Assortative mating

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Assortative mating Female mate choice 1.
Head-color match healthier chicks Sex ratio
5050 2. Mismatched matings Weak female chicks
(die) Shortage of compatible males 70 chicks
are male Head-dying experiments Sex ratio 5050
Sex determined by female Therefore, she must
be choosing what?
Gouldian finch
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Evolutionary effects of gene flow and genetic
drift
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Gene flow
Can counter natural selection
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Nerodia sipedon
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Allele diversity in Zosterops lateralis
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Genetic Drift

• Natural populations (unlike Hardy-Weinberg
  populations) are finite in size.
• Geographically structured so that mating is not
  random.
• Demes
• In small isolated populations, alleles can
  fluctuate by chance (genetic drift).
• Therefore, genetic drift is an evolutionary
  force.
• But GD is non-directional and cannot produce
  adaptations.

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• Sometimes a new population is established by a
  small number of COLONISTS or FOUNDERS, through
• 1. Dispersal (geographic)
• 2. Vicariance (geological)
• 3. Bottleneck (population is drastically
  decreased in size -- reestablishment of the
  population by a small number of founders.
• All such populations experience a loss of genetic
  variability.
• e.g., a gene locus has 25 alleles. A new
  population is founded by 10 individuals. No way
  that all of this allele variation can be
  represented.

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• E.g., Tristan da Cunha Island
• Most remote inhabited island on Earth
• Colonized in 1816 by William Glass, wife, two
  daughters
• Joined later by a few additional settlers from
  England

1,750 mi. from South Africa 2,088 mi. from South
America 38 mi2
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• 1961 volcanic eruption
• Population (294) evacuated and taken back to
  England
• Population had been isolated for 145 years
• All residents homozygous (fixed) for nine genetic
  markers
• e.g., clinodactyly (dominant) present in the
  Glass family.
• Gene Flow
• Alternative glucose-6-phosphate dehydrogenase
  allele arrived in 1827.

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• Because new small populations generally
  experience slow growth, genetic drift continues
  to reduce genetic variation.
• Heterozygosity decreases and homozygosity
  increases.
• Genetic drift operates independently on
  geographically isolated populations.
• Frequency of an allele might increase in some
  populations, decrease in others.
• Populations diverge as different alleles become
  fixed in each.

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Computer modeling of genetic drift Start with
heterozygous individuals
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Drift in a lab 107 experimental populations of
Drosophila Started with heterozygous individuals
bw75/bw Random draws of 8 males 8 females for
subsequent generations Population size kept at N
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Reduction in heterozygosity
Distributions of the numbers of bw75 alleles in
107 lines of D. melanogaster
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Natural Examples of Drift
Genotypic variation Pocket gopher Thomomys bottae
825 individuals 50 geographic localities Two
polymorphic gene loci
150 described subspecies
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Desert 8,000 to 4,000 ybp Contiguous with SW
deserts Then retreat of deserts to SW
Present oak-hickory forest Relictual
populations 12 or so individs per deme