Lecture 4 - Random genetic drift and NE Flashcards
Genetic Drift
Stochastic changes in allele frequency by random sampling of gametes to form offspring in finite population
For a single locus with two alleles, random sampling follows what distribution?
Binomial distribution
What are the changes in probabilities of allele frequencies in the next generation for different population sizes?
For smaller populations there is quite a high probability that it will change the frequency
In larger population, there is less variance
What is Buri’s study of genetic drift
A study carried out in Drosophila
Actual data for 107 populations
Randomly selected 8 males and 8 females for the next generation for 19 consecutive generations
Calculated the frequency of bw75 allele (eye colour) and generated a frequency distribution for the 107 populations
Allele frequencies tend to become different overtime
Rate of allele frequency change under genetic drift with population size?
Varies with population size
In larger population sizes allele frequencies have fluctuations with lower amplitude
Fixation takes longer in larger population size
What does the initial allele frequency determine in genetic drift?
Determines the probability of fixation or loss under genetic drift
What does genetic drift lead to?
Loss of variation
What is genetic drift caused by?
Variation in a random sampling of gametes in finite populations
Why can we not predict allele frequency change in genetic drift?
Random process (stochastic)
Unlike mutation pressure of natural selection which are deterministic
How does genetic drift differ across replicate populations?
Average allele frequency does not change but the variation in allele frequency changes overtime
Census population size (N)
A head count of the population
What are the characteristics of an ideal population?
Mating is random
All individuals are equally likely to have offspring and the number of offspring they have does vary more than would be expected by chance alone
The number of breeding individuals remains the same each generation
There are equal numbers of males and females involved in breeding
Effective population size
Number of individuals in an idealized population that undergo genetic drift at the same rate as the actual population under consideration
What are reasons for the effective population size to be lower than the census population size?
Variation in reproductive success (that is greater than that expected by chance)
Fluctuation in population size overtime
Variation between the number males and females that contribute offspring to the next generation
Overlapping generations/age-structured populations
What is the effective population size a function of?
The number of breeding males and females
What might cause low genetic diversity?
Population bottleneck
Cheetah population bottleneck
Migrated from north America through Asia to Africa around 100,000 years ago
Another bottle neck 10,000 years ago at the end of the last glaciation
Continued low population size due to recent human impact
Population bottleneck
Extreme reduction in population size that increases the effect of genetic drift
Ne is closer to bottleneck population size
Ne in population expansion
Ne is closer to the pre-expansion population size
Northern elephant seals genetic bottleneck event
Human hunting in the 1890 reduced their population to 20 at the end of the 19th centuary
New population 30,000
But genetically they resemble a much smaller population
They have much less genetic variation than southern elephant seals that were not hunted
Founder effect
The loss of genetic diversity when a new smaller population is founded from a larger population
Loss of diversity is due to the genetic drift associated with this type of bottlenecking event
Founder effect on blood type
Distribution of O type blood allele in native populations of the world reflect original settlements
Founder effect in the amish community
The amish community in eastern united states was founded by a small number of colonists (around 200 from Germany)
They continue to be genetically isolated
The founders had a recessive deleterious autosomal alles that underlies a form of dwarfism, Ellis,van Creveld (EvC) syndrome, that also involved polydactyly of the hands as well as short stature and congenital heart malformation
Heterozygotes now make up about 12.3% of the population
