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Cellular Control

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Title: Cellular Control


1
Cellular Control
  • Unit 1
  • Communication, Homeostasis and Energy

2
Meiosis
  • Module 1 Cellular Control

3
Learning outcomes
  • describe, with the aid of diagrams and
    photographs,
  • the behaviour of chromosomes during meiosis,
  • the associated behaviour of the nuclear envelope,
    cell membrane and centrioles.
  • (Names of the main stages are expected, but not
    the subdivisions of prophase)

4
Reproduction and variation
  • Asexual reproduction
  • Single organism divides by mitosis
  • New organism is genetically identical to the
    parent
  • Sexual reproduction
  • Meiosis produces haploid gametes
  • Which fuse at fertilisation to form a diploid
    zygote
  • This produces genetic variation amongst offspring

5
Human Life Cycle
Diploid Zygote 46
Mitosis
Haploid Sperm 23
Meiosis
Adult 46
Haploid Egg 23
fertilisation
6
Self assessment questions
  • The fruit fly Drosophila melangaster has eight
    chromosomes in its body cells. How many
    chromosomes will there be in a Drosophila sperm?
  • The symbol n is used to indicate the number of
    chromosomes in one set the haploid number of
    chromosomes. For example in humans n 23, in a
    horse n 32.
  • How many chromosomes are there in a gamete of a
    horse?
  • What is the diploid number of chromosomes (2n) of
    a horse?

7
Meiosis
  • Meiosis is a reduction division
  • Resulting daughter cells have half the original
    number of chromosomes
  • Daughter cells are haploid
  • Can be used for sexual reproduction
  • Source of genetic variation
  • Meiosis has two divisions
  • meiosis I and meiosis II
  • Each division has 4 stages
  • Prophase, metaphase, anaphase, telophase

8
Meiosis
  • You can view an animation of Meiosis at
    http//www.cellsalive.com/meiosis.htm

9
Meiosis I
Prophase I Chromatin condenses Homologous pairs form a bivalent Nucleolus disappears Spindle forms
Metaphase I Bivalents line up on equator of cell
Anaphase I Homologous chromosome in each bivalent are pulled to opposite poles
Telophase I Two new nuclear envelopes form Cell divides by cytokinesis
10
Early Prophase 1
11
Late Prophase 1
12
Metaphase 1
13
Anaphase 1
14
Telophase 1
15
Cytokinesis 1
16
Meiosis II
Prophase II Nucleolus disappears Chromosomes condense Spindle forms
Metaphase II Chromosomes arrange themselves on equator Attach by centromere to spindle fibres
Anaphase II Centromeres divide Chromatids pulled apart to opposite poles
Telophase II nuclear envelopes reform around haploid nuclei Cell divides by cytokinesis
17
Prophase II
18
Metaphase II
19
Anaphase II
20
Telophase II
21
Cytokinesis II
22
Learning outcomes
  • explain how meiosis and fertilisation can lead to
    variation through the independent assortment of
    alleles

23
Key words
  • Allele
  • Locus
  • Crossing over
  • Maternal chromosome
  • Paternal chromosome

24
Alleles, locus and homologous chromosomes
25
Meiosis and variation
  • Meiosis enables sexual reproduction to occur by
    the production of haploid gametes.
  • Sexual reproduction increases genetic variation
  • Genetic variation increases the chances of
    evolution through natural selection

26
Meiosis and Variation
  • Crossing over prophase I
  • Independent assortment of chromosomes metaphase
    I
  • Random assortment of chromatids metaphase II
  • Random fertilisation
  • Chromosome mutations
  • Number of chromosomes
  • Non-disjunction - polysomy or polyploidy
  • Structure of chromosomes
  • Inversion, deletion, translocation

27
Crossing over
28
During metaphase I
29
During metaphase I
30
No crossing over
31
Crossing over new combinations of alleles
32
Independent Assortment
33
(No Transcript)
34
Learning Outcomes
  • explain the terms allele, locus, phenotype,
    genotype, dominant, codominant and recessive
  • explain the terms linkage and crossing-over

35
Glossary
  • Gene
  • Locus
  • Allele
  • Genotype
  • Phenotype
  • Heterozygous
  • Homozygous
  • Monohybrid cross
  • Dominant allele
  • Recessive allele

36
Genetics
  • Genetics is the study of inheritance
  • Allele
  • different varieties of the same gene
  • Locus
  • position of a gene on a chromosome

37
Genetics
  • Dominant
  • An allele whose effect is expressed in the
    phenotype if one copy present
  • Recessive
  • An allele which only expresses as a homozygote
  • Co-dominant
  • Both alleles have an effect on the phenotype

38
  • Genotype
  • genetic constitution of the organism
  • Phenotype
  • appearance of character resulting from inherited
    information

39
  • Homozygous
  • Individual is true breeding
  • Possesses two alleles of a gene e.g. RR or rr
  • Heterozygous
  • Two different alleles for a gene e.g. Rr

40
Monohybrid inheritance
  • Mendels First Law
  • principle of segregation
  • The alleles of a gene exist in pairs but when
    gametes are formed, the members of each pair pass
    into different gametes, thus each gamete contains
    only one of each allele.

41
Inheritance of height in pea plants
gene Allele relationship Symbol
Height of pea plants Tall Dominant T
Height of pea plants dwarf recessive t
  • Follow out the following cross to the F2
    generation
  • Homozygous tall pea plant with a homozygous dwarf
    pea plant
  • Write out the genotypic and phenotypic ratios
    from the F2 generation

42
Inheritance of height in pea plants
  • Laying out the cross
  • P phenotype
  • P genotype
  • Gametes
  • F1 genotype
  • F1 phenotype
  • F1 self-fertilised
  • Gametes
  • Random fertilisation
  • F2 genotypic ratio
  • F2 phenotypic ratio

43
Pupil Activity
  • Answer the questions on monohybrid inheritance
  • Remember to write out each cross in full.

44
Cystic Fibrosis
  • Cystic Fibrosis is caused by a mutation to a gene
    on one of the autosomes.
  • Mutation
  • Changes the shape of the transmembrane chloride
    ion channels (CFTR protein)
  • The CFTR gene is found on Chromosome 7
  • The faulty gene is recessive

45
Genetic Cross conventions
  • Use symbols to represent two alleles
  • Alleles of the same gene should be given the same
    letter
  • Capital letter represents the dominant allele
  • Small letter represents the recessive allele
  • Choose letters where the capital and small letter
    look different
  • The examiner needs to be in no doubt about what
    you have written

46
Inheritance of cystic fibrosis
  • Three possible genotypes
  • FF unaffected
  • Ff unaffected
  • ff cystic fibrosis
  • Remember gametes can only contain one allele for
    the CFTR gene
  • At fertilisation, any gamete from the father can
    fertilise any gamete from the mother
  • This can be shown in a genetic diagram

47
Genetic diagram showing the chances of a
heterozygous man and a heterozygous woman having
a child with cystic fibrosis.
48
Phenotype ratio of offspring
  • Genotype ratio 1FF2Ff1ff
  • Phenotype ratio 3 unaffected1cystic fibrosis
  • Can also be expressed as
  • 25 chance of the child having cystic fibrosis
  • Probability of 0.25 that a child will inherit the
    disease
  • Probability that 1 in 4 that a child from these
    parents will have this disease.

49
Learning Outcome
  • Use genetic diagrams to solve problems involving
    sex-linkage and codominance.

50
Sex-Linkage
  • Sex-linked genes are genes whose loci are on the
    X or Y chromosomes
  • The sex chromosomes are not homologous, as many
    genes present on the X are not present on the Y.
  • Examples
  • Haemophilia
  • Fragile X syndrome
  • Red green colour blindness

51
Sex Chromosomes
52
Factor VIII and Haemophilia
  • Haemophilia is caused by a recessive allele of a
    gene that codes for a faulty version of the
    protein factor VIII
  • XH normal allele
  • Xh haemophilia allele

53
possible genotypes and phenotypes
54
Inheritance of Haemophilia
55
Pedigree for a sex linked recessive disease
56
Codominance
  • Codominance describes a pair of alleles, neither
    of which is dominant over the other.
  • This means both have an effect on the phenotype
    when present together in the genotype

57
Codominance example
  • Flower colour in plants
  • CR red
  • Cw white
  • Genotypes
  • CRCR red flowers
  • CRCW pink flowers
  • CWCW white flowers
  • Write out a genetic cross between a pure breeding
    red plant and a pure breeding white plant.
  • Carry out the cross to the F2 generation.
  • Write out the genotype and phenotype ratio for
    the F2 generation

58
Revision Question
  • Coat colour in Galloway cattle is controlled by a
    gene with two alleles. The CR allele produces red
    hairs and therefore a red coat colour. The Cw
    allele produces white hairs.
  • A farmer crossed a true-breeding, red-coated cow
    with a true-breeding white-coated bull. The calf
    produced had roan coat colouring (made up of an
    equal number of red and white hairs).
  • Explain the result and draw a genetic diagram to
    predict the outcome of crossing two roan coloured
    animals.

59
Inheritance of A, B, AB and O blood groups
  • Human blood groups give an example of codominance
    and multiple alleles
  • There are 3 alleles present
  • IA
  • IB
  • Io

60
  • IA and IB are codominant
  • Io is recessive
  • Remember each human will only have two alleles

61
Blood Groups
Genotype Phenotype
IAIA Blood Group A
IA Io Blood Group A
IAIB Blood Group AB
IBIB Blood Group B
IB Io Blood Group B
Io Io Blood Group o
62
Inheritance of blood groups
  • Carry out genetic crosses for the following
    examples
  • Two parents have blood groups A and B, the father
    is IAIo and the mother is IBIo
  • Father has blood group AB and the mother has
    blood group O
  • Mother is homozygous blood group A and the father
    is heterozygous B.

63
Learning Outcome
  • Describe the interactions between loci
    (epistasis).
  • Predict phenotypic ratios in problems involving
    epistasis.

64
Dihybrid Inheritance
  • Monohybrid cross
  • Inheritance of one gene
  • Dihybrid cross
  • Inheritance of two genes

65
Example dihybrid cross
  • Tomato plants
  • Stem colour
  • A purple stem a green stem
  • Leaf shape
  • D cut leaves d potato leaves
  • NOTE
  • In the heterozygote AaDd due to independent
    assortment in meiosis there are 4 possible gamete
    combinations
  • AD Ad aD ad

66
Crosses
  • Cross a heterozygous plant with a plant with a
    green stem and potato leaves
  • Cross two heterozygous tomato plants

67
Dihybrid Inheritance
  • A woman with cystic fibrosis has blood group A
    (genotype IAIo). Her partner does not have
    cystic fibrosis and is not a carrier for it. He
    has blood group O.
  • Write down the genotypes of these two people.
  • With the help of a full and correctly laid out
    genetic diagram, determine the possible genotypes
    and phenotypes of any children that they may have.

68
Autosomal linkage
  • Each Chromosome carries a large number of linked
    genes
  • If two genes are on the same chromosome then
    independent assortment can not take place.
  • The genes are transmitted together and are said
    to be linked.

69
Linked Genes
  • Where linked genes are involved the offspring of
    a dihybrid cross will result in a 31 ratio
    instead of the 9331 ratio.
  • Example
  • In peas, the genes for plant height and seed
    colour are on the same chromosome (i.e. linked)

70
Learning Outcome
  • Describe the interactions between loci
    (epistasis).
  • Predict phenotypic ratios in problems involving
    epistasis.

71
Flower colour in sweet pea
  • Flower colour
  • Colourless precursor of a pigment C
  • Gene that controls conversion of this pigment to
    purple P
  • Both dominant alleles need to be present for the
    purple colour to develop
  • Cross
  • Cross two white flowered plants with the
    genotypes CCpp and ccPP
  • Follow this cross through to the F2 generation

72
Interactions of unlinked genes
  • A single character maybe influenced by two or
    more unlinked genes.
  • E.g. determination of comb shape in domestic
    poultry
  • Dominant allele P pea comb
  • Dominant allele R rose comb
  • Two dominant alleles walnut comb
  • No dominant alleles single comb

73
Genetic Crosses
  • Carry out a genetic cross between a true-breeding
    pea comb and a true breeding rose comb
  • Follow this cross through to the F2 generation

74
Inheritance of coat colour in mice
  • Wild mice have a coat colour that is referred to
    as agouti.
  • Agouti (A) is dominant to black (a)
  • C is a dominant gene required for coat colour to
    develop
  • A homozygous recessive cc means that no pigment
    can be formed and the individual is albino

75
Inheritance of coat colour in mice
  • Carry out a cross between a pure-breeding black
    mouse (aaCC) and an albino (AAcc)
  • Follow this cross through to the F2 generation.

76
Epistasis
  • This is the interaction of different gene loci so
    that one gene locus masks or suppresses the
    expression of another gene locus.
  • Genes can
  • Work antagonistically resulting in masking
  • Work complementary

77
Epistasis ratios
  • 9 3 4 ratio
  • Suggests recessive epistasis
  • 9 7 ratio
  • Suggests epistasis by complementary action
  • 12 3 1 ratio or 13 3 ratio
  • Suggests dominant epistasis

78
Predicting phenotypic ratios
  • Read through pages 132 and 133 of your textbook
  • Answer questions 1 7
  • Complete the stretch and challenge question on
    eye colour in humans
  • Read through and complete the worksheet provided
    for you on epistasis

79
Learning outcome
  • Use the chi-squared (?2) test to test the
    significance of the difference between observed
    and expected results.

80
?2 (chi-squared) test
  • Allows us to compare observed and expected
    results and decide if there is a significant
    difference between them.

81
?2 (chi-squared) test
  • Where
  • S the sum of
  • O observed value
  • E expected value

82
?2 (chi-squared) test
  • Compare the ?2 value to a table of probabilities
  • The probability that the differences between our
    expected and observed values are due to chance.
  • If the ?2 value represents a probability of 0.05
    or larger, the differences are not significant
  • If the ?2 value represents a probability of less
    than 0.05, it is likely that the results are not
    due to chance and there is a significant
    difference.

83
Degrees of freedom
  • The degrees of freedom takes into account the
    number of comparisons made.
  • Degrees of freedom
  • number of classes of data - 1

84
Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Critical value 95 certain that the results are
not due to chance
85
Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Accept null hypothesis There is no significant
difference, results have occurred due to chance
86
Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Reject null hypothesis accept experimental
hypothesis Difference is significant, not due to
chance
87
Mammal question
  • ?2 value 51.8
  • Degrees of freedom 3
  • Critical value (p0.05) 7.82
  • Reject the null hypothesis
  • There is a significant difference between
    observed and expected results
  • Suggestions?
  • The two genes are linked

88
Variation
  • What did you learn at AS level?

89
Learning Outcomes
  • Define the term variation.
  • Discuss the fact that variation occurs within as
    well as between species.
  • Describe the differences between continuous and
    discontinuous variation, using examples of a
    range of characteristics found in plants, animals
    and microorganisms.
  • Explain both genetic and environmental causes of
    variation.

90
Variation
  • Variation is the differences that exist between
    individual organisms.
  • Interspecific variation (between species)
  • Differences that are used to assign individuals
    to different species
  • Intraspecific variation (within a species)
  • Individuals of the same species show variation
  • Variation can be inherited or influenced by the
    environment.

91
Types of variation
  • There are two main types of variation
  • Continuous variation
  • Discontinuous variation
  • There are two main causes of variation
  • Genetic variation
  • Environmental variation

92
Continuous variation
  • Existence of a range of types between two
    extremes
  • Most individuals are close to a mean value
  • Low numbers of individuals at the extremes
  • Both genes and the environment interact in
    controlling the features
  • Examples
  • Height in humans
  • Length of leaves on a bay tree
  • Length of stalk of a toad stool

93
Continuous variation
  • Use a tally chart and plot results in a histogram

94
Discontinuous variation
  • 2 or more distinct categories with no
    intermediate values
  • Examples
  • Earlobes attached or unattached
  • Blood groups A, B, AB or o
  • Bacteria flagella or no flagella
  • Flowers colour of petals
  • Genetically determined
  • The environment has little or no effect on
    discontinuous variation

95
Discontinuous variation
96
Causes of variation
  • Genetic Variation
  • Genes inherited from parents provide information
    used to define our characteristics
  • Environmental Variation
  • Gives differences in phenotype (appearance) but
    not passed on by parents to offspring
  • Examples
  • Skin colour tans with exposure to sunlight
  • Plant height determined by where the seed lands

97
Variation
  • What you need to know for A2!!

98
Learning outcomes
  • Describe the differences between continuous and
    discontinuous variation.
  • Explain the basis of continuous and discontinuous
    variation by reference to the number of genes
    which influence the variation.
  • Explain that both genotype and environment
    contribute to phenotypic variation.
  • Explain why variation is essential in selection.

99
variation
  • Variation can be
  • Discontinuous
  • Each organism falls into one of a few clear-cut
    categories, no intermediate values
  • Qualitative differences between phenotypes
  • Continuous
  • No definite categories
  • A continuous range of values between two extremes
  • Quantitative differences between phenotypes

100
Genes and variation
  • Discontinuous (qualitative) variation
  • Monogenic inheritance
  • Different alleles at same gene locus
  • Different gene loci have different effects
  • Epistasis, codominance, dominance and recessive
    patterns of inheritance

101
Genes and Variation
  • Continuous (quantitative) variation
  • Polygenic inheritance
  • Two or more genes
  • Each gene has an additive effect
  • Unlinked genes

102
Polygenic Inheritance
  • Example length of corn cobs
  • Three genes A/a, B/b and C/c
  • Each dominant allele adds 2 cm length
  • Each recessive allele adds 1 cm length
  • So
  • AABBCC 12 cm long
  • aabbcc 6 cm long
  • Hmmm!!
  • How long would AaBBCc be?
  • How long would aaBbCc be?

103
Genotype, environment and phenotype
  • The environment can affect the expression of the
    genotype
  • examples
  • AABBCC has the genetic potential to produce cobs
    12cm long
  • This could be affected by
  • Lack of water, light or minerals
  • Obesity in humans
  • Affected by diet and exercise

104
Genotype, environment and phenotype
  • The environment influences the expression of
    polygenic traits more than monogenic traits.

105
Learning Outcomes
  • Use the HardyWeinberg principle to calculate
    allele frequencies in populations.

106
Population genetics
  • What is a population?
  • Group of individuals of the same species that can
    interbreed
  • Populations are dynamic
  • The set of genetic information carried by a
    population is the gene pool.

107
Allele Frequency
  • To measure the frequency of an allele you need to
    know
  • Mechanism of inheritance of that trait
  • How many different alleles of the gene for that
    trait are in the population

108
Hardy-Weinberg principle
  • The Hardy-Weinberg principle is a fundamental
    concept of population genetics
  • It makes the following assumptions
  • Population is very large
  • Random mating
  • No selective advantage
  • No mutation, migration or genetic drift

109
The equations
  • p frequency of the dominant allele
  • q frequency of the recessive allele
  • The frequency of the allele will be in the range
    0 1.
  • 0 no one has the allele
  • 0.5 half the population has the allele
  • 1 only allele for that gene in the population

110
Ok the equations
  • Equation 1
  • p q 1
  • Equation 2
  • p2 2pq q2 1
  • Where
  • p2 frequency of genotype DD
  • 2pq frequency of genotype Dd
  • q2 frequency of genotype dd

111
Calculating the frequency of cystic fibrosis in
the population
  • 1 in 3300 babies are born with cystic fibrosis
  • All babies with cystic fibrosis have genotype nn
  • Calculate q2
  • Calculate q
  • Calculate p
  • Calculate frequency of genotype Nn
  • If we have 30,000 people in our population how
    many will be carriers of the cystic fibrosis
    allele

112
Question
  • Phenylketonuria, PKU, is a genetic disease caused
    by a recessive allele. About one in 15 000
    people in a population are born with PKU.
  • Use the hardy-Weinberg equations to calculate the
    frequency of the PKU allele in the population.
  • State the meaning of the symbols that you use,
    and show all your working.

113
The Answer
  • Calculate q2 1 / 15000 0.000067
  • Calculate q 0.0082

114
Another question
  • Explain why the Hardy-Weinberg principle does not
    need to be used to calculate the frequency of
    codominant alleles.

115
Pupil Activity
  • Answer the Hardy-Weinberg practice question.
  • You have 10 minutes
  • Starting NOW!!

116
The Answers
  • q2 0.52 / q 0.72
  • p 1 0.72 0.28
  • p q 1 p2 2pq q2 1
  • Answer 2pq / use of appropriate numbers
  • Answer 40

117
The other answers
  • Any three from
  • Small founder population / common ancestor
  • Genetic isolation / small gene pool / no
    immigration /
  • no migration / in-breeding
  • High probability of mating with person having
    H-allele
  • Reproduction occurs before symptoms of disease
    are apparent
  • Genetic argument Hh x hh 50 / Hh x Hh 75
    affected offspring
  • No survival / selective disadvantage

118
Learning Outcomes
  • Explain, with examples, how environmental factors
    can act as stabilising or evolutionary forces of
    natural selection.
  • Explain how genetic drift can cause large changes
    in small populations.

119
Variation and Natural Selection
  • The set of alleles in a population is its gene
    pool
  • Each individual can have any combination of
    alleles in the gene pool
  • producing variation
  • Some individuals more likely to survive
  • They reproduce and pass genes on to offspring
  • Advantageous alleles become more frequent in the
    population

120
Environmental Resistance
  • Environmental factors that limit the growth of a
    population offer environmental resistance
  • These factors can be biotic or abiotic

121
Selection pressures
  • An environmental factor that selects for some
    members of a population over others
  • Confers an advantage onto certain individuals

122
Stabilising Selection
  • If the environment stays stable
  • The same alleles will be selected for in
    successive generations
  • Nothing changes, this is called stabilising
    selection

123
Stabilising Selection
124
Stabilising Selection
125
Directional Selection
  • Change in the environment resulting in a change
    in the selection pressures on the population
  • Previously disadvantageous alleles maybe selected
    for
  • Change in the genetically determined
    characteristics of subsequent generations of the
    species
  • A.k.a. evolution

126
Directional Selection
127
Directional Selection
128
Genetic Drift
  • A change in the gene pool and characteristics
    within the population.
  • This change has occurred by chance rather than as
    the result of natural selection.

129
Genetic Drift and Islands
  • Genetic drift is thought to happen relatively
    frequently in populations on islands.
  • Small populations
  • Geographically separated from other members of
    their species
  • Evidence
  • Many isolated islands have their own endemic
    species of plants and animals

130
Genetic Drift
  • Reduces genetic variation
  • Reduce the ability of the population to survive
    in a new environment
  • May contribute to the extinction of a population
    or species
  • Could lead to the production of a new species

131
Genetic Drift Frog Hoppers
  • The colours of the common frog-hopper are
    determined by seven different alleles of a single
    gene.
  • The range of colours and their frequencies, on
    different islands in the Isles of Scilly, are
    very variable,
  • There are different selection pressures on the
    different islands

132
Genetic Drift Frog Hoppers
133
The answers
134
Learning Outcomes
  • Explain the role of isolating mechanisms in the
    evolution of new species, with reference to
    ecological (geographic), seasonal (temporal) and
    reproductive mechanisms.

135
Speciation
  • Speciation is the formation of a new species.
  • Species
  • Group of organisms, with similar morphology and
    physiology, which can interbreed with one another
    to produce fertile offspring.

136
Speciation
  • In the production of a new species, some
    individuals must
  • Becomes morphologically or physiologically
    different from members of the original species
  • No longer be able to breed with the members of
    the original species to produce fertile offspring.

137
Isolation
  • Splitting apart of a splinter group
  • Geographical isolation
  • Organisms are separated by a physical barrier
  • Reproductive isolation
  • Two groups have become so different that they are
    no longer able to interbreed
  • They are now a different species

138
Isolating Mechanisms
  • Large populations may be split into sub-groups by
  • Geographic barriers
  • Ecological barriers
  • Temporal barriers
  • Reproductive barriers

139
Geographical Barriers (AS recap)
  • Geographical barrier separates two populations of
    a species
  • Two groups evolve along different lines
  • Different selection pressures
  • Genetic drift
  • If barrier breaks down and two populations come
    together again, they may have changed so much
    that they can no longer interbreed
  • They are now two different species

140
Isolating Mechanisms
  • Speciation occurs when organisms live in the same
    place
  • The barriers which can prevent two closely
    related species from interbreeding include
  • Ecological
  • Temporal
  • Reproductive

141
Ecological Barriers
  • Ecological barriers exist where two species live
    in the same area at the same time, but rarely
    meet.
  • Example
  • Two different species of crayfish, Orconectes
    virilis and orconectes immunis, both live in
    freshwater habitats in North America

142
Meet the Crayfish
  • Orconectes virilis
  • Not good at digging, cant survive summer drying
  • Lives in streams and lake margins
  • Orconectes immunis
  • Lives in ponds and swamps,
  • Can easily burrow into the mud when the pond
    dries up
  • In streams and lake margins O. virillis is more
    aggressive and will drive O. immunis out of
    crevices where it tries to shelter

143
Temporal Barriers
  • Two species live in the same place, and may even
    share the same habitat
  • Do not interbreed as they are active at different
    times of the day, or reproduce at different times
    of year
  • Example flowering shrubs in Western Australia

144
Meet the shrubs
  • Banksia attenuata flowers in the summer
  • Banksia menziesii flowers in the winter
  • They can not interbreed

145
Reproductive barriers
  • Even if species share the same habitat and are
    reproductively active at the same time, they may
    not be able to interbreed
  • Different courtship behaviours
  • Mechanical problems with mating
  • Gamete incompatibility
  • Zygote inviability
  • Hybrid sterility

146
Meet the Mallards
  • Different courtship behaviours
  • A male mallard duck will only mate with a female
    who displays the correct courtship behaviour
  • Although the pintail female looks similar to the
    Mallard female, her courtship behaviour will only
    attract a pintail male.

147
Learning Outcomes
  • Explain the significance of the various concepts
    of the species, with reference to the biological
    species concept and the phylogenetic
    (cladistic/evolutionary) species concept.

148
The Species Concept
  • In AS biology you defined a species as
  • a group of organisms, with similar
    morphological, physiological, biochemical and
    behavioural features, which can interbreed to
    produce fertile offspring, and are reproductively
    isolated from other species

149
The two species concept
  • Group of organisms
  • Capable of interbreeding
  • Capable of producing fertile offspring
  • Reproductively isolated from other groups
  • This is the Biological Species concept
  • Group of organisms showing similarities in
    characteristics
  • Morphological
  • Physiological
  • biochemical
  • Ecological
  • Behavioural
  • This is the phylogenetic species concept

150
Biological Species concept
  • Group of organisms that can interbreed and
    produce fertile offspring.
  • Clear cut definition
  • Limitation
  • Can only be used for organisms that reproduce
    sexually

151
Phylogenetic species concept
  • Also known as the
  • Evolutionary species concept
  • Cladistic species concept
  • Different morphology between the two groups and
    certain that they evolved from a common ancestor
  • Not rigorous but allows decisions to be made

152
Comparing the genetics
  • Closely related organisms have similar molecular
    structures for DNA, RNA and proteins.
  • Biologists can compare specific base sequences
    (haplotypes)
  • The number of differences caused by base
    substitutions can be expressed as the divergence

153
Cladistics
  • Clade
  • Group of organisms with similar haplotypes
  • In cladistic classification systems is assumes
    that the taxa are monophyletic, this means that
    it includes an ancestral organism and all its
    descendents.

154
Cladistic classification
  • Focuses on evolution
  • Places importance on using molecular analysis
  • Uses DNA and RNA sequencing
  • Uses computer programmes
  • Makes no distinction between extinct and still
    existing species

155
Learning Outcomes
  • Compare and contrast natural selection and
    artificial selection.
  • Describe how artificial selection has been used
    to produce the modern dairy cow and to produce
    bread wheat (Triticum aestivum).

156
Selection
  • Natural Selection
  • Mechanism for evolution
  • Organisms best adapted to their environments more
    likely to survive to reproductive age
  • Favourable characteristics are passed on
  • Produces organisms that are well adapted to their
    environment

157
Artificial Selection
  • Humans select the favourable characteristics
  • Humans allow those organisms to breed
  • Produces populations that show one characteristic
    to an extreme
  • Other characteristics retained may be
    disadvantageous

158
Artificial Selection and the modern dairy cow
  • Breeds of cows with higher milk production have
    been artificially selected for
  • Milk yield from each cow is measured and recorded
  • Test progeny of bulls
  • Elite cows given hormones to produce many eggs
  • Eggs fertilised in vitro
  • Embryos implanted into surrogate mothers
  • A few elite cows produce more offspring than they
    would naturally

159
Disadvantage to high milk yields
  • Health costs for artificially selected cows is
    higher due to
  • Mastitis
  • Ketosis and milk fever
  • Lameness
  • Respiratory problems

160
Artificial selection and bread wheat (Triticum
aestivum)
  • Polyploidy
  • Nuclei contain more than one diploid set of
    chromosomes
  • Wild species of wheat have a diploid number (2n)
    of 14
  • Modern bread wheat is hexaploid (6n), It has 42
    chromosomes in the nucleus of every cell

161
Getting from the ancestors to modern bread wheat
Wild einkorn AUAU 2n 14
Domestication and artificial selection
Wild Grass BB 2n 14
Einkorn AUAU 2n 14
x
162
Wild Grass BB 2n 14
Einkorn AUAU 2n 14
x
Sterile hybrid P AuB
Mutation that double chromosome number
Wild Grass DD 2n 14
Emmer Wheat AUAUBB 4n 28
x
163
Wild Grass DD 2n 14
Emmer Wheat AUAUBB 4n 28
x
Sterile hybrid Q AuBD
Mutation that double chromosome number
Common Wheat AUAUBBDD 6n 42
164
Continuing selection in wheat
  • Breeders are continuing to try and improve wheat
    varieties
  • Resistance to fungal infections
  • High protein content
  • Straw stiffness
  • Resistance to lodging
  • Increased yield
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