BIOEE 1780 Lecture Notes - Lecture 28: Spatial Analysis, Zygosity, Genetic Drift
7 May 2018
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Department
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Landscape Genetics: combines population genetics, landscape ecology, and spatial statistics
Population Structure (or Population Subdivision): the occurrence of populations that are subdivided by
geography, behavior, or other influences that prevent individuals from mixing completely
• Leads to deviations from Hardy-Weinberg Equilibrium
• Increases the chance of genetic drift, because subpopulations are smaller than the entire
population
• Populations grow more distant from each other over time.
Genetic Distance: a measure of how different populations are from each other genetically
• Indicates levels of inbreeding
• Can reveal relationships between populations or species
• FST: a measure of genetic distance between subpopulations
o FST = 1 / [4Nm + 1]
• N = the size of the population
• Small N = lots of genetic drift
• m = the migration rate
• Small m = high differentiation
o Measures the reduction in heterozygotes at a locus attributable to the effects of population
subdivision
o When FST is near zero, allele frequencies are approximately the same from location to location.
• Indicates lots of gene flow and less differentiation
o When FST values increase, subpopulations have begun to diverge from each other (i.e. partially
isolated subpopulations).
• High value indicates less gene flow and high differentiation
o If FST = 1, all subpopulations are fixed for different alleles.
Habitat Fragmentation: previously large population becomes many small subpopulations
• Example of bottleneck (genetic drift)
• Genetic drift in each subpopulation is high.
• Immediate and catastrophic loss of variation
o Unable to adapt or evolve readily to different situations
• Can result in fixation of deleterious alleles
• Ex.) cheetahs
Which is stronger, natural selection of genetic drift?
• In very small populations, genetic drift is strong.
o It can either reinforce or counteract the effects of selection, depending upon whether the
beneficial or deleterious allele is drifting upward in frequency.
o Drift is never deterministic. It is always random.
• In very large populations, drift is minimal and selection is more powerful.
• We have to think about the direction of selection and/or drift in populations.
• The relative importance of genetic drift versus selection depends on the population size (N) and the
selection coefficient (s).
o If Ns < 1, drift rules
o If Ns > 1, selection rules
• The higher Ns is, the stronger selection is.
Document Summary
Landscape genetics: combines population genetics, landscape ecology, and spatial statistics. Population structure (or population subdivision): the occurrence of populations that are subdivided by geography, behavior, or other influences that prevent individuals from mixing completely. Increases the chance of genetic drift, because subpopulations are smaller than the entire population: populations grow more distant from each other over time. Genetic distance: a measure of how different populations are from each other genetically: can reveal relationships between populations or species. Fst: a measure of genetic distance between subpopulations: fst = 1 / [4nm + 1, n = the size of the population. Small n = lots of genetic drift: m = the migration rate. Small m = high differentiation: measures the reduction in heterozygotes at a locus attributable to the effects of population subdivision, when fst is near zero, allele frequencies are approximately the same from location to location.
