population genetics and human evolution
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Population Genetics and Human Evolution. Chapter 19. Gradients of Genetic Variation in Human Populations. Prior to genomics, evolutionary biologists surveyed populations and cataloged differences in allele frequencies - PowerPoint PPT PresentationTRANSCRIPT
Michael Cummings
David Reisman • University of South Carolina
Population Genetics and Human Evolution
Chapter 19
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.
http://www.genome.gov/25019968
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
Variations used in Genomic Studies
Table 19-5, p. 428
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)
Fig. 19-9, p. 429
Australian Caucasoid
Human genetic
variationNorth American African
‘‘Race’’ 2 ‘‘Race’’ 1
South American African‘‘Race’’ 3
Indian Mainland Asian
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.
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
Images from http://christpantokrator.blogspot.com/2011/06/human-evolution-diagrams.html
Variations on Rudolph Zallinger’s March of Progress
Evolution is not a linear morphing of one type of organism into another – a common misunderstanding
Rather, evolution is branching from a shared ancestor
http://pewresearch.org/pubs/1105/darwin-debate-religion-evolutionhttp://tolweb.org/onlinecontributors/app
Genome-based evolutionary relationships among hominoids
Fig. 19-10, p. 430
7 m.y.a. chimps and humans had a common ancestor
After human line split from the chimps, three different species groups appeared.
Collectively known as hominins• Australopithecines• Paranthropus• Homo – our ancestral group
Early humans emerged ~5 million years ago
Fig. 19-11, p. 430
Homo floresiensisHomo rudolfensisAustralopithecus
anamensis Homo habilis Homo sapiens
Australopithecus africanus Homo erectus
Australopithecus afarensis Australopithecus garhi
Homo neanderthalensisParanthropus
aethiopicus Paranthropus robustus
Paranthropus boisei
4 3 2 1 PresentTime (millions of years ago)
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.
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.
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
Fig. 19-13, p. 432
European population Origin: 40,000 to 50,000 years ago
Asian population Origin: 50,000 to 70,000 years ago
New World population Origin: 20,000 to 30,000 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
The origin and spread of modern H. sapiens, reconstructed from genetic and fossil evidence.
*All human populations are derived from African populations**Colors correspond to major continental regions.
Fig. 19-12, p. 431
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
19.7 Genomics and Human Evolution
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
Fig. 19-15, p. 434
–440,000 to 270,000 y. a. Split of ancestral human
and Neanderthal populations
–41,000 y. a. Earliest modern
humans in Europe
–706,000 y. a. Coalescence of human
and Neanderthal reference sequences
–195,000 y. a. Earliest known anatomically
modern humans
–28,000 y. a. Most recent known
Neanderthal remains
Modern human
NeanderthalGenomic 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.
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
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
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.html With 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.