Lecture 17.docx

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Ecology & Evolutionary Biology
Stephen Wright

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.  A Draft Sequence of the Neanderthal Genome: provided a very comprehensive picture of what the Neanderthal genome sequence looks like  3 samples of bone from Croatia (Vindija): where Neanderthal DNA was sequenced and where they were from and when their dates are  Nuclear genome: Pairwise divergence of Neanderthal bone sample and human reference genome: wide range of percent divergence. There is
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