Download Chapter 19 Power Point Slides

Michael Cummings
Chapter 19
Population Genetics and
Human Evolution
David Reisman • University of South Carolina
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)
Australian
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.
Caucasoid
Human
genetic
variation
North American
‘‘Race’’ 1
‘‘Race’’ 2
South American
African
‘‘Race’’ 3
Indian
African
Mainland Asian
Fig. 19-9, p. 429
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
Variations on Rudolph Zallinger’s
March of Progress
Images from http://christpantokrator.blogspot.com/2011/06/human-evolution-diagrams.html
Evolution is not a linear morphing of one type
of 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
Genome-based evolutionary relationships
among hominoids
7 m.y.a.
chimps and
humans had
a common
ancestor
Fig. 19-10, p. 430
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
Homo floresiensis
Homo rudolfensis
Australopithecus
anamensis
Homo
sapiens
Homo habilis
Australopithecus africanus
Australopithecus afarensis
Homo erectus
Australopithecus garhi
Paranthropus
aethiopicus
Homo
neanderthalensis
Paranthropus robustus
Paranthropus boisei
4
3
2
1
Present
Time (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.
Fig. 19-11, p. 430
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
The origin and spread of modern H. sapiens, reconstructed
from genetic and fossil evidence.
European population
Origin: 40,000 to
50,000 years ago
Asian population
Origin: 50,000 to
70,000 years ago
Immigration
from Africa
African
About 137,000
populations
years ago; 200
Origin: 130,000
to 500 or more
to 170,000 years
individuals
ago Population:
23,000 to 45,000
New World population
Origin: 20,000 to
30,000 years ago
Australo-Melanesian
population Origin: 40,000
to 60,000 years ago
Fig. 19-13, p. 432
*All human populations are
derived from African
populations
**Colors correspond to major
continental regions.
Fig. 19-12, p. 431
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
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
–440,000 to 270,000 y. a.
Split of ancestral human
and Neanderthal
populations
–706,000 y. a.
Coalescence of human
and Neanderthal
reference sequences
–41,000 y. a.
Earliest modern
humans in Europe
–195,000 y. a.
Earliest known
anatomically
modern humans
–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
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/genographi
c/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.