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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 genetics2
Population Genetics
  • The study of genetic variation in populations.
population
Population
  • A localized group of individuals of the same species.
species
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.
microevolution
Microevolution
  • 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
genotypes
Genotypes
  • 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%

result
Result
  • 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.
example
Example
  • 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?
microevolution30
Microevolution
  • 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.
result36
Result
  • Some alleles lost.
  • Other alleles are over-represented.
  • Genetic variation usually lost.
importance
Importance
  • 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
result39
Result
  • Genetic variation reduced.
  • Some alleles increase in frequency while others are lost (as compared to the parent population).
importance40
Importance
  • 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
result42
Result
  • Changes in gene frequencies.
mutations
Mutations
  • Inherited changes in a gene.
result44
Result
  • 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.
causes
Causes
  • Inbreeding within the same “neighborhood”.
  • Assortative mating (like with like).
result47
Result
  • 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.
comment
Comment
  • As the Environment changes, so does Natural Selection and Gene Frequencies.
result50
Result
  • 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.

polymorphism
Polymorphism
  • 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.

example60
Example
  • Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.
result61
Result
  • Can\'t bred "true“ and the diversity of the population is maintained.
  • Ex – Sickle Cell Anemia
comment62
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
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

stabilizing
Stabilizing
  • 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.
diversifying
Diversifying
  • Selection toward both extremes and against the norm.
  • Ex: bill size in birds
comment73
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.
sexual mate selection
Sexual Mate selection
  • May not be adaptive to the environment, but increases reproduction success of the individual.
result76
Result
  • Sexual dimorphism.
  • Secondary sexual features for attracting mates.
comments
Comments
  • 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.
example86
Example
  • 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.

result90
Result
  • Gene pool of isolate changes from the parent population.
  • New Species can form.
comment91
Comment
  • Populations separated by geographical barriers may not evolve much.
  • Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus.
examples92
Examples
  • 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
mechanism
Mechanism
  • 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.
plants
Plants
  • 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
animals
Animals
  • 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.
problem
Problem
  • 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
predictions
Predictions
  • Speciation can occur over a very short period of time (1 to 1000 generations).
  • Fossil record will have gaps or missing links.
predictions110
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.
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.
comment112
Comment
  • 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.
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