Population Genetics

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Population Genetics
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What is Population Genetics

• Population genetics is the study of the
  differences in alleles and genotypes within the
  gene pool, and how this causes variation changes
  from one generation to the next.
• Population genetics explains how genetic
  mutations lead to speciation.

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Population Genetics

• Population genetics asks what happens when a new
  allele or an increased number of an old allele
  move into a population.
• New allele an individual with black hair enters
  the population and breeds with the rest. How
  does this change the population?
• Old allele a bunch of blonde haired people move
  into the population. How does this change the
  population?

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Gene Pool

• The collection of all the alleles of all of the
  genes found within a freely interbreeding
  population is known as the gene pool of the
  population.
• Each member of the population receives its
  alleles from other members of the gene pool (its
  parents) and passes them on to other members of
  the gene pool (its offspring).
• What is an allele?

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• Basic Genetics Review

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• The Hardy-Weinberg Equilibrium Equation describes
  and predicts a balanced equilibrium in the
  frequencies of alleles and genotypes within a
  freely interbreeding population.
• (p²  2pq  q²  1)
• For a trait controlled by a pair of alleles (A
  and a).
• p is defined as the frequency of the dominant
  allele
• q as the frequency of the recessive allele  
• In other words, p equals all of the alleles in
  individuals who are homozygous dominant (AA) and
  half of the alleles in people who are
  heterozygous (Aa) for this trait in a population.

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Hardy Weinberg Equilibrium

• The Hardy-Weinberg Equilibrium Equation assumes
  the following
• A large population size
• No mutation
• No genetic drift
• No natural selection
• No gene flow between populations
• Random mating patterns.

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Hardy Weinberg

• Factors influencing the genetic diversity within
  a gene pool include
• population size
• Mutation
• genetic drift
• natural selection
• environmental diversity
• migration
• non-random mating patterns.

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Variation

• Variations are the phenotypic expression of the
  different alleles in a population.
• Mutations are the genetic changes in alleles that
  lead to variations.
• Genetic diversity is another way of referring to
  the gene pool.

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Speciation Mechanisms
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Natural Selection

• Natural selection states that those individuals
  best adapted to their environment will survive
  and reproduce.
• The population will then reflect the fittest
  individuals within the population.

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Natural Selection Concepts

• Fitness
• Stabilizing selection
• Disruptive selection
• Directional selection

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Fitness

• The theory of natural selection relies on the
  concept of fitness.
• Fitness is measured by the number of offspring
  that an individual has that survive and
  reproduce.
• The greater the percentage of offspring that you
  contribute to the population, the higher your
  fitness score and the more the population will
  reflect you.

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Stabilizing Selection

• Stabilizing selection favors the norm, the
  common, average traits in a population.

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Example of Stabilizing Selection

• Look at the Siberian Husky, a dog bred for
  working in the snow.
• The Siberian Husky is a medium dog, males
  weighing 16-27kg (35-60lbs).
• These dogs have strong pectoral and leg muscles,
  allowing it to move through dense snow.
• The Siberian Husky is well designed for working
  in the snow.

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Example of Stabilizing Selection

• If the Siberian Husky had heavier muscles, it
  would sink deeper into the snow, so they would
  move slower or would sink and get stuck in the
  snow.
• Yet if the Siberian Husky had lighter muscles, it
  would not be strong enough to pull sleds and
  equipment, so the dog would have little value as
  a working dog.
• So stabilizing selection has chosen a norm for
  the the size of the Siberian Husky.

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Directional Selection

• Directional selection favors those individuals
  who have extreme variations in traits within a
  population.

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Directional Selection

• A useful example can be found in the breeding of
  the greyhound dog.
• Early breeders were interested in dog with the
  greatest speed.
• They carefully selected from a group of hounds
  those who ran the fastest.
• From their offspring, the greyhound breeders
  again selected those dogs who ran the fastest.
• By continuing this selection for those dogs who
  ran faster than most of the hound dog population,
  they gradually produced a dog who could run up to
  64km/h (40mph).

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Disruptive Selection

• Disruptive selection, like directional selection,
  favors the extremes traits in a population.

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What is Gene flow
Gene flow is the transfer of genetic material
between different populations of the same
species. Gene flow is a fancy name for
migration. When an organism emigrates from the
rest of the population it takes its genes with
it, decreasing the gene pool and the genetic
diversity of the population. When an organism
immigrates into a population it adds genes to the
population, increasing the genetic diversity.
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Allele Frequency Review

• Example
• Brown hair (B) allele is dominant over blonde
  hair (b) allele.
• In a normal population with normal allelic
  frequencies, what percentage of the population
  would have
• B allele
• b allele
• Brown hair
• Blonde hair
• Brown hair but carry the blonde hair gene
  (heterozygote)

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Allele Frequency

• B allele50
• b allele50
• Brown hair75
• Blonde hair25
• Heterozygotes (have both the B and b allele)50
  of the 75 with brown hair

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How Does Gene Flow Occur

• Gene flow results from the movement of
  individuals (migration) or their gametes (sexual
  or asexual reproduction).

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Gene Flow Prevents Speciation

• In other words, if gene flow occurs between
  populations, speciation will not occur.
• Why?

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Gene Flow and Speciation

• Gene flow connects populations of a species. It
  can cause two populations to share genes enabling
  them to evolve collectively (as a unit).
• Would speciation occur in a population with
  constant gene flow?

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Speciation

• Reductions in gene flow may lead to speciation.
• Why?

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Allele Frequencies

• Gene flow may add new alleles to a population or
  change the frequencies of alleles already
  present.
• What does this mean?

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Allopatric (Geographic) Isolation

• Allopatric isolation means that a population
  becomes geographically isolated from the original
  population.
• In order for speciation to occur a population
  must become genetically isolated from the
  original population.
• The gene pool is reduced because of lack of gene
  flow.
• An individual with different alleles that might
  never have the highest fitness in the larger
  population now has the highest fitness in the new
  population.
• How is Vail High School an example of allopatric
  isolation?

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Sympatric Isolation

• Sympatric isolation occurs when two populations
  are not separated by geography but still will not
  interbreed.
• Generally it is believed that allopatric
  (geographical) isolation leads to speciation but
  sympatric isolation keeps the two species
  separated.
• Types of sympatric isolation include
• Temporal
• Behavioral

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Temporal Isolation

• Temporal isolation is a form of sympatric
  isolation fertilization because the two different
  species reproduce at different times.
• The time periods could differ simply by hours, or
  by seasons.
• If one species reproduces in the spring, while
  the other reproduces in the fall, the two species
  arent able to breed.

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Behavioral Isolation

• An isolating mechanism in which two species do
  not mate because of differences in courtship
  behavior.
• Therefore the two species do not recognize each
  other as possible mates even though they could
  physically reproduce.

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Genetic Drift

• Once populations are geographically isolated into
  small populations, small changes in alleles can
  greatly change a population.

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Genetic Drift

• In natural populations, however, the genetic
  composition of a population's gene pool may
  change over time.
• Mutation is the primary source of new alleles in
  a gene pool, but the other factors act to
  increase or decrease the occurrence of alleles.
• Genetic drift occurs as the result of random
  fluctuations in the transfer of alleles from one
  generation to the next, especially in small
  populations formed, say, as the result adverse
  environmental conditions (the bottleneck effect)
  or the geographical separation of a subset of the
  population (the founder effect).

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Bottlenecks

• Population bottlenecks occur when a populations
  size is reduced for at least one generation.
• Because genetic drift acts more quickly to reduce
  genetic variation in small populations,
  undergoing a bottleneck can reduce a populations
  genetic variation by a lot, even if the
  bottleneck doesnt last for very many
  generations.

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Bottleneck
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Bottleneck

• Reduced genetic variation means that the
  population may not be able to adapt to new
  selection pressures, such as climatic change or a
  shift in available resources, because the genetic
  variation that selection would act on may have
  already drifted out of the population.

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Bottleneck Example

• Northern elephant seals have reduced genetic
  variation probably because of a population
  bottleneck humans inflicted on them in the 1890s.
  
• Hunting reduced their population size to as few
  as 20 individuals at the end of the 19th century.
• Their population has since rebounded to over
  30,000but their genes still carry the marks of
  this bottleneck they have much less genetic
  variation than a population of southern elephant
  seals that was not so intensely hunted

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Founder Effect

• A founder effect occurs when a new colony is
  started by a few members of the original
  population.
• This small population size means that the colony
  may have
• reduced genetic variation from the original
  population.
• a non-random sample of the genes in the original
  population.

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Founder Effect Example

• For example, the Afrikaner population of Dutch
  settlers in South Africa is descended mainly from
  a few colonists.
• Today, the Afrikaner population has an unusually
  high frequency of the gene that causes
  Huntingtons disease, because those original
  Dutch colonists just happened to carry that gene
  with unusually high frequency.
• This effect is easy to recognize in genetic
  diseases, but of course, the frequencies of all
  sorts of genes are affected by founder events.

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Speciation

• The result of genetic drift tends to be a
  reduction in the variation within the population,
  and an increase in the divergence between
  populations.
• Usually this will cause populations to go
  extinct.
• If two populations of a given species survive and
  become genetically distinct enough that they can
  no longer interbreed, they are regarded as new
  species (a process called speciation).

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Citations

• www.eeob.iastate.edu/classes/Biol304/docs/Biol203
  0420Genetic20Diversity.pdf
• www3.botany.ubc.ca/rieseberglab/plantevol/migratio
  nKHJan8.ppt
• http//users.rcn.com/jkimball.ma.ultranet/BiologyP
  ages/E/Evolution.html
• http//www.le.ac.uk/ge/genie/vgec/he/population.ht
  ml
• http//evolution.berkeley.edu/evosite/evo101/IIID3
  Bottlenecks.shtml