Week 5 -- Lecture 10
Week 5 -- Lecture 10 Genetic Drift and Inbreeding • relax HWE assumption of infinite population size • Ne and genetic drift definition o do not memorize these equations!!!!!!!! • how to estimate Ne from Nc • inbreeding o from non-random mating o does not change allele frequencies • identity by descent for an allele • extinction vortex of small population Recall Notations • Homozygous: AA, aa • Heterozygous: Aa •
2 alleles in population: A1, A2 o 3 genotypes: A1A1, A1A2, A2A2
•
3 alleles in population: A1, A2 ,A3 o 6 genotypes: A1A1, A1A2, A1A3, A2A2, A2A3, A3A3
The Effect of Genetic Drift is Inversely Related to Population
Large populations = small effects. large effects.
Small populations =
(10% to 9%)
(10% to 0%! loss in
variation) ** be careful with populations below 50
Genetic drift during prolonged population bottlenecks can reduce genetic variation
• • • •
mtDNA control region (has snips, only inherited from mother) o Antarctica population: 23 “alleles” Nc minimum o 30 in California population: 2 “alleles” genetic bottleneck, a historical case o creates random genetic changes without regard to adaption o severe genetic bottleneck occurred in Northern elephant seals (seen above) drift causes evolution but inbreeding does not
Simulation with 3 colours of jelly beans (genetic bottleneck with N=2 diploids) • the effects of genetic drift is inversely related to population size • Hardy Weinberg Equilibrium (HWE) o current allele frequencies predict allele frequencies in the next generation o red jellybean allele p=1/3, “not red” (o/y) allele q =2/3 o if infinite population, p will be constant (HWT)
o p (A1A1, 1/3 x 1/3), pq (A1A2, 2 x 1/3 x 2/3), q2 (A2A2, 2/3 x 2/3) Definition of Genetic Drift and Ne • genetic drift is the random change in the genetic composition of a population arising as a consequence of sampling gametes from a finite population • effective population size (Ne) is the size of an ideal population that would undergo the same amount of genetic drift as the population under consideration (below: 50 to 500 rule) o Ne > 50 prevents inbreeding o Ne > 500 prevents loss of alleles from drift 2
Genetic drift for eye colour in 107 populations of 16 Drosophila
• •
NO DOMINANCE (2 copies = red eyes, 0 copies = yellow/white eyes…. 1 copy = orange eyes!!!!!) 19 generations… becoming completely homozygous vs heterozygous
Genetic drift causes evolution in finite populations (same thing as above, from textbook)
Estimating the effective population size, decline in heterozygosity for eye colour over 20 generations
•
declined must faster than expected/theorized o was a mystery… o effective population number of 9 worked well!
Estimating the effective population size, Ne, from Nc, the census population size
Ne < Nc EXAMPLE 1:
EXAMPLE 2: Male to male Competition leads to extreme variance in reproductive success ** add slide • most had
2 alleles in population: A1, A2 o 3 genotypes: A1A1, A1A2, A2A2
•
3 alleles in population: A1, A2 ,A3 o 6 genotypes: A1A1, A1A2, A1A3, A2A2, A2A3, A3A3
The Effect of Genetic Drift is Inversely Related to Population
Large populations = small effects. large effects.
Small populations =
(10% to 9%)
(10% to 0%! loss in
variation) ** be careful with populations below 50
Genetic drift during prolonged population bottlenecks can reduce genetic variation
• • • •
mtDNA control region (has snips, only inherited from mother) o Antarctica population: 23 “alleles” Nc minimum o 30 in California population: 2 “alleles” genetic bottleneck, a historical case o creates random genetic changes without regard to adaption o severe genetic bottleneck occurred in Northern elephant seals (seen above) drift causes evolution but inbreeding does not
Simulation with 3 colours of jelly beans (genetic bottleneck with N=2 diploids) • the effects of genetic drift is inversely related to population size • Hardy Weinberg Equilibrium (HWE) o current allele frequencies predict allele frequencies in the next generation o red jellybean allele p=1/3, “not red” (o/y) allele q =2/3 o if infinite population, p will be constant (HWT)
o p (A1A1, 1/3 x 1/3), pq (A1A2, 2 x 1/3 x 2/3), q2 (A2A2, 2/3 x 2/3) Definition of Genetic Drift and Ne • genetic drift is the random change in the genetic composition of a population arising as a consequence of sampling gametes from a finite population • effective population size (Ne) is the size of an ideal population that would undergo the same amount of genetic drift as the population under consideration (below: 50 to 500 rule) o Ne > 50 prevents inbreeding o Ne > 500 prevents loss of alleles from drift 2
Genetic drift for eye colour in 107 populations of 16 Drosophila
• •
NO DOMINANCE (2 copies = red eyes, 0 copies = yellow/white eyes…. 1 copy = orange eyes!!!!!) 19 generations… becoming completely homozygous vs heterozygous
Genetic drift causes evolution in finite populations (same thing as above, from textbook)
Estimating the effective population size, decline in heterozygosity for eye colour over 20 generations
•
declined must faster than expected/theorized o was a mystery… o effective population number of 9 worked well!
Estimating the effective population size, Ne, from Nc, the census population size
Ne < Nc EXAMPLE 1:
EXAMPLE 2: Male to male Competition leads to extreme variance in reproductive success ** add slide • most had
