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
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Inversions:
Crossing-over is reduced in heterozygotes for inversions:A
C DEF
A
E DCF
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
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Frequency of the AA genotype = # of individuals with AA genotype
total # of individuals in the population
Genotypic frequencies
AA
AA
Aa
Aa
Aa
Aa
Aa aaaa
aa
Frequency (AA) = 2/10 = 0.2 = 20%
Frequency (Aa) = 5/10 = 0.5 = 50%
Frequency (aa) = 3/10 = 0.3 = 30%
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
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Frequency of the a allele = # of copies of the a allele
total # of copies of the A gene
Allelic frequencies
AA
AA
Aa
Aa
Aa
Aa
Aa aaaa
aa
Frequency (A) = 9/20 = 0.45 = 45%
Frequency (a) = 11/20 = 0.55 = 55 %
Note: The total = 1.0 or 100%
Allele frequencies can also be calculated from genotypic frequencies
AA
AA
Aa
Aa
Aa
Aa
Aa aaaa
aa
Frequency (A) = f(AA) + 1/2 f(Aa) = 0.2 + 1/2(0.5) = 0.45
Frequency (a) = f(aa) + 1/2 f(Aa) = 0.3 + 1/2(0.5) = 0.45
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
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?
• 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
The Hardy-Weinberg Principle
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=0.45q = 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
The gene pool of this populationcan be pictured as a container fullof gametes.
The frequency of gametescarrying the A allele = 0.2
The frequency of gametes carrying the a allele = 0.8
A
A
A
A
a
a a
aa
a
aa
a
a
a
aaaa
a
How does it work? -Genotypic frequencies
When gametes fuse to produce offspring:
Eggs (generation 0)
A (freq.=p) a (freq.=q)
A (
freq
.=p)
a (f
req.
=q)
Sp
erm
(ge
ner
atio
n 0
)
Freq (AA) = p x p
Freq (Aa) = p x q
Freq (aA) = q x p
Freq (aa) = q x q
Genotypic frequency (we’ll call this generation 1)
f(AA) = p2
f(Aa) = 2pqf(aa) = q2
Since these are all the possible genotypes:
p2 + 2pq + q2 = 1
The next generation…
Genotype frequencies in Generation 1: f(AA) = p2 f(Aa) = 2pq f(aa) = q2
Allele frequencies in Generation 1?
p’ = f(A) in generation 1
p’ =
Gametes of Generation 0: f(A) = p f(a) = q
• 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
What’s the point?
Other consequences of H-W• Genotypic/ phenotypic frequencies depend on allele
frequencies, not on which allele is dominant or recessiveExample: Achondroplasia gene: D =dwarfism, d= normal height
p = f(D) = 0.00005; q = f(d) = 0.99995Frequency 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 itExample:Cystic fibrosis: C = normal allele, c = cystic fibrosis
p = f(C) = 0.978; q = f(c) = 0.022Freq. of cc individuals = q2 = 0.00048 (1 in 2000)Freq.of Cc individuals = 2pq = 0.043 (almost 1 in 25)