variation in natural populations. overview of evolutionary change natural selection: variation among...

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:

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



Drosophila subobscura: same inversions are found in similar frequencies in similar locations along an environmental cline

New genes can arise from gene duplications



“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 %


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


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







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)

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