The Evolution of Populations: Population Genetics and Microevolution

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The Evolution of PopulationsPopulation
Genetics and Microevolution

• IB HL Biology 2

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Two types of evolution

• Microevolution
• Change that occurs within a species
• Change in gene frequencies.
• Focus of Darwins theory of evolution via natural
  selection.

• Macroevolution
• Evolution that takes place over LONG periods of
  time
• Change on a grand scale
• Mass extinctions.

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Microevolution and Natural Selection

• Natural selection results in development of
  features which increase an organisms likelihood
  for survival and reproductionadaptations.
• Therefore, Darwins explanation of evolution goes
  like this
• Adaptation natural selection ? change within
  species.

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How can we study microevolution?

• The study of population genetics helps us to
  understand changes that occur within a species.

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Gene Pool

• The collection of all alleles in a population
• We know that variety exists among organisms and
  among species
• Blood groups in humans
• Enzymes among some vertebrate species have
  striking similarities.

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But why does variety persist?
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But why does variety persist?
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• Godfrey Hardy, a mathematician.

• Wilhelm Weinberg, MD

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By Jove, I think Ive got it!
Ach du lieber! Ich denke, dass ich es habe!

• Independently, the two men devised what would
  come to be called the Hardy-Weinberg Theorem

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H-W Theorem

• Allele frequencies and genotypes in a
  populations gene pool do not change over the
  generations unless acted upon by agents other
  than Mendelian segregation and recombination of
  alleles.

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H-W Equilibrium

• Allele frequencies and genotypes in a
  populations gene pool stay the same generation
  to generation
• In other words, evolution does NOT occur unless
  there are outside forces acting on the population

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Conditions that must be true for H-W equilibrium
to apply

• Population must be LARGE
• Mating must be totally random
• No gene flow
• No mutations
• No natural selection

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Hardy-Weinberg Equation

• p2 2pq q2 1
• Where
• p2 represents homozygous dominant individuals
  (AA)
• 2pq represents individuals heterozygous for
  alleles A and a, so Aa
• q2 represents homozygous recessive for alleles a,
  so aa.

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A few things you need to know about the H-W
equation

• P always represents the dominant allele.
• Q always represents the recessive allele.
• Commonality of each allele does not matter a
  recessive allele can be the most common, while a
  dominant allele can be the least common.

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Other things you need to know

• The equation is derived from
• (p q)2 1
• p q must always equal 1!
• Think what are the chances an organism has of
  inheriting a gene if there are only two alleles
  for a specified trait?

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Genetic Drift

• This is a random increase or decrease in alleles.
• This has a greater effect if the population is
  very small.

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Genetic Drift

• Bottleneck the population experiences a huge
  decrease in size.
• Result severe reduction in diversity of the
  original gene pool.
• Endangered species can experience this.

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Genetic Drift

• Founder effect when allele frequencies in a
  group of migrating individuals are not the same
  as that of their population of origin.

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Nonrandom Mating

• When individuals choose mates based on their
  particular traits. Nonrandom mating can also
  occur when mates choose only nearby individuals.
  Two kinds
• Inbreeding individuals mate with relatives
• Sexual selection females choose males based on
  appearance or behavior.

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Gene Flow

• This is the introduction or removal of alleles
  from a population that occurs when individuals
  enter (immigrate) or leave (emigrate) that
  population.

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Mutations

• Mutations introduce new alleles that can be
  selectively advantageous, but most are harmful.

Polydactyly, or extra digits.
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Natural Selection

• This is a change in gene frequencies due to
  differential reproductive success.

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In other words, for a H-W equilibrium to hold
true

• There can be NO mutations.
• There can be NO gene flow.
• There can be NO genetic drift. In other words,
  the population is BIG.
• Mating must be TOTALLY random.
• There must be NO natural selection of any kind.

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Preservation of Variation Diploidy

• The presence of homologous chromosomes (one from
  mom, one from dad) maintains variety in the
  population
• A dominant allele can cover up a harmful
  recessive one

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Balanced Polymorphisms

• Polymorphism multiple physical forms of one
  trait
• Heterozygote advantage heterozygotes more
  likely to survive than homozygotes
• Sickle-cell anemia and malaria resistance

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Next time

• Population Genetics Lab
• Fishy Frequencies