Population Genetics Ch' 16

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Population Genetics (Ch. 16)
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• Why should ecologists care about evolution and
  genetics?
• Genetics influences who lives, who dies, who
  mates, whether populations grow or shrink
• Changes in the environment produce evolutionary
  responses that can influence ecological responses
• Genetics can be a big problem for small
  populations

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• Genes are made of DNA and carry instructions for
  making proteins
• Most organisms have two sets of each chromosome
  therefore, two copies of each gene
• genes come in different variants (alleles)
• individuals can have one or two alleles for any
  gene
• two of the same allele homozygous
• two different alleles heterozygous

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• Dominant alleles are expressed whether there is
  one copy or two
• Recessive alleles must be present in two copies
  to be expressed

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• Two sources of new variation in genes
• Mutation accidental change in genetic code,
  usually during cell division
• most mutations are harmful
• rate of mutation is very low
• Recombination exchange of genetic material
  between different chromosomes during meiosis

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• Gene pool the sum of all genes in a population
• Imagine a gene with 2 alleles, A B
• 20 AA homozygotes 40 A alleles
• 10 BB homozygotes 20 B alleles
• 15 AB heterozygotes 15 A, 15 B alleles
• Total gene pool 55 A, 35 B alleles
• frequency of A 55/90 0.61
• frequency of B 35/90 0.39

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The gene pool determines the proportions of
genotypes in the next generation.
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• Hardy-Weinberg equilibrium
• Frequencies of alleles and genotypes will stay
  the same from generation to generation given
• large N
• random mating
• no selection
• no mutation
• no migration between populations

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• With 2 alleles
• frequency of allele A p
• frequency of allele B q
• q p 1, so p 1 q
• At equilibrium, genotype frequencies can be
  predicted based on allele frequencies
• Genotype AA AB BB
• Frequency p2 2pq q2

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• Most populations deviate from Hardy-Weinberg
  equilibrium due to
• genetic drift
• assortative mating
• gene flow
• selection

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• Genetic drift change in allele frequencies due
  to random chance
• results in loss of uncommon alleles
• most important in very small populations
• Importance of genetic drift is proportional to 1/N

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• 2 times when genetic drift can be important
• Founder events small number of individuals
  found a new population
• contain a reduced sample of the alleles in the
  parent population

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• Population bottleneck a large population is
  reduced to small numbers
• can occur with highly endangered species

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• Assortative mating when individuals chose mates
  nonrandomly with respect to genotype
• positive assortative mating like with like
• negative assortative mating prefer different
  genotype
• Inbreeding mating with close relatives

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• Positive assortative mating and inbreeding
• reduce the number of heterozygotes
• lead to expression of harmful, recessive
  mutations
• Inbreeding is a major problem for small, isolated
  populations

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• Spatial variation in gene frequencies
• Gene flow movements of alleles between
  subpopulations
• Subpopulations often have different allele
  frequencies due to
• barriers to gene flow
• different selection pressures
• result in locally adapted genotypes

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Optimal outcrossing distance
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Ecotypes genetically distinct subpopulations in
different locations
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Cline gradual change in a trait over distance
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Geographic variation without a cline
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Geographic barriers
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• Today
• Finish population genetics
• Start community ecology (consumer-resource
  interactions)

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• Three kinds of natural selection
• Stabilizing intermediate phenotypes are best
• selection for an optimal phenotype

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• Directional more extreme phenotypes (in one
  direction) do best
• new optimum for the population

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• Disruptive more extreme phenotypes (both
  directions) do best
• selection for specialization

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• Summary of factors influencing genetic variation
• Forces increasing genetic variation
• recombination
• mutation
• migration
• Forces reducing genetic variation
• genetic drift
• directional and stabilizing selection
• inbreeding
• positive assortative mating