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

MECHANISMS OF EVOLUTION:GENETIC VARIATION

Recall the following terms:

• Genes• Alleles• Homozygous• Heterozygous• Genome• Genotype• Phenotype• Population

Wilhelm Weinberg

GODFREY HAROLD HARDY

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

2) Mating opportunities are equal

3) No mutations occur

4) No migration occurs

5) No natural selection occurs – all individuals have an equal chance of reproduction

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

MECHANISMS OF EVOLUTION:HARDY-WEINBURG PRINCIPLE

p2 = frequency of AA genotype

2pq = frequency of Aa genotype

q2 = frequency of aa genotype

MECHANISMS OF EVOLUTION:HARDY-WEINBURG SQUARE

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 ?

PROBLEM #1

Got a problem ?

No problem… Step 1…..

Write down all givens

Step #3 How is what you are

being asked, related to what you know ? Then

solve !

Step #2What are you being asked ?

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

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 !

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

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

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

MECHANISMS OF EVOLUTION:Genetic Drift

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

FOUNDER EFFECT

• Founder effect = genetic drift that results when a small number of individuals separate from their original population and find a new population

PATTERNS OF SELECTION• 3 types of selection

pressures

1) Stabilizing Selection=

selection against individuals exhibiting variations in a trait deviate from the current average

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

PATTERNS OF SELECTION

3. Disruptive Selection Pressure= Selection pressure that favours two or more variations or traits that differ from the current average

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

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

ALTRUISM

Altruism = behaviour that decreases the fitness of an individual that is assisting or cooperating with a recipient whose individual fitness is increased

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

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

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

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

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:

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 ?

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

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

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

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

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.

Mountain range is the barrier

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

12