Population Genetics and Human Evolution

1. Population Genetics and Human Evolution Chapter 19

2. Gradients of Genetic Variation in Human Populations • Prior to genomics, evolutionary biologists surveyed populations and cataloged differences in allele frequencies • The distribution of a specific allele as a gradient across continents is called a cline. • The gradient of an allele reflects waves of migration • This data provides clues to the origin of humans—the older populations have had more generations to evolve and thus have greater diversity of alleles.

3. http://www.genome.gov/25019968

4. Are There Human Races? • Race—term used in the 19th century to describe phenotypic differences among populations. • Studies of variations in proteins, microsatellites regions of DNA, and expressed genes show more genetic variation within populations than between populations • Main Conclusion: There is no clear genetic basis for dividing our species into races

5. Variations used in Genomic Studies Table 19-5, p. 428

6. Conclusions of Population Studies • Based on studies from the 1990’s to the present… • There is very little genetic variation in the human genome • Variation in the human genome is continuously distributed • Most genetic variation is widely shared, but a small amount is geographically clustered • Some genetic analyses can sometimes allow the identification of an individuals continent of ancestry (see Genographic Project slide later)

7. Australian Caucasoid Each circle in the center represents genetic variation within a population defined as a race. The variations overlap greatly as shown by the dark grey in the center. Few to no genetic differences belong to a single racial group. Human genetic variation North American African ‘‘Race’’ 1 ‘‘Race’’ 2 South American African ‘‘Race’’ 3 Mainland Asian Indian Fig. 19-9, p. 429

8. 19.6 The Evolutionary History and Spread of Our Species (Homo sapiens) • A combination of anthropology, paleontology, archaeology, and genetics is being used to reconstruct the dispersal of human populations around the globe • Our evolutionary history begins with the hominoid lineage about 25 million years ago • Hominoid – superfamily of primates, including apes and humans

10. Variations on Rudolph Zallinger’sMarch of Progress Images from http://christpantokrator.blogspot.com/2011/06/human-evolution-diagrams.html

11. Evolution is not a linear morphing of one typeof organism into another – a common misunderstanding http://pewresearch.org/pubs/1105/darwin-debate-religion-evolution http://tolweb.org/onlinecontributors/app Rather, evolution is branching from a shared ancestor

12. Genome-based evolutionary relationships among hominoids 7 m.y.a. chimps and humans had a common ancestor Fig. 19-10, p. 430

13. Early humans emerged ~5 million years ago • After human line split from the chimps, three different species groups appeared. • Collectively known as hominins • Australopithecines • Paranthropus • Homo – our ancestral group

14. Homo floresiensis Homo rudolfensis Australopithecus anamensis Homo sapiens Homo habilis Australopithecus africanus Homo erectus Australopithecus garhi Australopithecus afarensis Homo neanderthalensis Paranthropus aethiopicus Paranthropus robustus Estimates of the dates of origin and extinction of the three main groups of hominins (green, blue, and orange). The australopithecines split into two groups about 2.5 to 2.7 million years ago. Paranthropus boisei 4 3 2 1 Present Time (millions of years ago) Fig. 19-11, p. 430

15. Two Theories Differ on How and Where Homo sapiens Originated • Hypothesis 1: Modern Homo sapiens arose once, in one place, from its ancestral species. Members of this species then migrated from there to all parts of the globe. (Out-of-Africa hypothesis) • Hypothesis 2: Modern Homo sapiens arose in a number of different locations from similar ancestral populations at roughly the same time. The hypothesis assumes that although the populations were in different locations, they did interbreed and exchange genes. (Multiregional hypothesis) • Genetic evidence supports model #1.

16. Humans Have Spread Across the World • Available evidence suggests that • H. sapiens emigrated from Africa about 137,000 years ago • H. sapiens spread through Southeast Asia and Australia 40,000 to 60,000 years ago • H. sapiens replaced Neanderthals in Europe 40,000 to 50,000 years ago • North America and South America were populated in waves 15,000 to 30,000 years ago

17. The origin and spread of modern H. sapiens, reconstructed from genetic and fossil evidence. European population Origin: 40,000 to 50,000 years ago New World population Origin: 20,000 to 30,000 years ago Asian population Origin: 50,000 to 70,000 years ago Immigration from Africa About 137,000 years ago; 200 to 500 or more individuals African populations Origin: 130,000 to 170,000 years ago Population: 23,000 to 45,000 Australo-Melanesian population Origin: 40,000 to 60,000 years ago Fig. 19-13, p. 432

18. *All human populations are derived from African populations **Colors correspond to major continental regions. Fig. 19-12, p. 431

19. 19.7 Genomics and Human Evolution • Although separated for about 7 million years, analysis of human and chimp genomes shows many similarities and subtle differences • The DNA sequences are 98.8% identical • Variations due to insertions, deletions and duplications differ, ultimately change gene dosage • There is a 1% difference in coding sequence of genes • Phenotypic differences cannot be explained by differences in coding sequences • The important differences may involve gene regulation and genes that control body structure

20. Neanderthals are not Closely Related to Humans • H. neanderthalensis lived in the Middle East, Asia, and Europe 300,000 to 30,000 years ago. • Analysis of DNA recovered from Neanderthal remains clearly show that humans did not descend from them. (Neanderthal genome sequenced in 2010. • Some interbreeding did occur most likely in the Middle East before humans expanded into Europe and Asia • 1-4% of genes carried by non-africans are from Neanderthals

21. –440,000 to 270,000 y. a. Split of ancestral human and Neanderthal populations –41,000 y. a. Earliest modern humans in Europe –195,000 y. a. Earliest known anatomically modern humans –706,000 y. a. Coalescence of human and Neanderthal reference sequences –28,000 y. a. Most recent known Neanderthal remains Modern human Neanderthal Genomic data Fossil data Genomic and fossil evidence has been used to estimate the time of divergence of human and Neanderthal lines relative to landmark events evolution. Genomic analyses trace evolution back much farther than fossils can. Fig. 19-15, p. 434

22. DNA as a Molecular Clock • The rate of mutation between two DNA sequences can be used as a clock to provide a relative measure of time since divergence from a common ancestor • Assumes that mutation rate is constant • Can be calibrated by comparison to the fossil record

23. The Genetic Revolution: Tracing Ancient Migrations • How can we map out events that occurred thousands of years ago? • The answers are written in the genomes of present day populations • Genetic markers on the Y chromosome are passed from father to son • Markers in mitochondrial DNA are passed from mother to all offspring • These markers do not undergo recombination in meiosis—individuals carry these markers to new locations as the migrate • The Genographic Project:https://genographic.nationalgeographic.com/genographic/index.html

24. An Important Gene in Language Development • The gene is called FOXP2 • It is present in chimpanzees, modern humans, and Neanderthals • Genes and pathways controlled by FOXP2 differ among these groups • http://www.physorg.com/news188139245.htmlWith the help of a little singing bird, Penn State physicists are gaining insight into how the human brain functions, which may lead to a better understanding of complex vocal behavior, human speech production and ultimately, speech disorders and related diseases.