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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


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Population Genetics

  • The study of genetic variation in populations.


Population l.jpg

Population

  • A localized group of individuals of the same species.


Species l.jpg

Species

  • A group of similar organisms.

  • A group of populations that could interbreed.


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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.


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Microevolution

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


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Hardy-Weinberg Theorem

  • Developed in 1908.

  • Mathematical model of gene pool changes over time.


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Basic Equation

  • p + q = 1

  • p = % dominant allele

  • q = % recessive allele


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Expanded Equation

  • p + q = 1

  • (p + q)2 = (1)2

  • p2 + 2pq + q2 = 1


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Genotypes

  • p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives


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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%


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A and a in HW equation

  • Cross: Aa X Aa

  • Result = AA + 2Aa + aa

  • Remember: A = p, a = q


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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%


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Recessive Allele

  • a = pq + q2

    = .16 + .04

    = .20

    = 20%


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Result

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

  • No Evolution has occurred.


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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.


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Example

  • 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


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Expanded Equation

  • p2 + 2pq + q2 = 1

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

    .9801 + .0198 + .0001 = 1


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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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Microevolution

  • 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


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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


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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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Result

  • Some alleles lost.

  • Other alleles are over-represented.

  • Genetic variation usually lost.


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Importance

  • 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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Result

  • Genetic variation reduced.

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


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Importance

  • 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


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Result

  • Changes in gene frequencies.


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Mutations

  • Inherited changes in a gene.


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Result

  • 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.


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Causes

  • Inbreeding within the same “neighborhood”.

  • Assortative mating (like with like).


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Result

  • 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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Comment

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


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Result

  • 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.


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Polymorphism

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

  • Usually inherited as Discrete Characteristics.


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Garter Snakes

Gaillardia

Examples


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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


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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.


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Example

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


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Result

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

  • Ex – Sickle Cell Anemia


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Comment

  • 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.


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Modes of Natural Selection

1. Stabilizing

2. Directional

3. Diversifying

4. Sexual


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Stabilizing

  • Selection toward the average and against the extremes.

  • Ex: birth weight in humans


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Directional Selection

  • Selection toward one extreme.

  • Ex: running speeds in race animals.

  • Ex. Galapagos Finch beak size and food source.


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Diversifying

  • Selection toward both extremes and against the norm.

  • Ex: bill size in birds


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Comment

  • 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.


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Result

  • Sexual dimorphism.

  • Secondary sexual features for attracting mates.


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Comments

  • 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.


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Key Points

  • Could interbreed.

  • Fertile offspring.


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Speciation Requires:

1. Variation in the population.

2. Selection.

3. Isolation.


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Reproductive Barriers

  • Serve to isolate a populations from other gene pools.

  • Create and maintain “species”.


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Modes of Speciation

1. Allopatric Speciation

2. Sympatric Speciation

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


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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.


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Example

  • 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.


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Result

  • Gene pool of isolate changes from the parent population.

  • New Species can form.


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Comment

  • Populations separated by geographical barriers may not evolve much.

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


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Examples

  • 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


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Mechanism

  • 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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Plants

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


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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


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Autopolyploid


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Allopolyploid


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Animals

  • 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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Problem

  • 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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Predictions

  • 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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Predictions

  • 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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Comment

  • 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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