Evolutionary effects of migration and genetic drift

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Evolutionary effects of migration and genetic
drift
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Gene flow
Can counter natural selection
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Nerodia sipedon
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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, 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. Ten
  individuals found a new population. No way that
  all of this allele variation can be represented.

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• E.g.,
• Tristan da Cunha Island in southern Atlantic
  one of most isolated places on earth
• Colonized in 1816 by William Glass, wife, two
  daughters
• Joined later by a few additional settlers from
  England
• 1961 volcanic eruption population (294) taken
  back to England, tested for various genetic
  traits.
• 145 years in isolation
• 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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Allele diversity in Zosterops lateralis
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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
bw25/bw Random draws of 8 males 8 females for
subsequent generations Population size kept at N
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Natural Examples of Drift
Genotypic variation Pocket gopher Thomomys bottae
825 individuals 50 geographic localities Two
polymorphic gene loci
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Greater Prairie Chicken
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Habitat loss
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Bottleneck
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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