Population Genetics

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Population Genetics
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The Gene Pool

• Members of a species can interbreed produce
  fertile offspring
• Species have a shared gene pool
• Gene pool all of the alleles of all individuals
  in a population

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The Gene Pool

• Different species do NOT exchange genes by
  interbreeding
• Different species that interbreed often produce
  sterile or less viable offspring e.g. Mule

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Populations

• A group of the same species living in an area
• No two individuals are exactly alike (variations)
• More Fit individuals survive pass on their
  traits

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Speciation

• Formation of new species
• One species may split into 2 or more species
• A species may evolve into a new species
• Requires very long periods of time

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Modern Evolutionary Thought
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Modern Synthesis Theory

• Combines Darwinian selection and Mendelian
  inheritance
• Population genetics - study of genetic variation
  within a population
• Emphasis on quantitative characters (height, size
  )

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Modern Synthesis Theory

• 1940s comprehensive theory of evolution (Modern
  Synthesis Theory)
• Introduced by Fisher Wright
• Until then, many did not accept that Darwins
  theory of natural selection could drive evolution

S. Wright
A. Fisher
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Modern Synthesis Theory

• TODAYS theory on evolution
• Recognizes that GENES are responsible for the
  inheritance of characteristics
• Recognizes that POPULATIONS, not individuals,
  evolve due to natural selection genetic drift
• Recognizes that SPECIATION usually is due to the
  gradual accumulation of small genetic changes

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Microevolution

• Changes occur in gene pools due to mutation,
  natural selection, genetic drift, etc.
• Gene pool changes cause more VARIATION in
  individuals in the population
• This process is called MICROEVOLUTION
• Example Bacteria becoming unaffected by
  antibiotics (resistant)

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Hardy-Weinberg Principle
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The Hardy-Weinberg Principle

• Used to describe a non-evolving population.
• Shuffling of alleles by meiosis and random
  fertilization have no effect on the overall gene
  pool. 
•  Natural populations are NOT expected to actually
  be in Hardy-Weinberg equilibrium.

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The Hardy-Weinberg Principle

• Deviation from Hardy-Weinberg equilibrium usually
  results in evolution
• Understanding a non-evolving population, helps us
  to understand how evolution occurs

•                        
• .

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5 Assumptions of the H-W Principle

1. Large population size - small populations have
   fluctuations in allele frequencies (e.g., fire,
   storm).
2. No migration- immigrants can change the
   frequency of an allele by bringing in new alleles
   to a population.
3. No net mutations- if alleles change from one to
   another, this will change the frequency of those
   alleles

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5 Assumptions of the H-W Principle

1. Random mating- if certain traits are more
   desirable, then individuals with those traits
   will be selected and this will not allow for
   random mixing of alleles.
2. No natural selection- if some individuals
   survive and reproduce at a higher rate than
   others, then their offspring will carry those
   genes and the frequency will change for the next
   generation.

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Traits Selected for Random Mating
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The Hardy-Weinberg Principle

• The gene pool of a NON-EVOLVING population
  remains CONSTANT over multiple generations
  (allele frequency doesnt change)
•  
• The Hardy-Weinberg Equation 
•                1.0 p2 2pq q2
•  Where
• p2 frequency of AA genotype
• 2pq frequency of Aa
• q2 frequency of aa genotype

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The Hardy-Weinberg Principle

• Determining the Allele Frequency using
  Hardy-Weinberg 
•               
• 1.0 p q
•  Where
• p frequency of A allele
• q frequency of a allele

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Allele Frequencies Define Gene Pools
500 flowering plants
480 red flowers
20 white flowers
320 RR
160 Rr
20 rr
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and
females) 320 x 2 (RR) 160 x 1 (Rr) 800 R
800/1000 0.8 (80) R 160 x 1 (Rr) 20 x 2 (rr)
200 r 200/1000 0.2 (20) r
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Microevolution of Species
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Causes of Microevolution

• Genetic Drift
• - the change in the gene pool of a small
  population due to chance
• Natural Selection
• - success in reproduction based on heritable
  traits results in selected alleles being passed
  to relatively more offspring (Darwinian
  inheritance)
• - Cause ADAPTATION of Populations
• Gene Flow
• -is genetic exchange due to the migration of
  fertile individuals or gametes between populations

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Causes of Microevolution

• Mutation
• a change in an organisms DNA
• Mutations can be transmitted in gametes to
  offspring
• Non-random mating
• - Mates are chosen on the basis of the best traits

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Genetic Drift
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Factors that Cause Genetic Drift

• Bottleneck Effect
• a drastic reduction in population (volcanoes,
  earthquakes, landslides )
• Reduced genetic variation
• Smaller population may not be able to adapt to
  new selection pressures
• Founder Effect
• occurs when a new colony is started by a few
  members of the original population
• Reduced genetic variation
• May lead to speciation

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Loss of Genetic Variation

• Cheetahs have little genetic variation in their
  gene pool
• This can probably be attributed to a population
  bottleneck they experienced around 10,000 years
  ago, barely avoiding extinction at the end of the
  last ice age

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Founders Effect
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Modes of Natural Selection
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Modes of Natural Selection

• Directional Selection
• Favors individuals at one end of the phenotypic
  range
• Most common during times of environmental change
  or when moving to new habitats
• Disruptive selection
• Favors extreme over intermediate phenotypes
• Occurs when environmental change favors an
  extreme phenotype

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DirectionalSelection
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Disruptive Selection
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Modes of Natural Selection

• Stabilizing Selection
• Favors intermediate over extreme phenotypes
• Reduces variation and maintains the cureent
  average
• Example Human birth weight

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Variations in Populations
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Geographic Variations

• Variation in a species due to climate or another
  geographical condition
• Populations live in different locations
• Example Finches of Galapagos Islands South
  America

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Heterozygote Advantage

• Favors heterozygotes (Aa)
• Maintains both alleles (A,a) instead of removing
  less successful alleles from a population
• Sickle cell anemia
• gt Homozygotes exhibit severe anemia, have
  abnormal blood cell shape, and usually die before
  reproductive age.
• gt Heterozygotes are less susceptible to malaria

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Sickle Cell and Malaria
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Other Sources of Variation

• Mutations
• In stable environments, mutations often result in
  little or no benefit to an organism, or are often
  harmful
• Mutations are more beneficial (rare) in changing
  environments  (Example  HIV resistance to
  antiviral drugs)
• Genetic Recombination
• source of most genetic differences between
  individuals in a population
• Co-evolution
• -Often occurs between parasite host and flowers
  their pollinators

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Coevolution
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