Download Computer modeling of genetic drift

Evolutionary effects of emigration and
genetic drift
• Sometimes a new population is established by a small
number of COLONISTS or FOUNDERS, through:
• 1. Dispersal (geographic)
• 2. Bottleneck (population is drastically decreased in size -reestablishment of the population by a small number of
founders.
• Small populations lose genetic variability.
• e.g., a gene locus has 25 alleles. Ten individuals found a
new population. This allelic variation cannot be fully
represented in the new population.
Gene flow
Can counter natural selection
Nerodia sipedon
Allele diversity in
Zosterops lateralis
Founder effect
and genetic drift
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, allele frequency can
fluctuate randomly: independent of selection (genetic
drift).
• Therefore, genetic drift is an evolutionary force.
• But, GD is non-directional and adaptations would be
accidental.
Genetic Drift Example
Island: Tristan da Cunha
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
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1816  1961: population of 294 individuals
Volcanic eruption
Population evacuated and 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.
• 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.
Computer modeling of
genetic drift
Start with heterozygous
individuals
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 = 16
Natural Examples of Drift
Genotypic variation
Pocket gopher
Thomomys bottae
825 individuals
50 geographic
localities
Two polymorphic
gene loci
150 described
subspecies
Greater Prairie Chicken
Habitat loss
Bottleneck
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