Genetic Bottlenecks

Unfortunately, we haven’t sequenced the genomes of very many critters, but we have mapped out our own which has revealed a long history of human migrations that match up (once again) with archaeological discoveries. Which brings us back to genetic bottlenecks.

A genetic, or population, bottleneck occurs when some event reduces a population drastically.

When this happens genetic drift will encourage a decrease in genetic diversity. Here’s one way to visualize this using bags of marbles to represent genetic diversity, each generation is represented as a random “drawing” of marbles from the bag.

In generation 2 there is a bottleneck event in which most of the population is killed off, resulting in a less diverse “drawing” of marbles for generation 3. Thus even when the number of individuals within a population begins to recover genetic diversity will still be reduced, this is a genetic bottleneck.

Now let’s look at the actual genetic history of human beings to see if we see such a reduction of genetic variation 4,350 years ago. Copying errors are an inevitable part of reproduction. As this process continues some parts of our genome have mutated while others have remained pretty much the same and thus can serve as genetic markers for tracking human populations.

It’s important to focus on what genetic markers actually are – they are not things that evil evolutionists make up in order to support common descent – instead they are specific variations of the dna sequence among members of the same species.  Let’s say, for example, that these are two segments of dna from the same place on a human chromosome from two different individuals:

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As you can see there is a difference in one of the nucleotides, this is what’s known as a Single Nucleotide Polymorphism.
Here’s a simple explanation for the visual learners:

By looking at the genomes of people all over the world geneticists have discovered that there are certain groups of SNP’s that are unique to certain groups of people, these groups are called haplotypes.

It’s important to note that we are not talking about the theory of evolution here, this is population genetics.  Additionally, it is important to understand that haplotypes, haplogroups, and genetic bottlenecks are direct results of the fact of evolution and do not, in any way, rely upon the the theory of evolution.

• The process (fact) of evolution is the change in allele frequencies over time.
• The theory of evolution is that the process of evolution can account for the observed diversity of life.

Geneticists have also learned that when you look at lots of different haplotypes you start to find some which all share at least one SNP, and thus likely a common ancestor – these groups of related haplotypes are called haplogroups. By grouping similar haplogroups we can see a much larger pattern of common descent for all modern humans back to a common ancestor:

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By “common ancestor” we’re not talking about human evolution (as in “the common ancestor between chimpanzees and humans”), we’re talking about human lineages (as in “the common ancestor between you and your cousin”) so it’s important to once again remind ourselves that we are talking about the process, or fact, of evolution here. As human beings reproduce each generation will be more genetically diverse than the previous one unless there is a large extinction event which causes a bottleneck. Since genetic mutations occur at a fairly constant rate we should then be able to measure the time it would take to go from one genetic marker to the next.

By looking at these differences in genetic markers, and also at where we find groups of people with similar genetic markers, we can build a history of human activity.

How do genes tell the story of our ancient ancestors’ migrations?

When DNA is passed from one generation to the next, most of it is mixed by the processes that make each person unique from his or her parents. Some special pieces of DNA, however, remain virtually unaltered as they pass from parent to child. One of these pieces is carried by the Y chromosome, which is passed only from father to son. Another piece, mitochondrial DNA (mtDNA), is passed (with few exceptions) only from mother to child. Since the DNA in the Y chromosome does not mix with other DNA, it is like a genetic surname that allows men to trace their paternal lineages. Similarly, mtDNA allows both men and women to trace their maternal lineages.

Both the Y chromosome DNA and mtDNA are subject to occasional harmless mutations that become inheritable genetic markers. After several generations, a particular genetic marker is carried by almost all male and female inhabitants of the region in which it arose. When people leave that region, they carry the marker with them. By studying the genes of many different indigenous populations, scientists can trace when and where a particular marker arose. Each marker contained in a person’s DNA represents a location and migration pattern of that person’s ancient ancestors.
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While the rate of mutation may be fairly constant, the rate at which certain mutations are passed down through time to reach us in the present certainly is not due to natural selection. Therefore, genetic or molecular clocks are inherently rather “sloppy”. That being said, recent advancements in techniques comparing the genomes of thousands of individuals as well as comparisons with known historical events (such as the colonization of certain Polynesian islands), have allowed for more and more accurate molecular dating.

New ‘molecular clock’ aids dating of human migration history
Researchers at the University of Leeds have devised a more accurate method of dating ancient human migration – even when no corroborating archaeological evidence exists. Estimating the chronology of population migrations throughout mankind’s early history has always been problematic. The most widely used genetic method works back to find the last common ancestor of any particular set of lineages using samples of mitochondrial DNA (mtDNA), but this method has recently been shown to be unreliable, throwing 20 years of research into doubt.The new method refines the mtDNA calculation by taking into account the process of natural selection – which researchers realised was skewing their results – and has been tested successfully against known colonisation dates confirmed by archaeological evidence, such as in Polynesia in the Pacific (approximately 3,000 years ago), and the Canary Islands (approximately 2,500 years ago).. . .working with a published database of more than 2,000 fully sequenced mtDNA samples, Soares’ calculation, for the first time, uses data from the whole of the mtDNA molecule. This means that the results are not only more accurate, but also more precise, giving narrower date ranges.

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Now, lets take all this background information and put it all together:

However, when we use these genetic markers to track our genetic history out between 10,000 to 5,000 years ago we get a picture of migration, not near extinction.

Furthermore, we can look further back to 30,000 years ago,

and even to 50,000 years ago without seeing a genetic bottleneck. . .in fact, we haven’t even found where all of humanity was concentrated into a single geographic area yet.

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