost exclusively within community B. The third semi-isolated breeding population is the community of A/B interbreeders. Since the A/B interbreeders are not well adapted to either community, they will fail to sustain themselves. If one of the communities becomes extinct, so will the community of interbreeders and the remaining community will be left more or less genetically pure.

This hypothesis involves the application of Probability Theory to Population Genetics equations. It is far too complex to evaluate through algebraic proofs. Consequently, we are utilizing monte carlo simulations to determine whether it can be supported using reasonable assumptions. This hypothesis has surfaced only in the past couple of months, so we are very much in the initial stages of analysis. Preliminary results, however, look promising.

A Note on Polymathic Studies

As Polymaths we do not challenge the genetic evidence supporting evolutionary bottlenecks nor the paleoanthropological evidence supporting interbreeding. In this case we are incorporating information and methodologies from Population Genetics, Game Theory, Psychology and Cultural Anthropology in an effort to help resolve the controversy.

Michael Ferguson
Executive Director
American Polymathic Institute




Note: If you are interested in gaining a deeper understanding of these issues, we strongly suggest you read Population and Evolutionary Genetics: A Primer by Francisco J. Ayala. It requires only basic high school algebra.

Assortive Mating
Assortive mating means non-random mating with respect to a trait. If a population shows no preference in total for brown eyes over blue eyes or vice versa, then the population sortively mates with respect to eye color. If the population shows a preference for one or the other, then it assortively mates with respect to eye color.
Evolutionary Bottleneck
When a large contemporary population is descended from a small original population. When a large population is descended from a very small founding population, geneticists can tell by measuring the degree of genetic variation. It will be less than expected due to the founder effect.
Exclusively Matrilineal Ancestor
Mitochondria are threadlike bodies in the cytoplasm of a cell that function in cellular metabolism. They contain DNA that is inherited exclusively from the mother. Consequently, Mitochondrial Eve was not only mother to us all, she was so exclusively through an uninterrupted line of mothers. Because Y-chromosomes are passed down exclusively from father to son, Y-chromosomal Adam is father to us all through an uninterrupted line of fathers.
Founder Effect
When all members of a population possess the same genetic trait, having received it from a single or small group of ancestors through purely random processes. The founder effect is only likely to occur when there is a very small, isolated initial breeding population.
Monte Carlo Simulations
This is a technique for evaluating systems of probability equations too complex to be easily solved directly. The system of equations is computer modeled so that the equations will randomly select from within each probability distribution. Then the model is run repeatedly to determine a probability distribution of outcomes for the whole system.


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