Neutral evolution & Genetic distance
Neutral evolution
Undirected changes in allele frequencies; arise from the fact that
offspring is a sample of parental genomes
Neurtral evolution depends on population size (important in small
populations) and breeding structure (harems
Examples:
Genetic drift
Colonisations (founder effects)
Bottleneck effects: drastische afname in populatiegrootte door
omgevingsfactoren (uitbraak dodelijke ziekte)
o Demographic event after origin of H. sapiens – bottleneck effect ±
70000 y BP
Small populations can evolve quickly in large populations selection is necessary
Diffusion model of Kimura
Fixation: loss of polymorphism, a locus becomes monomorphic by random genetic
drift
Fixation is strongly dependent on effective population size (Ne) (this can be much
smaller than real population size)
Ne = 4NfNm/Nf + Nm
The fate of new alleles depends strongly on Ne. In many cases the Ne is too small to
prevent that mutation and drift determine the course of evolution
2N alleles in population of diploid organisms
All alleles have same probability to fix – likelihood of fixation is K = 1/(2N)
If allele emerge by mutation at frequency µ the probability of a new allele emerging is
2Nµ per generation time
Fixation is now K= 1/(2N) * 2Nµ = µ
Under equilibrium conditions fixation frequency is numerically equal to mutation
frequency
Harems
Ne << N
Polygynous social structure is accompanied by sexual dimorphism
Polydactyly (hebben van meer vingers/tenen) among Amish
Congenital dominant defect
The initial colonizers of Amish carried this allele already
Population which is established by single colonization often shows deviating genetic
composition
Neutral evolution
Undirected changes in allele frequencies; arise from the fact that
offspring is a sample of parental genomes
Neurtral evolution depends on population size (important in small
populations) and breeding structure (harems
Examples:
Genetic drift
Colonisations (founder effects)
Bottleneck effects: drastische afname in populatiegrootte door
omgevingsfactoren (uitbraak dodelijke ziekte)
o Demographic event after origin of H. sapiens – bottleneck effect ±
70000 y BP
Small populations can evolve quickly in large populations selection is necessary
Diffusion model of Kimura
Fixation: loss of polymorphism, a locus becomes monomorphic by random genetic
drift
Fixation is strongly dependent on effective population size (Ne) (this can be much
smaller than real population size)
Ne = 4NfNm/Nf + Nm
The fate of new alleles depends strongly on Ne. In many cases the Ne is too small to
prevent that mutation and drift determine the course of evolution
2N alleles in population of diploid organisms
All alleles have same probability to fix – likelihood of fixation is K = 1/(2N)
If allele emerge by mutation at frequency µ the probability of a new allele emerging is
2Nµ per generation time
Fixation is now K= 1/(2N) * 2Nµ = µ
Under equilibrium conditions fixation frequency is numerically equal to mutation
frequency
Harems
Ne << N
Polygynous social structure is accompanied by sexual dimorphism
Polydactyly (hebben van meer vingers/tenen) among Amish
Congenital dominant defect
The initial colonizers of Amish carried this allele already
Population which is established by single colonization often shows deviating genetic
composition
