EHJ352H1 Lecture Notes - Lecture 17: Reproductive Isolation, Denisovan, Neanderthal Genome Project

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26 Jan 2013
Lecture 17: Human History and Speciation
Conclusions About Human History From Human Population Genetics:
Recent shared ancestry from relatively small founding population in Africa
Increased genetic bottlenecks with distance from Africa, consistent with model of serial
founder events
Real world likely more complicated, e.g. long-distance migration
Models of Human Evolution:
1. Single origin model: The other lineages are here but they did not have anything to do with
humans. Still a lot of shared polymorphisms.
2. High-migration model: Some gene flow between different species, modern humans
extend from one of these lineages
3. Low-migration model: Still humans extend from a single lineage
4. Isolation and Admixture model: after the origin of modern humans, some subset of
humans experience gene flow from another lineage. Out of Africa event, humans that left
Africa exchanged genes with Neanderthal and other lineages
Coalescent Times Are Highly Variable Across Genes:
A lot of coalescent time frames goes much further back to 2000 years ago - the origin of
modern humans. A lot of alleles that are segregating now ware already segregating back
in other hominoid species. Gene flow? We expect allele sharing between humans and
others, even if there wasn’t any subsequent gene flow
Mitochondria DNA: coalescent time was around origin of Homo sapiens, within 2000 years.
Many nuclear genes are not showing that pattern
A lot of polymorphism occurred before the origin of humans
Thus if Neanderthal are still around today, and if there was never any gene flow between
us and Neanderthal, we still expect a lot of shared polymorphism as a lot of our alleles
coalesce back further than speciation
Coalescence Rarely Occurs at Time of Speciation:
If isolation happens, there are still a lot of cases where my allele is going to match the
Neanderthal allele before either one does to your allele due to history of coalescence, that
does not shut down right at the time of isolation
A long time period of isolation, all the alleles within humans will be monophyletic: all the
alleles we share in common ancestor within humans before sharing with another species
A lot of our alleles might be shared with other lineages before with our own lineage
Did Modern Humans Exchange Genes with Neanderthals: Speciation in the presence or absence
of gene flow
Results from mtDNA: DNA from Neanderthal and isolate it as much as possible. Pairwise
difference between 1) two humans, 2) H-N and 3) H-C. No cases where H-N comparison is
closer than H-H comparison, early work that argues that there were no sign of gene flow.
No sign of some humans looking more like N than other humans. However mitochondria
have a very small Ne, all the mtDNA coalescence before with the Neanderthals. Not telling
us much. Separate lineage from mtDNA of Neanderthal than humans.
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