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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Evolution

• The process of change in the genetic makeup of populations.

• The basis of the change is change in gene frequencies over time.

• How the frequency of a mutant allele change in time under various evolutionary forces.

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Population

• A population is any group of members of the same species in a given geographical area.

• Gene pool refers to the collection of all alleles in the members of the population.

• Population genetics refers to the study of the genetics of a population and how the alleles vary with time.

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Allele Frequencies

# of particular allele /total # of alleles

• count both chromosomes of each individual

• Allele frequencies affect the genotype frequencies or the frequency of each type of homozygote and heterozygote in the population.

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Fixation of an allele:

An allele must increase in frequency and ultimately become fixed in the population (all individuals have the same allele).

Fitness: of a genotype, a measure of individual’s ability to survive and reproduce (it is rather relative with respect to other individuals).

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Allele frequencies can change creating microevolution

• Individuals of one genotype reproduce more often with each other

• Individuals migrate between populations

• Population size is small or a group becomes reproductively isolated within a larger population

• Mutation introduces new alleles or new copies of alleles

• Individuals with a particular genotype are more likely to have viable, fertile offspring (negative, neutral, or positive selection)

Nonrandom mating

Migration

Genetic drift

Mutation

Selection

Conditions in which allele frequencies can change:

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Hardy-Weinberg EquilibriumA condition in which allele frequencies remain constant is called Hardy-Weinberg equilibrium

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Hardy-Weinberg Equilibrium

A condition in which allele frequencies remain

constant is called Hardy-Weinberg equilibrium

p + q = 1

p allele frequency of one alleleq allele frequency of a second allele

p2 + 2pq + q2 = 1

p2 and q2 genotype frequencies for each homozygote

2pq genotype frequency for heterozygotes

All of the allele frequencies together equals 1 or the whole collection of alleles

All of the genotype frequencies together equals 1

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Hardy-Weinberg EquilibriumGeneration 1p allele frequency of D normal finger length = .7q allele frequency of d short middle finger = .3

DDp2 = (.7)2 = .49

Genotype frequencies

Dd 2pq = 2 (.7)(.3) = .42

ddq2 = (.3)2= .09

Gamete frequencies

.49 .21 .21 .09

Frequency D gamete= .7 frequency d gamete = .3

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Hardy-Weinberg EquilibriumGeneration 1p allele frequency of D normal finger length = .7q allele frequency of d short middle finger = .3

Frequency D gamete= .7 frequency d gamete = .3

dd

q2=.09

Dd

pq=.21

Dd

pq=.21

DD

p2=.49

d

q=.3

D

p=.7

Female

gametes

d

q=.3

D

p=.7

Male gametes

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Possible matings

Proportion

In population

Frequency of offspring

genotypes

Male Female DD Dd dd

.49 DD .49 DD .2401 (DDxDD) .2401

.49 DD .42 Dd .2058 (DDxDd) .1029 .1029

.49 DD .09 dd .0441 (DDxdd) .0441

.42 Dd .49 DD .2058 (DDxDd) .1029 .1029

.42 Dd .42 Dd .1764 (DdxDd) .0441 .0882 .0441

.42 Dd .09 dd .0378 (Ddxdd) .0189 .0189

.09 dd .49 DD .0441 (DDxdd) .0441

.09 dd .42 Dd .0378 (Ddxdd) .0189 .0189

.09 dd .09 dd .0081 (ddxdd) .0081

Resulting offspring frequencies .49 .42 .09

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Application of Hardy-Weinberg Equilibrium:calculating risk

Frequency of disease in population = 1 / 2000

q2 = .0005

Frequency of CF disease allele = q

Frequency of wildtype CF allele = p p + q = 1, so

q2 = .022 =

= 1 - q = .977

Frequency of being heterozygote = 2pq

= 2 (.977)(.022) = .043 1 in 23

Risk of being a carrier of cystic fibrosis

for an Caucasian American depends upon

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How to calculate p and q from genotypic frequencies:

If you don’t know p and q, but you can distinctly identify homozygotes from heterozygotes, then p:(2 times # of homozygotes + number of heterozygotes)/2N

If you don’t know p and q, but you know (p2+2pq) and q2; then assume equilibrium and calculate q from q2:q = square root of q2

p = 1-q

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Carrier Frequency for Cystic Fibrosis

PopulationCarrier

Frequency

African American 1 in 66

Asian American 1 in 150

Caucasian American 1 in 23

Hispanic American 1 in 46

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Females: p2 + 2pq + q2 = 1

Males: p + q = 1

All of the women in the population

All of the men in the population

Application of Hardy-Weinberg Equilibrium:calculating risk with X-linked traits

Hemophilia is X-linked and occurs in 1 in 10,000 malesp= 1/10,000 = .0001q= .9999

Carrier females = 2pq = 2 (.0001) (.9999) = .0002 1 in 5000 are carriersAffected females = p2 = (.0001) 2 = .00000001 1 in 100 million women

will have hemophilia

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Application of Hardy-Weinberg Equilibrium:DNA identification

SNPs or Single nucleotide polymorphisms Single base differences between chromosomes

Repeated sequences Variation in the number of repeats present

Variation in DNA sequences outside of genes are subject to Hardy-Weinberg equilibrium.

Noncoding variation is useful as it is not subject to as many impacts that lead to deviations in H-W equilibrium namely selection and assortative mating.

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Forces that alter allele frequencies

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Migration

• Changes in allele frequency can be mapped across geographical or linguistic regions.

• Allele frequency differences between current populations can be correlated to certain historical events.

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Mapping a trait geographically can suggest patterns of migration

Frequencies of galactokinase deficiency decrease westward from Bulgaria.

Gradients in allele frequencies between successive neighboring populations are called clines.

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Genetic variation in space and time & natural variation in populations:

• Genetic structure of populations and frequency of alleles varies in space or time.

• Allele frequency cline = allele frequencies change in a systematic way geographically.

Fig. 22.6, Allele frequency clines in the blue mussel.

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Genetic Drift

Events that create small populations enhance the effect of genetic drift.

Founding a new population

Bottlenecks (natural disaster, famine)

Geographic separation (islands)

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Genetic driftA population bottleneck occurs when a large population is drastically reduced in size.

Rebounds in population size occur with descendants of a limited number of survivors.

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Genetic drift:

• Chance alone may result in changes in allele frequency, including fixation and extinction.

• Genetic drift is analogous to sampling effect.

• Genetic drift has important consequences for small populations.

Example:

• Island population of 10 individuals; 5 with brown eyes (BB) and 5 with green eyes (bb); f(B) = 0.5, f(b) = 0.5.

• Typhoon devastates the island; 5 people with brown eyes (BB) die.

• Allelic frequency of b , f(b) = 1.0; chance events have radically changed the allele frequencies and the population evolves.

• Now imagine the same scenario for an island of 10,000 inhabitants.

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Genetic drift:

• Chance deviations from expected ratios of gametes and zygotes also produce genetic drift.

• Cross Aa x aa expect 50% Aa and 50% aa, but not all of the time insofar that sampling is limited (sampling error).

• Sampling variance: sp2 = pq/2N

*N = number of individuals in the population.

• Variance is large for small populations, and small for large populations.

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Effective population size (Ne)

When number of males and females are not equal, the Ne is:Ne = (4 x Nf x Nm)/(Nf + Nm), where Nf and Nm

are breeding females and males, respectively.

If Nf = 36 and Nm = 36, Ne equals Ntotal:Ne = (4 x 36 x 36)/(36+36) = 72

If Nf = 70 and Nm = 2:Each male contributes ½ x ½ = 0.25 of the alleles to the

next generation (both males 0.5 of all alleles)

Each female contributes ½ x 1/70 = 0.0071 of all alleles.

Ne = (4 x 70 x 2)/(70+2) = 7.8 (~8 breeding adults).

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Sampling variance of p

Remember sampling variance: sp

2 = pq/2N

Consider unequal number of breeding males and females:sp

2 = pq/2Ne

Standard error: sp = √pq/2Ne

95% confidence limit = p 2sp

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Buri’s study of genetic drift in Drosophila

Actual data for 107 experimental populations.

Randomly selected 8 males and 8 females (N = 16) from each population for the next generation for 19 consecutive generations.

Calculated the frequency of bw75 allele, and generated a frequency distribution among 107 populations.

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MutationAllele frequencies change in response to mutation.

Mutation can introduce new alleles.Mutation can convert one allele to another.

Mutation has a minor impact unless coupled with another effect (small population size, selection).

•Dominant deleterious alleles disappear quickly.•Recessive deleterious alleles are eliminated when homozygotes appear and fail to reproduce.

The collection of recessive deleterious alleles present in a population is called the genetic load.

Selection acts to eliminate deleterious alleles.

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Mutation:

• Heritable changes within DNA.

• Source of all truly new genetic variation.

• Raw material for evolution.

Mutation rate varies between loci and among species: • ~10-4 to 10-8 mutations/gene/generation.

• Mutation rate is abbreviated .

• Some mutations are neutral (no effect on reproductive fitness).

• Others are detrimental or lethal (depends on environment).

• If population size is large, effects of mutation act slowly.

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Mutation:

Irreversible mutation:

Allele A is fixed (p =1.0) and mutates A a at rate of = 10-4:

Hartl & Clark (1997) Principles of Population Genetics

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Fig. 22.3, Frequencies of genotypes AA, Aa, and aa relative to the frequencies of alleles A and a in populations at Hardy-Weinberg equilibrium.

Max. heterozygosity@ p = q = 0.5

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