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Ch. 13 Population genetics
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Ch. 13 Population genetics

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  • 1. Population
    Genetics
  • 2. Populations and Variation
    2
    Population
    ... Is a group of the same species, living within a particular geographical area, at a given time.
    Variation exists between members of a population and may be:
  • Structural Variation
    3
    Eg. Length of hair in dogs
  • 7. Biochemical Variation
    4
    Eg. Coat colour in quolls
    Eg. Human ABO blood groups
    Ability to produce enzyme phenylalanine hydroxylase
  • 8. Physiological Variation
    Eg. Red-green colourblindness
    Eg. Ability to taste PTC or other toxins
    Interestingly, brussel sprouts contain a very similar plant tannin and appear to have the same bitter taste to some people.
    5
  • 9. Behavioural Variation
    6
    Eg. Horses: trotters vs pacers
    Eg. What certain dog breeds can be trained to do
    Eg. Domesticable animals
    Eg,
  • 10. Developmental Variation
    7
    Eg. Adult vs juvenile appearance
    Pythons
    Human proportions
  • 11. Variation
    8
    Geographical variation
    Not a way in which species will vary, but often a result of one of the aforementioned types of variation occurring in geographically isolated populations
  • 12. Variations on Variation
    9
    How many variants?
    Monomorphic (only one type, eg. Galahs)
    Polymorphic (more than 1 type)
    Continuous or discontinuous
    Continuous (eg. Height in humans)
    Discontinuous (eg. ABO blood groups)
    B
    O
    AB
    A
    Height in cm
  • 13. Causes of Variation
    10
    Environmental
    Eg. Identical twins looking different
    Eg. Bees: caste determination by food
    Eg. The arrowleaf plant
    Eg. Hydrangeas
    Grown in water
    Grown in soil
    Alkaline soil
    Acidic soil
  • 14. Causes of variation
    11
    Genetic
    Monogenic traits (controlled by one gene)
    Eg. ABO and Rh blood groups
    Eg. Cleft chin, detached ear lobes
    No of alleles and relationship between them determines the number of variations possible
    Polygenic traits (controlled my more than one gene)
    Eg. Height and skin tone in humans
  • 15. Skin tone (simplification)
    12
    Hypothetically controlled by two genes each with two alleles (+ / -).
    (+ = dark, - = light), Incomplete dominance
    How many possible outcomes?
  • 16. Genes in populations
    13
    Gene pool
    All the alleles in a given population
    Allele frequency
    The proportions of each allele for a given gene in a population
    Calculating allele frequency
    Divide number of particular allele by total number of alleles.
    All allelic frequencies must add up to a total of 1.0
  • 17. Calculating Allele Frequency
    Alleles are assigned the letters p and q
    In this population of sheep
    Total no. of alleles is 20
    W = 14, w = 6
    Allele frequency for W (p)
    p = 14/20 = 0.7
    Allele frequency for w (q)
    q = 6/20 = 0.3
    14
  • 18. Calculating Allele Frequency
    We don’t need to be given both p & q
    If only given p or q, we know that p + q = 1.0
    The real world
    Unfortunately we rarely know the actual genotype for most individuals displaying the dominant phenotype
    Calculating expected allele frequency
    We are able to count the number of homozygous recessive individuals and assign them the value q2
    The Hardy-Weinberg formula predicts that √ q2 will provide us with an approximation of q
    15
  • 19. Hardy-Weinberg Equilibrium
    A population in H-W equilibrium will be expected to maintain near-identical allelic frequencies from one generation to the next.
    A population is said to be in H-W equilibrium if:
    The population is large
    Mating is completely random
    All matings are fertile
    The population is closed
    A population will maintain H-W equilibrium unless an agent of change enacts upon it.
    16
  • 20. Agent of change #1 - Selection
    Selective pressure can be as a result of many things
    Competition for food, habitat or mates
    Pressure exerted through predation
    Death or illness do to parasitic organisms or infectious disease
    As a result of these pressures, due to genetic variability, some phenotypes may have a selective advantage
    Greater contribution to next gen = greater fitness
    No phenotype has a set fitness level – depends on circumstances
    17
  • 21. 18
    It would appear that these beetles are at a distinct disadvantage
  • 22. 19
    ... and now?
  • 23. Selection in human populations
    Case study – Malaria and Sickle Cell Anaemia
    Sickle cell anaemia is a debilitating genetic disease that causes the red blood cells to take on a sickle shape that is particularly unconducive to carrying oxygen
    The alleles
    Haemoglobin A is found in normal RBCs
    Haemoglobin S is found in sickle cell RBCs
    The effect
    Malarial parasites can inhabit only non-sickled RBCs
    The HA and HS display incomplete dominance
    20
  • 24. To whom goes the advantage?
    Non-malarial environment
    Most to least successful genotypes
    HAHA – no sickling, plenty of oxygen
    HAHS – some sickling, less oxygen
    HSHS – complete sickling, very little oxygen
    Malarial environment
    Most to least successful genotypes
    HAHS – some sickling, but resistant to malaria
    HSHS – complete sickling, quite debilitating
    HAHA – no sickling, high risk of malaria
    21
  • 25. Natural Selection
    When an environmental agent enacts on a wild population causing differential reproduction
    When one phenotype produces more viable offspring than another
    Agents of natural selection
    Same as the sources of selective pressure
    Results over time
    In the short term can result in one phenotype being more common than another
    Over longer periods can result in phenotypically variant groups becoming so different that they can no longer mate = speciation
    22
  • 26. Natural Selection
    When an environmental agent enacts on a wild population causing differential reproduction
    When one phenotype produces more viable offspring than another
    Agents of natural selection
    Same as the sources of selective pressure
    Results over time
    In the short term can result in one phenotype being more common than another
    Over longer periods can result in phenotypically variant groups becoming so different that they can no longer mate = speciation
    23
  • 27. Artificial Selection
    Individuals are selected for desired traits and used as parents for the following generation
    Often the traits for which these animals have been selected would be disadvantageous in a natural environment.
    Not even going to go there
    Masters of predator evasion
    So what if he can’t breath or smell, he looks so cuuute!
    24
  • 28. Artificial selection
    Further difficulties arise when a species reaches its desired form.
    In the case of crops, this creates a “monoculture” where each individual has the same advantages and disadvantages.
    An example of this going wrong was in the case of the great potato famine in Ireland
    The outbreak of the fungus that causes potato blight decimated the crop of the entire country.
    Over one million people died of starvation
    International seed and sperm banks are being created in an effort to maintain genetic diversity
    25
  • 29. Migration (aka gene flow)
    Capable of changing allele frequencies far more rapidly than selection
    Immigration
    Disproportionate quantity of certain alleles are brought in to a population
    Emmigration
    The departing group do not represent the population as a whole with regard to allelic proportions
    26
  • 30. Human Migration
    The first great migration in hominid history was Homo Erectus’ departure from sub-Saharan Africa approx. 2 million years ago
    The second was H. Sapiens making the same journey approx. 130,000 years ago
    Interesting results of human migration
    People of Celtic ancestry adapted to an environment with far less solar radiation than Australia
    The HS allele is in drastic decline in US Black populations due to lack of selective pressure.
    27
  • 31. Chance events: Genetic Drift
    When a population experiences a calamitous event that decimates the population indiscriminately, the repercussions can be interesting.
    Examples of such events are fires, floods, earthquakes, etc.
    28
  • 32. Bottleneck Effect
    Natural disasters do not favour any particular phenotype
    The resultant reduced population may be unrepresentative of the original population
    A bottleneck essentially eliminates thousands of years of divergent evolution.
    The next generation have very few mating options and as a result the growing population will be genetically very simillar
    Time
    29
  • 33. Founder Effect
    At times members of a population migrate, to another location and become isolated.
    These new populations may not be representative of the population from which they originated.
    eg. On the Antarctic peninsula most macaroni penguins have black faces, a very few have white faces.
    All the macaroni penguins on Macquarie Island have white faces
    30
  • 34. Evolution within a species
    Once there was a population of red circles
    They were a fairly homogenous population but they used to make fun of the “pinkies”
    One day the pinkies got sick of this and left
    31
  • 35. Evolution within a species
    A few generations later
    The pinks met a really nice clan of blues and started having a fling here and there
    Meanwhile, a dark red had some mutant oval offspring
    32
  • 36. Evolution within a species
    A few generations later
    The introduced alleles were producing some varying phenotypes in the formerly pink population
    Meanwhile, skinny was the new black with the reds and the streamline mutants were quite popular
    33
  • 37. Evolution within a species
    A few generations later
    The green offspring’s photosynthetic abilities gave them a great upper hand and they grew big and strong
    Meanwhile, skinny was the new black with the reds and the streamline mutants were quite popular
    34
  • 38. Evolution within a species
    A few generations later
    The most successful greens were the ones with a larger surface area. They could just sit on their ass and photosynthesize all day
    The reds just kept hooking up with skinny chicks
    35
  • 39. Evolution within a species
    A few generations later
    One day members of the divided populations decided to check out what sort of action they could get from across the river
    Apart from the fact that they found each other incredibly
    Unattractive, their bits didn’t even match any more!
    36
  • 40. Species vs Sub-species
    Two populations that are isolated are often exposed to different agents of change
    They may stay biologically compatible for thousands of years, but will not be attracted to each other. They are now different sub-species
    Speciation only occurs once the two populations become reproductively isolated (can no longer produce viable offspring).
    37
  • 41. mtDNA
    Is only maternally inherited
    Therefore does not recombine
    Each cell contains hundreds of copies
    Some regions have a high mutation rate
    Can be used to trace evolutionary origins
    38
  • 42. mtDNA
    The longer two populations are geographically isolated, the more uniquedifferences they accumulate.
    mtDNA sequence only found in certain populations are known as haplogroups
    Haplogroups are compared against the originally sequenced Cambridge Reference Sequence (CRS)
    Haplogroups can be traced back to their point of origin
    39
  • 43. Origin and movement of haplogroups
    40
  • 44. Homo neanderthalensis
    In 1997 it was confirmed via mtDNA that neanderthals were a separate species to modern humans
    In a sequence of mtDNA 397 base pairs long, there were 27 differences.
    This is in contrast to the average of 8 differences between human populations
    41