Popula-tion

Genetics

Populations and VariationPopulations and Variation

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• 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:StructuralBiochemicalPhysiological

DevelopmentalBehavioural

Structural VariationStructural Variation

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• Eg. Length of hair in dogs

Biochemical VariationBiochemical Variation

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• Eg. Coat colour in quolls• Eg. Human ABO blood groups• Ability to produce enzyme

phenylalanine hydroxylase

Physiological VariationPhysiological 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.

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Behavioural VariationBehavioural Variation

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• Eg. Horses: trotters vs pacers• Eg. What certain dog breeds can be

trained to do• Eg. Domesticable animals• Eg,

Developmental VariationDevelopmental Variation

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• Eg. Adult vs juvenile appearance– Pythons– Human proportions

VariationVariation

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• Geographical variation

– Not a way in which species will vary, but often a result of one of the aforemen-tioned types of variation occurring in geographically isolated populations

Variations on VariationVariations on Variation

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• 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 ABA Height in cm

Causes of VariationCauses of Variation

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• Environmental– Eg. Identical twins looking different– Eg. Bees: caste determination by food– Eg. The arrowleaf plant– Eg. Hydrangeas

Acidic soil

Alkaline soil

Grown in soilGrown in water

Causes of variationCauses of variation

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• 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

Skin tone (simplification)Skin tone (simplification)

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• Hypothetically controlled by two genes each with two alleles (+ / -).– (+ = dark, - = light), Incomplete domin-

ance– How many possible outcomes?

Genes in populationsGenes in populations

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• 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

Calculating Allele FrequencyCalculating Allele Frequency

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• 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

Calculating Allele FrequencyCalculating Allele Frequency

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• 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

Hardy-Weinberg EquilibriumHardy-Weinberg Equilibrium

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• 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.

Agent of change #1 - Selec-tion

Agent of change #1 - Selec-tion

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• 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

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It would appear that these beetles are at a distinct disadvantage

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... and now?

Selection in human popula-tions

Selection in human popula-tions

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• 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

To whom goes the advant-age?

To whom goes the advant-age?

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• 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

Natural SelectionNatural Selection

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• 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

Natural SelectionNatural Selection

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• 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

Artificial SelectionArtificial Selection

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• 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.So what if he

can’t breath or smell, he looks so cuuute!

Masters of predator evasion

Not even going to go

there

Artificial selectionArtificial selection

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• 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

Migration (aka gene flow)Migration (aka gene flow)

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• 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

Human MigrationHuman Migration

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• 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.

Chance events: Genetic DriftChance events: Genetic Drift

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• 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.

Bottleneck EffectBottleneck Effect

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• 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

Founder EffectFounder Effect

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• 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

Evolution within a speciesEvolution within a species

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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

Evolution within a speciesEvolution within a species

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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

Evolution within a speciesEvolution within a species

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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

Evolution within a speciesEvolution within a species

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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

Evolution within a speciesEvolution within a species

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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

Evolution within a speciesEvolution within a species

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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 incrediblyUnattractive, their bits didn’t even match any more!

Species vs Sub-speciesSpecies vs Sub-species

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• 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).

mtDNAmtDNA

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• 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

mtDNAmtDNA

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• The longer two populations are geographically isolated, the more unique differences 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

Origin and movement of haplogroups

Origin and movement of haplogroups

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Homo neanderthalensisHomo neanderthalensis

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• 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