Population Genetics and Speciation

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Population Genetics and Speciation
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Population genetics

• Study of evolution from a genetic point of view.
• Microevolution
• Change in the collective genetic material of a
  population
• Population
• A group of individuals of the same species that
  routinely interbreed

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Causes of variation

• Mutation
• Random change in a gene that is passed on to
  offspring
• Recombination
• Reshuffling of genes in a diploid individual
• Occurs during meiosis
• Random pairing of gametes

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Gene pool

• The total genetic information available in a
  population

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Allelic frequency
N total no. of alleles in a population
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Phenotypic frequency

• No. of individuals with a particular phenotype
  divided by the total number of individuals in the
  population

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Hardy-Weinberg Genetic Equilibrium

• Genotype frequencies remain the same from
  generation to generation unless acted on by
  outside influences
• Assumptions
• No net mutations occur alleles remain the same
• Individuals neither enter nor leave the
  population
• The population is large (infinitely large)
• Individuals mate randomly
• Selection does not occur

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Hardy-Weinberg Genetic Equilibrium

• (pq)2 p2 2pq q2
• The total number of genes in a population is its
  gene pool.
• p frequency of one gene in the pool
• q frequency of its single allele.
• So, p q 1
• p2 the fraction of the population homozygous
  for p
• q2 the fraction homozygous for q
• 2pq the fraction of heterozygotes

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• Equilibrium no evolution
• No equilibrium evolution

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Disruption to Equilibrium

• Mutation
• Some harmful, others favorable
• Gene flow
• Immigration
• Movement of individuals into a population
• Emigration
• Movement of individuals out of a population

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• Genetic drift
• Allele frequencies in a population change as a
  result of random events, or chance
• Significant only in small and medium-sized
  populations

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Nonrandom mating

• Mate selection is influenced by geographic
  proximity
• Selection of mate with similar characteristics
• Sexual selection
• A male must be selected by a female to produce an
  offspring

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Natural selection

• Acts on variations of traits within a population

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Patterns of natural selection

• Stabilizing selection
• Individuals with the average form of a trait has
  the highest fitness
• Ex. Plants (height)

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Disruptive selection

• Individuals with either extreme variations of a
  trait have greater fitness
• Ex. Limpets

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Directional selection

• Individuals that display a more extreme form of a
  trait have greater fitness
• Ex. Selection for longer beaks in woodpeckers

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Speciation

• Process of species formation
• Biological species concept
• Ernst Mayr
• A species is a population of organisms that can
  successfully interbreed but cannot breed with
  other groups
• Members of a species are morphologically similar
  and can interbreed to produce fertile offspring

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Isolation and Speciation

• Geographic isolation
• Physical separation of members of a population
• Droughts in forests, lava flows

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• Allopatric speciation
• Species arise as a result of geographic isolation
• Occurs in small populations
• Smaller gene pool

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• Reproductive isolation
• Results from barriers to successful breeding
  between population groups in the same area
• Prezygotic premating isolation (before
  fertilization)
• Postzygotic postmating isolation (after
  fertilization)

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• Sympatric isolation
• Occurs when two subpopulations become
  reproductively isolated within the same
  geographic area

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Rates of Speciation

• Gradualism
• Speciation occurs at a regular, gradual rate

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• Punctuated equilibrium
• Speciation happens in bursts relative to the
  geologic time scale

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Gradualism vs Punctuated Equilibrium
Gradualism
Punctuated Equilibrium