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Population genetics
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Population Genetics. Population Genetics. The study of genetic variation in populations. Population. A localized group of individuals of the same species . Species. A group of similar organisms. A group of populations that could interbreed. Gene Pool.

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Population Genetics

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Population genetics l.jpg

Population Genetics

Population genetics2 l.jpg

Population Genetics

  • The study of genetic variation in populations.

Population l.jpg


  • A localized group of individuals of the same species.

Species l.jpg


  • A group of similar organisms.

  • A group of populations that could interbreed.

Gene pool l.jpg

Gene Pool

  • The total aggregate of genes in a population.

  • If evolution is occurring, then changes must occur in the gene pool of the population over time.

Microevolution l.jpg


  • Changes in the relative frequencies of alleles in the gene pool.

Hardy weinberg theorem l.jpg

Hardy-Weinberg Theorem

  • Developed in 1908.

  • Mathematical model of gene pool changes over time.

Basic equation l.jpg

Basic Equation

  • p + q = 1

  • p = % dominant allele

  • q = % recessive allele

Expanded equation l.jpg

Expanded Equation

  • p + q = 1

  • (p + q)2 = (1)2

  • p2 + 2pq + q2 = 1

Genotypes l.jpg


  • p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives

Example calculation l.jpg

Example Calculation

  • Let’s look at a population where:

    • A = red flowers

    • a = white flowers

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Starting Population

  • N = 500

  • Red = 480 (320 AA+ 160 Aa)

  • White = 20

  • Total Genes = 2 x 500 = 1000

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Dominant Allele

  • A = (320 x 2) + (160 x 1)

    = 800

    = 800/1000

    A = 80%

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Recessive Allele

  • a = (160 x 1) + (20 x 2)

    = 200/1000

    = .20

    a = 20%

A and a in hw equation l.jpg

A and a in HW equation

  • Cross: Aa X Aa

  • Result = AA + 2Aa + aa

  • Remember: A = p, a = q

Substitute the values for a and a l.jpg

Substitute the values for A and a

  • p2 + 2pq + q2 = 1

    (.8)2 + 2(.8)(.2) + (.2)2 = 1

    .64 + .32 + .04 = 1

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Dominant Allele

  • A = p2 + pq

    = .64 + .16

    = .80

    = 80%

Recessive allele19 l.jpg

Recessive Allele

  • a = pq + q2

    = .16 + .04

    = .20

    = 20%

Result l.jpg


  • Gene pool is in a state of equilibrium and has not changed because of sexual reproduction.

  • No Evolution has occurred.

Importance of hardy weinberg l.jpg

Importance of Hardy-Weinberg

  • Yardstick to measure rates of evolution.

  • Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold.

  • Way to calculate gene frequencies through time.

Example l.jpg


  • What is the frequency of the PKU allele?

  • PKU is expressed only if the individual is homozygous recessive (aa).

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PKU Frequency

  • PKU is found at the rate of 1/10,000 births.

  • PKU = aa = q2

    q2 = .0001

    q = .01

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Dominant Allele

  • p + q = 1

    p = 1- q

    p = 1- .01

    p = .99

Expanded equation25 l.jpg

Expanded Equation

  • p2 + 2pq + q2 = 1

    (.99)2 + 2(.99x.01) + (.01)2 = 1

    .9801 + .0198 + .0001 = 1

Final results l.jpg

Final Results

  • Normals (AA) = 98.01%

  • Carriers (Aa) = 1.98%

  • PKU (aa) = .01%

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AP Problems Using Hardy-Weinberg

  • Solve for q2 (% of total).

  • Solve for q (equation).

  • Solve for p (1- q).

  • H-W is always on the national AP Bio exam (but no calculators are allowed).

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Hardy-Weinberg Assumptions

1. Large Population

2. Isolation

3. No Net Mutations

4. Random Mating

5. No Natural Selection

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If H-W assumptions hold true:

  • The gene frequencies will not change over time.

  • Evolution will not occur.

  • But, how likely will natural populations hold to the H-W assumptions?

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  • Caused by violations of the 5 H-W assumptions.

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Causes of Microevolution

1. Genetic Drift

2. Gene Flow

3. Mutations

4. Nonrandom Mating

5. Natural Selection

Genetic drift l.jpg

Genetic Drift

  • Changes in the gene pool of a small population by chance.

  • Types:

    • 1. Bottleneck Effect

    • 2. Founder's Effect

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By Chance

Bottleneck effect l.jpg

Bottleneck Effect

  • Loss of most of the population by disasters.

  • Surviving population may have a different gene pool than the original population.

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  • Some alleles lost.

  • Other alleles are over-represented.

  • Genetic variation usually lost.

Importance l.jpg


  • Reduction of population size may reduce gene pool for evolution to work with.

  • Ex: Cheetahs

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Founder's Effect

  • Genetic drift in a new colony that separates from a parent population.

  • Ex: Old-Order Amish

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  • Genetic variation reduced.

  • Some alleles increase in frequency while others are lost (as compared to the parent population).

Importance40 l.jpg


  • Very common in islands and other groups that don't interbreed.

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Gene Flow

  • Movement of genes in/out of a population.

  • Ex:

    • Immigration

    • Emigration

Result42 l.jpg


  • Changes in gene frequencies.

Mutations l.jpg


  • Inherited changes in a gene.

Result44 l.jpg


  • May change gene frequencies (small population).

  • Source of new alleles for selection.

  • Often lost by genetic drift.

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Nonrandom Mating

  • Failure to choose mates at random from the population.

Causes l.jpg


  • Inbreeding within the same “neighborhood”.

  • Assortative mating (like with like).

Result47 l.jpg


  • Increases the number of homozygous loci.

  • Does not in itself alter the overall gene frequencies in the population.

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Natural Selection

  • Differential success in survival and reproduction.

  • Result - Shifts in gene frequencies.

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  • As the Environment changes, so does Natural Selection and Gene Frequencies.

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  • If the environment is "patchy", the population may have many different local populations.

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Genetic Basis of Variation

1. Discrete Characters – Mendelian traits with clear phenotypes.

2. Quantitative Characters – Multigene traits with overlapping phenotypes.

Polymorphism l.jpg


  • The existence of several contrasting forms of the species in a population.

  • Usually inherited as Discrete Characteristics.

Examples l.jpg

Garter Snakes



Human example l.jpg

Human Example

  • ABO Blood Groups

  • Morphs = A, B, AB, O

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Other examples

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Quantitative Characters

  • Allow continuous variation in the population.

  • Result –

    • Geographical Variation

    • Clines: a change along a geographical axis

Yarrow and altitude l.jpg

Yarrow and Altitude

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Sources of Genetic Variation

  • Mutations.

  • Recombination though sexual reproduction.

    • Crossing-over

    • Random fertilization

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Preserving Genetic Variation

1. Diploidy - preserves recessives as heterozygotes.

2. Balanced Polymorphisms - preservation of diversity by natural selection.

Example60 l.jpg


  • Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.

Result61 l.jpg


  • Can't bred "true“ and the diversity of the population is maintained.

  • Ex – Sickle Cell Anemia

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  • Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.

  • Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

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Fitness - Darwinian

  • The relative contribution an individual makes to the gene pool of the next generation.

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Relative Fitness

  • Contribution of one genotype to the next generation compared to other genotypes.

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Rate of Selection

  • Differs between dominant and recessive alleles.

  • Selection pressure by the environment.

Modes of natural selection l.jpg

Modes of Natural Selection

1. Stabilizing

2. Directional

3. Diversifying

4. Sexual

Stabilizing l.jpg


  • Selection toward the average and against the extremes.

  • Ex: birth weight in humans

Directional selection l.jpg

Directional Selection

  • Selection toward one extreme.

  • Ex: running speeds in race animals.

  • Ex. Galapagos Finch beak size and food source.

Diversifying l.jpg


  • Selection toward both extremes and against the norm.

  • Ex: bill size in birds

Comment73 l.jpg


  • Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.

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Sexual Mate selection

  • May not be adaptive to the environment, but increases reproduction success of the individual.

Result76 l.jpg


  • Sexual dimorphism.

  • Secondary sexual features for attracting mates.

Comments l.jpg


  • Females may drive sexual selection and dimorphism since they often "choose" the mate.

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The Origin of Species

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Biological Species

  • A group of organisms that could interbreed in nature and produce fertile offspring.

Key points l.jpg

Key Points

  • Could interbreed.

  • Fertile offspring.

Speciation requires l.jpg

Speciation Requires:

1. Variation in the population.

2. Selection.

3. Isolation.

Reproductive barriers l.jpg

Reproductive Barriers

  • Serve to isolate a populations from other gene pools.

  • Create and maintain “species”.

Modes of speciation l.jpg

Modes of Speciation

1. Allopatric Speciation

2. Sympatric Speciation

Both work through a block of gene flow between two populations.

Allopatric speciation l.jpg

Allopatric Speciation

  • Allopatric = other homeland

  • Ancestral population split by a geographical feature.

  • Comment – the size of the geographical feature may be very large or small.

Example86 l.jpg


  • Pupfish populations in Death Valley.

  • Generally happens when a specie’s range shrinks for some reason.

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Another Example

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Conditions Favoring Allopatric Speciation

1. Founder's Effect - with the peripheral isolate.

2. Genetic Drift – gives the isolate population variation as compared to the original population.

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Conditions Favoring Allopatric Speciation

3. Selection pressure on the isolate differs from the parent population.

Result90 l.jpg


  • Gene pool of isolate changes from the parent population.

  • New Species can form.

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  • Populations separated by geographical barriers may not evolve much.

  • Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus.

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  • Fish - 72 identical kinds.

  • Crabs - 25 identical kinds.

  • Echinoderms - 25 identical kinds.

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Adaptive Radiation

  • Rapid emergence of several species from a common ancestor.

  • Common in island and mountain top populations or other “empty” environments.

  • Ex – Galapagos Finches

Mechanism l.jpg


  • Resources are temporarily infinite.

  • Most offspring survive.

  • Result - little Natural Selection and the gene pool can become very diverse.

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When the Environment Saturates

  • Natural Selection resumes.

  • New species form rapidly if isolation mechanisms work.

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Sympatric Speciation

  • Sympatric = same homeland

  • New species arise within the range of parent populations.

  • Can occur In a single generation.

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  • Polyploids may cause new species because the change in chromosome number creates postzygotic barriers.

Polyploid types l.jpg

Polyploid Types

1. Autopolyploid - when a species doubles its chromosome number from 2N to 4N.

2. Allopolyploid - formed as a polyploid hybrid between two species.

  • Ex: wheat

Autopolyploid l.jpg


Allopolyploid l.jpg


Animals l.jpg


  • Don't form polyploids and will use other mechanisms.

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Gradualism Evolution

  • Darwinian style evolution.

  • Small gradual changes over long periods time.

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Gradualism Predicts:

  • Long periods of time are needed for evolution.

  • Fossils should show continuous links.

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  • Gradualism doesn’t fit the fossil record very well. (too many “gaps”).

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Punctuated Evolution

  • New theory on the “pacing” of evolution.

  • Elridge and Gould – 1972.

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Punctuated Equilibrium

  • Evolution has two speeds of change:

    • Gradualism or slow change

    • Rapid bursts of speciation

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  • Speciation can occur over a very short period of time (1 to 1000 generations).

  • Fossil record will have gaps or missing links.

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  • New species will appear in the fossil record without connecting links or intermediate forms.

  • Established species will show gradual changes over long periods of time.

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Possible Mechanism

  • Adaptive Radiation, especially after mass extinction events allow new species to originate.

  • Saturated environments favor gradual changes in the current species.

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  • Punctuated Equilibrium is the newest ”Evolution Theory”.

  • Best explanation of fossil record evidence to date.

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Evolutionary Trends

  • Evolution is not goal oriented. It does not produce “perfect” species.

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Future of Evolution ?

  • Look for new theories and ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.

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