Population genetics
1 / 115

Population Genetics - PowerPoint PPT Presentation

  • Updated On :

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.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' Population Genetics' - medwin

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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

Starting population l.jpg
Starting Population

  • N = 500

  • Red = 480 (320 AA+ 160 Aa)

  • White = 20

  • Total Genes = 2 x 500 = 1000

Dominant allele l.jpg
Dominant Allele

  • A = (320 x 2) + (160 x 1)

    = 800

    = 800/1000

    A = 80%

Recessive allele l.jpg
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

Dominant allele18 l.jpg
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).

Pku frequency l.jpg
PKU Frequency

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

  • PKU = aa = q2

    q2 = .0001

    q = .01

Dominant allele24 l.jpg
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%

Ap problems using hardy weinberg l.jpg
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 l.jpg
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 l.jpg
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 l.jpg

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

Causes of microevolution l.jpg
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

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.

Result36 l.jpg

  • 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

Founder s effect l.jpg
Founder's Effect

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

  • Ex: Old-Order Amish

Result39 l.jpg

  • 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.

Gene flow l.jpg
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.

Nonrandom mating l.jpg
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.

Natural selection l.jpg
Natural Selection

  • Differential success in survival and reproduction.

  • Result - Shifts in gene frequencies.

Comment l.jpg

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

Result50 l.jpg

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

Genetic basis of variation l.jpg
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

Quantitative characters l.jpg
Quantitative Characters

  • Allow continuous variation in the population.

  • Result –

    • Geographical Variation

    • Clines: a change along a geographical axis

Sources of genetic variation l.jpg
Sources of Genetic Variation

  • Mutations.

  • Recombination though sexual reproduction.

    • Crossing-over

    • Random fertilization

Preserving genetic variation l.jpg
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

Comment62 l.jpg

  • 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 l.jpg
Fitness - Darwinian

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

Relative fitness l.jpg
Relative Fitness

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

Rate of selection l.jpg
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.

Sexual mate selection l.jpg
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.

Biological species l.jpg
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.

Conditions favoring allopatric speciation l.jpg
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 l.jpg
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.

Comment91 l.jpg

  • Populations separated by geographical barriers may not evolve much.

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

Examples92 l.jpg

  • Fish - 72 identical kinds.

  • Crabs - 25 identical kinds.

  • Echinoderms - 25 identical kinds.

Adaptive radiation l.jpg
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.

When the environment saturates l.jpg
When the Environment Saturates

  • Natural Selection resumes.

  • New species form rapidly if isolation mechanisms work.

Sympatric speciation l.jpg
Sympatric Speciation

  • Sympatric = same homeland

  • New species arise within the range of parent populations.

  • Can occur In a single generation.

Plants l.jpg

  • 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

Animals l.jpg

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

Gradualism evolution l.jpg
Gradualism Evolution

  • Darwinian style evolution.

  • Small gradual changes over long periods time.

Gradualism predicts l.jpg
Gradualism Predicts:

  • Long periods of time are needed for evolution.

  • Fossils should show continuous links.

Problem l.jpg

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

Punctuated evolution l.jpg
Punctuated Evolution

  • New theory on the “pacing” of evolution.

  • Elridge and Gould – 1972.

Punctuated equilibrium l.jpg
Punctuated Equilibrium

  • Evolution has two speeds of change:

    • Gradualism or slow change

    • Rapid bursts of speciation

Predictions l.jpg

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

  • Fossil record will have gaps or missing links.

Predictions110 l.jpg

  • 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 l.jpg
Possible Mechanism

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

  • Saturated environments favor gradual changes in the current species.

Comment112 l.jpg

  • Punctuated Equilibrium is the newest ”Evolution Theory”.

  • Best explanation of fossil record evidence to date.

Evolutionary trends l.jpg
Evolutionary Trends

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

Future of evolution l.jpg
Future of Evolution ?

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