mechanisms of evolution: genetic variation
DESCRIPTION
MECHANISMS OF EVOLUTION: GENETIC VARIATION. Quantity of DNA and the number of genes are highly variable among species Variation within a species is the result of the variety & random recombination of alleles possessed by individuals - PowerPoint PPT PresentationTRANSCRIPT
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MECHANISMS OF EVOLUTION:GENETIC VARIATION
Quantity of DNA and the number of genes are highly variable among species
Variation within a species is the result of the variety & random recombination of alleles possessed by individuals
Results in a high degree of genetic diversity within most populations
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MECHANISMS OF EVOLUTION:GENETIC VARIATION
Recall the following terms:
• Genes• Alleles• Homozygous• Heterozygous• Genome• Genotype• Phenotype• Population
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Wilhelm Weinberg
GODFREY HAROLD HARDY
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MECHANISMS OF EVOLUTION:Hardy-Weinburg Principle
Allele frequencies will not change from generation to generation as long as the following conditions are met:
1) Large population
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2) Mating opportunities are equal
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3) No mutations occur
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4) No migration occurs
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5) No natural selection occurs – all individuals have an equal chance of reproduction
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Hardy-Weinburg Principle• For a gene with two alleles (A and a), the
Hardy-Weinburg Principle can be expressed using the equations below:
If p = frequency for allele A and q = frequency of allele a Then…. p + q = 1 (p + q)2 = 12
p2 + 2pq + q2 = 1
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MECHANISMS OF EVOLUTION:HARDY-WEINBURG PRINCIPLE
p2 = frequency of AA genotype
2pq = frequency of Aa genotype
q2 = frequency of aa genotype
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MECHANISMS OF EVOLUTION:HARDY-WEINBURG SQUARE
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PROBLEM #1
A population has only two alleles, R and r for a particular gene. The allele frequency of R is 20 %a) What are the frequencies for RR ?b) What are the frequencies for Rr ?c) What are the frequencies for rr ?
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PROBLEM #1
Got a problem ?
No problem… Step 1…..
Write down all givens
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Step #3 How is what you are
being asked, related to what you know ? Then
solve !
Step #2What are you being asked ?
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SOLUTION
GIVENS:p = 0.20q = 0.80
I KNOW THAT :p2 + 2pq + q2 = 1
Substitute values for p and q (.20)2 + 2(.20)(.80) + (.80)2 = 10.40 + 0.32 + 0.64 = 1
Frequency of RR genotype = 0.04 or 4 %
Frequency of Rr genotype = 0.32 or 32 %
Frequency of rr genotype = 0.64 0r 64 %
Congratulations !You did it
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PROBLEM # 2In a moth population of 100 individuals there are 5 individuals who are recessive (light grey ww) for wing color:
a) How many are homozygous (WW) dominant ?
b) How many are heterozygous (Ww) dominant ?
Write out your
givens !
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SOLUTIONGiven:q2 = 5_ 100
q2 = 0.05
q = 0.2236
p + q = 1
p= 1 - q
p = 1 – 0.2236
p = 0.7764
a) p2 = (0.7764)2
p2 = 0.6028
Therefore 60 individuals are homozygous dominant
b) 2pq = 2(0.7764) (0.2236)FIND p2 FIND 2pq
2pq = 0.3472
Therefore there are 35 individuals who are heterozygous
Grade 12 Biology Rocks
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MECHANISMS OF EVOLUTION:RANDOM CHANGE
• Genetic Drift = changes to allele frequency as a result of chance: such changes are much more pronounced in small populations
• Evolution = defined in genetic terms as any change in gene (and allele) frequencies within a population
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Genetic Drift• In small populations, chance can play a huge
role in altering allele frequencies• When a severe event results in a drastic
reduction in numbers, a population may experience a bottle-neck effect
• In this form of genetic drift occurs, a very small number of sample alleles survives to establish a new population
• Their relative frequency may differ those in the original population & additional genetic drift may result in further changes in the gene pool
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MECHANISMS OF EVOLUTION:Genetic Drift
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BOTTLENECK EFFECTOccurred in the northern elephant seal populationPopulation was reduced to 20 individuals in 1890’s and has rebounded to over 20 000 by 1974Genetic tests show 24 loci tests showed total homozygosity
Northern Elephant Seal Distribution
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FOUNDER EFFECT
• Founder effect = genetic drift that results when a small number of individuals separate from their original population and find a new population
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PATTERNS OF SELECTION• 3 types of selection
pressures
1) Stabilizing Selection=
selection against individuals exhibiting variations in a trait deviate from the current average
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PATTERNS OF SELECTION
2. Directional Selection Pressure= Selection that favours an increase or decrease in the value of a trait from the current population average
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PATTERNS OF SELECTION
3. Disruptive Selection Pressure= Selection pressure that favours two or more variations or traits that differ from the current average
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PATTERNS OF SELECTION4. Sexual Selection Pressure: Different reproductive success that results from variation in the ability to obtain mates: results in sexual dimorphism and mating courtship behaviours
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PATTERNS OF SELECTION• Not all species show
obvious sexual dimorphism
• In some species of penguin, males & females look the same and they have a hard time telling each other apart
• A male picks up a stone & drops it at the feet of a would-be-mate. If the other penguin is a male, the offering is rejected
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ALTRUISM
Altruism = behaviour that decreases the fitness of an individual that is assisting or cooperating with a recipient whose individual fitness is increased
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SPECIATION: The formation of a new species
• Species = members or breeding groups or populations that are reproductively isolated from other groups and evolve independently
• Speciation evolutionary formation of a new species
• Reproductive Isolating Mechanisms are any behavioral, structural or biochemical traits that prevent individuals from different species from reproducing together
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REPRODUCTIVE ISOLATING MECHAMISMS: Prezygotic Isolating Mechanisms
Pre-zygotic mechanisms include:1. Ecological isolation: species
occupy different habitats or separate niches
Ex) lions = savannah animals
tigers = forest animalsCould mate and hybridize if found
together and produce ligars
Lion
Tiger
Liger
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REPRODUCTIVE ISOLATING MECHAMISMS:Prezygotic Isolating Mechanisms
2. Temporal Isolation = similar plants bloom at different times of the day, night, month/seasons. This leaves no change of hybridization since reproductive cycles or mating occurs at different times
Spring Iris
Summer Iris
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REPRODUCTIVE ISOLATING MECHAMISMS: Prezygotic Isolating Mechanisms
3. Behavioural Isolation = Each species may use different signals for attracting a mate. Use the wrong dance, misinterpret the dance, and you are breakfast, lunch, or dinner
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REPRODUCTIVE ISOLATING MECHAMISMS
• Ecological isolation, temporal isolation & behavioural isolation are all pre- zygotic isolating mechanisms
• These mechanisms prevent interspecies mating
• Another prezygotic isolating mechanism is the prevention of fertilization. There are two mechanisms that prevent fertilization:
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REPRODUCTIVE ISOLATING MECHAMISMS:Prevention of Fertilization
1. Mechanical Isolation = structural differences in reproductive organs can prevent copulation.
Square Peg/Round Hole ?
Who Is He Kidding ?
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REPRODUCTIVE ISOLATING MECHAMISMS: Prevention of Fertilization
Shapes of floral features in plants can affect the transfer of pollen is also an example of mechanical isolation
This orchid looks like a wasp
The pink lady’s slipper orchid exhibits a mechanical isolating mechanism
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REPRODUCTIVE ISOLATING MECHAMISMS: Gametic Isolation
• In coral reefs, many species release sperm & eggs into the water at the same time.
• Sperm & egg of the SAME species recognize each other by molecular markers
• In many species sperm cannot survive inside the female
Coral spawning a cloud of sperm
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REPRODUCTIVE ISOLATING MECHAMISMS:Post Zygotic mechanisms preventing hybrids from mating
Prevents maturation & reproduction in offspring from interspecies reproduction
A spotted mule is sterile, resulting from horse & donkey matingHybrid trilliums are sterile
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REPRODUCTIVE ISOLATING MECHAMISMS:Post Zygotic Isolating Mechanisms
• Zygote mortality & Hybrid inviability ensure reproductive isolation of a gene pool by preventing the exchange of alleles between the parent species
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MODES OF SPECIATION
1. Allopatric Speciation is the evolution of populations into separate species as a result of geographic isolation
The canyon is a barrier to dispersal by small mammals, (Grand Canyon Squirrels) and as a consequence the isolated populations can diverge.
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Mountain range is the barrier
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MODES OF SPECIATION
2. Sympatric Speciation is the evolution of populations within the same geographic area into separate species. Largely due to genetic drift.
Evidence suggests stickleback fish species evolved by sympatric speciation
Good Bye Chapter
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