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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 genetics2
Population Genetics
  • The study of genetic variation in populations.
  • A localized group of individuals of the same species.
  • A group of similar organisms.
  • A group of populations that could interbreed.
gene pool
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.
  • Changes in the relative frequencies of alleles in the gene pool.
hardy weinberg theorem
Hardy-Weinberg Theorem
  • Developed in 1908.
  • Mathematical model of gene pool changes over time.
basic equation
Basic Equation
  • p + q = 1
  • p = % dominant allele
  • q = % recessive allele
expanded equation
Expanded Equation
  • p + q = 1
  • (p + q)2 = (1)2
  • p2 + 2pq + q2 = 1
  • p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives
example calculation
Example Calculation
  • Let’s look at a population where:
    • A = red flowers
    • a = white flowers
starting population
Starting Population
  • N = 500
  • Red = 480 (320 AA+ 160 Aa)
  • White = 20
  • Total Genes = 2 x 500 = 1000
dominant allele
Dominant Allele
  • A = (320 x 2) + (160 x 1)

= 800

= 800/1000

A = 80%

recessive allele
Recessive Allele
  • a = (160 x 1) + (20 x 2)

= 200/1000

= .20

a = 20%

a and a in hw equation
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
Substitute the values for A and a
  • p2 + 2pq + q2 = 1

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

.64 + .32 + .04 = 1

dominant allele18
Dominant Allele
  • A = p2 + pq

= .64 + .16

= .80

= 80%

recessive allele19
Recessive Allele
  • a = pq + q2

= .16 + .04

= .20

= 20%

  • Gene pool is in a state of equilibrium and has not changed because of sexual reproduction.
  • No Evolution has occurred.
importance of hardy weinberg
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.
  • What is the frequency of the PKU allele?
  • PKU is expressed only if the individual is homozygous recessive (aa).
pku frequency
PKU Frequency
  • PKU is found at the rate of 1/10,000 births.
  • PKU = aa = q2

q2 = .0001

q = .01

dominant allele24
Dominant Allele
  • p + q = 1

p = 1- q

p = 1- .01

p = .99

expanded equation25
Expanded Equation
  • p2 + 2pq + q2 = 1

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

.9801 + .0198 + .0001 = 1

final results
Final Results
  • Normals (AA) = 98.01%
  • Carriers (Aa) = 1.98%
  • PKU (aa) = .01%
ap problems using hardy weinberg
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).
hardy weinberg assumptions
Hardy-Weinberg Assumptions

1. Large Population

2. Isolation

3. No Net Mutations

4. Random Mating

5. No Natural Selection

if h w assumptions hold true
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?
  • Caused by violations of the 5 H-W assumptions.
causes of microevolution
Causes of Microevolution

1. Genetic Drift

2. Gene Flow

3. Mutations

4. Nonrandom Mating

5. Natural Selection

genetic drift
Genetic Drift
  • Changes in the gene pool of a small population by chance.
  • Types:
    • 1. Bottleneck Effect
    • 2. Founder's Effect
bottleneck effect
Bottleneck Effect
  • Loss of most of the population by disasters.
  • Surviving population may have a different gene pool than the original population.
  • Some alleles lost.
  • Other alleles are over-represented.
  • Genetic variation usually lost.
  • Reduction of population size may reduce gene pool for evolution to work with.
  • Ex: Cheetahs
founder s effect
Founder's Effect
  • Genetic drift in a new colony that separates from a parent population.
  • Ex: Old-Order Amish
  • Genetic variation reduced.
  • Some alleles increase in frequency while others are lost (as compared to the parent population).
  • Very common in islands and other groups that don't interbreed.
gene flow
Gene Flow
  • Movement of genes in/out of a population.
  • Ex:
    • Immigration
    • Emigration
  • Changes in gene frequencies.
  • Inherited changes in a gene.
  • May change gene frequencies (small population).
  • Source of new alleles for selection.
  • Often lost by genetic drift.
nonrandom mating
Nonrandom Mating
  • Failure to choose mates at random from the population.
  • Inbreeding within the same “neighborhood”.
  • Assortative mating (like with like).
  • Increases the number of homozygous loci.
  • Does not in itself alter the overall gene frequencies in the population.
natural selection
Natural Selection
  • Differential success in survival and reproduction.
  • Result - Shifts in gene frequencies.
  • As the Environment changes, so does Natural Selection and Gene Frequencies.
  • If the environment is "patchy", the population may have many different local populations.
genetic basis of variation
Genetic Basis of Variation

1. Discrete Characters – Mendelian traits with clear phenotypes.

2. Quantitative Characters – Multigene traits with overlapping phenotypes.

  • The existence of several contrasting forms of the species in a population.
  • Usually inherited as Discrete Characteristics.
human example
Human Example
  • ABO Blood Groups
  • Morphs = A, B, AB, O
quantitative characters
Quantitative Characters
  • Allow continuous variation in the population.
  • Result –
    • Geographical Variation
    • Clines: a change along a geographical axis
sources of genetic variation
Sources of Genetic Variation
  • Mutations.
  • Recombination though sexual reproduction.
    • Crossing-over
    • Random fertilization
preserving genetic variation
Preserving Genetic Variation

1. Diploidy - preserves recessives as heterozygotes.

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

  • Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.
  • Can't bred "true“ and the diversity of the population is maintained.
  • Ex – Sickle Cell Anemia
  • 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
fitness darwinian
Fitness - Darwinian
  • The relative contribution an individual makes to the gene pool of the next generation.
relative fitness
Relative Fitness
  • Contribution of one genotype to the next generation compared to other genotypes.
rate of selection
Rate of Selection
  • Differs between dominant and recessive alleles.
  • Selection pressure by the environment.
modes of natural selection
Modes of Natural Selection

1. Stabilizing

2. Directional

3. Diversifying

4. Sexual

  • Selection toward the average and against the extremes.
  • Ex: birth weight in humans
directional selection
Directional Selection
  • Selection toward one extreme.
  • Ex: running speeds in race animals.
  • Ex. Galapagos Finch beak size and food source.
  • Selection toward both extremes and against the norm.
  • Ex: bill size in birds
  • 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.
sexual mate selection
Sexual Mate selection
  • May not be adaptive to the environment, but increases reproduction success of the individual.
  • Sexual dimorphism.
  • Secondary sexual features for attracting mates.
  • Females may drive sexual selection and dimorphism since they often "choose" the mate.
biological species
Biological Species
  • A group of organisms that could interbreed in nature and produce fertile offspring.
key points
Key Points
  • Could interbreed.
  • Fertile offspring.
speciation requires
Speciation Requires:

1. Variation in the population.

2. Selection.

3. Isolation.

reproductive barriers
Reproductive Barriers
  • Serve to isolate a populations from other gene pools.
  • Create and maintain “species”.
modes of speciation
Modes of Speciation

1. Allopatric Speciation

2. Sympatric Speciation

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

allopatric speciation
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.
  • Pupfish populations in Death Valley.
  • Generally happens when a specie’s range shrinks for some reason.
conditions favoring allopatric speciation
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.

conditions favoring allopatric speciation89
Conditions Favoring Allopatric Speciation

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

  • Gene pool of isolate changes from the parent population.
  • New Species can form.
  • Populations separated by geographical barriers may not evolve much.
  • Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus.
  • Fish - 72 identical kinds.
  • Crabs - 25 identical kinds.
  • Echinoderms - 25 identical kinds.
adaptive radiation
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
  • Resources are temporarily infinite.
  • Most offspring survive.
  • Result - little Natural Selection and the gene pool can become very diverse.
when the environment saturates
When the Environment Saturates
  • Natural Selection resumes.
  • New species form rapidly if isolation mechanisms work.
sympatric speciation
Sympatric Speciation
  • Sympatric = same homeland
  • New species arise within the range of parent populations.
  • Can occur In a single generation.
  • Polyploids may cause new species because the change in chromosome number creates postzygotic barriers.
polyploid types
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
  • Don't form polyploids and will use other mechanisms.
gradualism evolution
Gradualism Evolution
  • Darwinian style evolution.
  • Small gradual changes over long periods time.
gradualism predicts
Gradualism Predicts:
  • Long periods of time are needed for evolution.
  • Fossils should show continuous links.
  • Gradualism doesn’t fit the fossil record very well. (too many “gaps”).
punctuated evolution
Punctuated Evolution
  • New theory on the “pacing” of evolution.
  • Elridge and Gould – 1972.
punctuated equilibrium
Punctuated Equilibrium
  • Evolution has two speeds of change:
    • Gradualism or slow change
    • Rapid bursts of speciation
  • Speciation can occur over a very short period of time (1 to 1000 generations).
  • Fossil record will have gaps or missing links.
  • 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.
possible mechanism
Possible Mechanism
  • Adaptive Radiation, especially after mass extinction events allow new species to originate.
  • Saturated environments favor gradual changes in the current species.
  • Punctuated Equilibrium is the newest ”Evolution Theory”.
  • Best explanation of fossil record evidence to date.
evolutionary trends
Evolutionary Trends
  • Evolution is not goal oriented. It does not produce “perfect” species.
future of evolution
Future of Evolution ?
  • Look for new theories and ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.