Week 8 Flashcards
forces of evolution
Directional change - selection
Random change - genetic drift
Wright Fisher Model assumptions
Haploid
Constant population size
Discrete generations
Asexual clonal reproduction
Genes are transmitted to next generation by random sampling (no selection)
Sampling with replacement
how do we simulate the production of the next generation?
Sample with replacement
Make sure event is independent from another
Sample from t (first generation) to form t+1 (second population)
Large population: frequency will be the same in both generations
Small population: can produce very different generations - genetic drift occurs!
rules about drift
The probability that a neutral allele will fix is equal to its frequency in the population
The probability of fixation of a new arising neutral allele = 1/population size (Pfix = 1/N)
The capacity for selection to discern between the fitness effects of alleles also depends on population size - if s is smaller that 1/N then natural selection cannot distinguish between that allele and a neutral allele (cannot act on it)
The probability of fixation for a newly arising neutral allele is lower in larger populations due to the decreased influence of genetic drift
selection vs drift
When a population is very small, selection is not effective and drift is very strong (random sampling)
bottlenecks
A genetic bottle neck is a mechanism by which drift can drastically shape a new break-away population
This can happen due to a big shift in population size or migration
founder effects
A few individuals found a new population
assumptions of HW
Equal number of male and females
Random mating
No selection
No migration
Very large population (no drift)
Discrete generations
what causes deviation from HW?
Selection
Population structure
Assortative mating and inbreeding
If one genotype is lethal
molecular clock
Genetic variation is always coming and going
Mutation rates in nucleotide bases are low, about 10^-7 to 10^-9
Genome rates are higher
U is used to measure mutation rate
Mutation rate is pretty constant
U is the genomic mutation rate or the expected number of mutation to occur in each new offspring (U = 100 in humans)
Can be wrong - mutation rates are not always that consistent or generation times or population size may have been small for a long time
neutral mutations
The number of mutations entering a diploid population = 2NU
N is population size
Rate of substitution (fixed) - the probability of fixation of a new neutral allele is 1/N
In diploid population 2NU and so fixation is 1/2N so expected number of neutral mutations we expect to be fixed every generation is 1/2N x 2NU = 2NU/2N = U (allows us to have a molecular clock!)
how do we use molecular clocks?
Get DNA from species to compare
Align
Find a way to calibrate the number of mutations over time
