Transcript Natural Selection - Indiana University

Sewall Wright
21 Dec 1889 Melrose, MA
3 March 1988 Madison, WI
R. A. Fisher
17 Feb 1890 London, England
29 July 1962 Adelaide, Australia
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R. A. Fisher
•1890: Born in East Finchley, London.
•1909: Student at Gonville and Caius College, Cambridge.
•1919: Statistician at Rothamsted Experimental Station.
•1933: Chair of Eugenics at University College, London.
•1943: Balfour Professor of Genetics, Cambridge University.
•1957: President of Gonville and Caius College.
•1962: Died Adelaide, South Australia.
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R. A. Fisher
• 1929, Elected a Fellow of the Royal Society
• 1938, Royal Medal of the Society
• 1948, Darwin Medal of the Royal Society:
... in recognition of his distinguished contributions to the theory of
natural selection.
• 1955, Copley Medal of the Royal Society:
... in recognition of his numerous and distinguished contributions
to developing the theory and application of statistics for making
quantitative a vast field of biology.
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S. Wright
• 1889, Born in Melrose Massachusetts
• 1911, Student, Cold Spring Harbor, summer
• 1915, PhD Harvard University
• 1915-1925, Researcher, USDA
• 1926-1954, Professor, University of Chicago
• 1949-1950 Fulbright Professor, University of Edinburgh,
• 1954-88, Emeritus Professor, University of Wisconsin
• 1988, Died in Madison, Wisconsin
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S. Wright
• 1947,Weldon Medal of the Royal Society
• 1947, Elliott Award of National Academy of Sciences
• 1956, Kimber Award of National Academy of Sciences
• 1966, National Medal of Science
• 1980, Medal of the Royal Society of London
• 1984, Balzan Prize
• Lewis Prize of the American Philosophical Society.
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Sickle Cell Disease
•
A heritable blood disorder that affects red blood
cells.
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The sickle cell allele changes the normal hemoglobin A
into hemoglobin S (the “S” stands for Sickle)
 hemoglobin: the oxygen carrying molecule in red
blood cells
red blood cells that contain mostly hemoglobin S
become stiff and sickle shaped rather than the normal
soft round cells
sickle cells have difficulty passing through small blood
vessels and cause blockages
blockages allow less blood to reach that part of the body
and result in tissue damage.
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Sickle Cell Disease
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Complications include
– anemia
– heart failure
– increased susceptibility to pneumonia
– kidney failure
– enlargement of the spleen
Many people with sickle cell disease do not
survive long enough or are not healthy
enough to have children
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Sickle Cell Disease
•
Most common in West and Central Africa where as
many as 25% of the people have sickle cell trait
and 1-2% of all babies are born with a form of
Sickle Cell disease.
– In the U.S. with an estimated population of 300
million, about 1,000 babies are born with sickle
cell disease each year.
– In Nigeria, with an estimated population of ~90
million, 45,000-90,000 babies are born with
sickle cell disease each year.
– Frequency of S hemoglobin allele is higher in
Nigeria than in the U.S.
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Population Genetics of SCD in the U.S.
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Homozygotes for the normal hemoglobin A gene (AA)
– Do NOT suffer from SCD
– Viability, WAA = 1.00
Homozygotes for the sickle cell form, hemoglobin S (aa)
– suffer from SCD
– Viability, Waa = 1 – s, where ‘s’ is the selection
coefficent.
Heterozygotes (Aa)
– Do not suffer from SCD
– Viability, WAa = 1.00
SCD is a Recessive Genetic Disease, because the viability of
the heterozygote equals that of the normal homozygote.
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Population Genetics of SCD in U.S.
Viability
Fitness
Phenotypes
Genotypes
Normal
AA
WAA = 1.0
Normal
Aa
WAa = 1.0
SCD
Waa = 0.1
aa
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s, selection coefficient
of an allele
+/-
Natural Selection ~ s
0.1000
0.0100
Alleles with s in this range
have a history determined
largely by natural selection
Population Size
Threshold
Ne = 500
0.0010
0.0001
0.00001
Genes with s in this range
are effectively neutral
have a history determined
largely by RGD
Random Genetic Drift
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s, selection coefficient
of an allele
+/-
R. A. Fisher’s
World View
Natural Selection
0.1000
0.0100
0.0010
Alleles with s in this range
have an evolutionary history
determined largely by
natural selection.
Essentially ALL alleles
fall in this region.
Domain of
Natural Selection
is very large.
Domain of
of RGD
is very small.
Random Genetic Drift
Population Size
Threshold
Ne > >1,000,000 12
0.0001
0.00001
s, selection coefficient
of an allele
+/-
Natural Selection
S. Wright’s
World View
0.1000
Population Size
Threshold
Ne = 50
0.0100
0.0010
Alleles with s in this range
are effectively neutral
and have an evolutionary history
0.0001
determined
largely by RGD
0.00001
For a neutral
allele, s = 0,
and it does not
experience
natural selection
Random Genetic Drift
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s, selection coefficient
of an allele
+/-
Natural Selection
S. Wright’s
World View
0.1000
Population Size
Threshold
Ne = 50
0.0100
0.0010
Alleles with s in this range
are effectively neutral
and have an evolutionary history
0.0001
determined
largely by RGD
0.00001
Domain of
Natural Selection
is comparable to
Domain of
of RGD.
Random Genetic Drift
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Interaction of the three evolutionary forces:
Mutation, Selection, and RGD
Only naive theories about evolution assume that
Natural Selection leads a population
to achieve an optimal level of adaptation.
Because Mutation introduces harmful alleles into
populations and because they can become fixed
by Random Genetic Drift,
Natural Selection simply cannot produce the
best of all possible worlds.
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