Download Variation in Natural Populations

Variation in Natural Populations
Overview of Evolutionary Change
• Natural Selection: variation among
individuals in heritable traits lead to variation
among individuals in reproductive success
• Evolution: change in genetic composition of a
population over time
Sooo, understanding evolution reduces to
understanding how gene frequencies change over time
Where does the genetic variation that
natural selection acts on come from?
• Mutation is ultimate source of new alleles
• Types of Mutations
– Point mutations
– Chromosome alterations
Point mutations
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Base substitutions
could be:
1) Missense
mutations
2) Silent
mutations
3) Neutral
mutations
Chromosome Alterations
Inversions:
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Crossing-over is reduced in heterozygotes for inversions:
A
C
D
E
F
A
E
D
C
F
Alleles in an inversion are
“locked together” and may be
selected together as one
Selection for Inversions
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Drosophila subobscura: same inversions are found in similar
frequencies in similar locations along an environmental cline
New genes can arise from gene duplications
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“gene families”: genes that
have arisen from gene
duplications
Measuring Genetic Variation in
Natural Populations
• Population genetics incorporates
Mendelian Genetics into the study of
Evolution
• The goal of population genetics is to
understand the genetic composition of a
population and the forces that determine
and change that composition
So what exactly is a population?
• A population = a group of interbreeding
individuals of the same species living within a
prescribed geographical area
• A Gene Pool = the complete set of genetic
information contained within all the individuals
in a population
Describing the genetic composition
of a population
• Genotypic frequencies: the proportion of
individuals in a population with a given genotype
Example: Gene A with two alleles, A and a
Frequency of the AA genotype
# of individuals with AA genotype
=
total # of individuals in the population
Genotypic frequencies
aa
Aa
aa
Frequency (AA) = 2/10 = 0.2 = 20%
AA
Aa
Aa
Frequency (Aa) = 5/10 = 0.5 = 50%
aa
Frequency (aa) = 3/10 = 0.3 = 30%
AA
Aa
Aa
Note: The total = 1.0 or 100%
Describing the genetic composition
of a population
• Allelic frequencies: the proportion of alleles of a
particular gene locus in a gene pool that are of a
specific type
Example: Gene A with two alleles, A and a
# of copies of the a allele
Frequency of the a allele =
total # of copies of the A gene
Allelic frequencies
aa
Aa
aa
Frequency (A) = 9/20 = 0.45 = 45%
AA
Aa
Aa
Frequency (a) = 11/20 = 0.55 = 55 %
aa
AA
Aa
Aa
Note: The total = 1.0 or 100%
aa
Allele frequencies can also be calculated
from genotypic frequencies
Aa
aa
Frequency (A) = f(AA) + 1/2 f(Aa) =
0.2 + 1/2(0.5) = 0.45
AA
Aa
Aa
Frequency (a) = f(aa) + 1/2 f(Aa) =
0.3 + 1/2(0.5) = 0.45
aa
AA
Aa
Aa
Note: The total = 1.0 or 100%
Measures of Genetic Diversity
A genetic locus is said to be polymorphic
if that locus has more than one allele occurring
at a frequency greater than 5% (for example: if
for gene A, f(A) = 0.06, f(a) = 0.94
Heterozygosity: the fraction of individuals in a
population that are heterozygotes
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Most species show
considerable
genetic diversity
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Why do we have polymorphic loci?
Shouldn’t dominant alleles replace
recessive ones?
Shouldn’t natural selection eliminate
genetic variation?
The Hardy-Weinberg Principle
• Allele frequencies and genotypic frequencies will
remain constant from generation to generation as long
as:
–
–
–
–
–
The population size is large
Mating is random
No mutation takes place
There is no migration in or out of the population
There is no natural selection
• If these conditions are met, the population is said to
be in Hardy-Weinberg Equilibrium
How does it work?-Allelic frequencies
• By convention, for a given gene the frequency of the dominant
allele is symbolized by p, the frequency of the recessive allele
is represented by q
• So for our previous example,
p = f(A) = 9/20=0.45
q = f(a) = 11/20=0.55
• If these are the only two alleles for the gene in the population
then
p + q = 1.0
How does it work? -Genotypic frequencies
Imagine a population in which p = 0.2, q = 0.8
a
A
a
a
a
a
a
a
a
A
a
a
a
A
a
a
a
a
a
A
The gene pool of this population
can be pictured as a container full
of gametes.
The frequency of gametes
carrying the A allele = 0.2
The frequency of gametes
carrying the a allele = 0.8
How does it work? -Genotypic frequencies
When gametes fuse to produce offspring:
a (freq.=q) A (freq.=p)
Sperm (generation 0)
Eggs (generation 0)
A (freq.=p)
a (freq.=q)
Genotypic frequency
(we’ll call this generation 1)
Freq (AA) =
pxp
Freq (Aa) =
pxq
f(AA) = p2
f(Aa) = 2pq
f(aa) = q2
Freq (aA) =
qxp
Freq (aa) =
qxq
Since these are all the
possible genotypes:
p2 + 2pq + q2 = 1
The next generation…
Gametes of Generation 0: f(A) = p
f(a) = q
Genotype frequencies in Generation 1:
f(AA) = p2 f(Aa) = 2pq
Allele frequencies in Generation 1?
p’ = f(A) in generation 1
p’ =
f(aa) = q2
What’s the point?
• Hardy-Weinberg tells us that if certain conditions are
met, there will be no change in gene frequencies--> no
evolution
– The population size is large
– Mating is random
– No mutation takes place
– There is no migration in or out of the population
– There is no natural selection
• If one or more of these assumptions is violated, gene
frequencies will change --> evolution occurs
Other consequences of H-W
• Genotypic/ phenotypic frequencies depend on allele
frequencies, not on which allele is dominant or recessive
Example: Achondroplasia gene: D =dwarfism, d= normal height
p = f(D) = 0.00005; q = f(d) = 0.99995
Frequency of dwarfs = p2 + 2pq =0.0001 (one in ten thousand)
• For rare recessive alleles, most individuals with the allele
will be heterozygotes, and will not express it
Example:Cystic fibrosis: C = normal allele, c = cystic fibrosis
p = f(C) = 0.978;
q = f(c) = 0.022
Freq. of cc individuals = q2 = 0.00048 (1 in 2000)
Freq.of Cc individuals = 2pq = 0.043 (almost 1 in 25)