- What does Heterozygosity depend on? How do they each affect the degree of heterozygosity?
Mutation & population size. Increase in both mutation rate and population size will increase heterozygosity
genetic drift
change in allele frequency over time due to random chance
how drift affects small populations
higher probability of large jumps in allele frequency any given generation; shorter time to fixation or loss
probability of brand new mutation becoming fixed
1/ (2N)
relationship between drift and mutation
H = (4Nu)/(1+4Nu)
When N gets larger, H ….
increases
When u gets larger, H…
increases
Define the Bottleneck effect and predict its outcome
Sudden external factor (e.g. natural disaster) that kills most of the individuals in a population, only leaving a
randomly selected small group of individuals. Genetic drift will cause it to evolve from the previous genotypic
frequencies
Selection happens when individuals
vary in fitness, due to some having higher rates of survival and/or reproduction
Modes of natural selection
- Purifying selection
- Directional selection
- Disruptive selection (favors extreme
phenotypes) - Overdominance (heterozygote advantage)
- Underdominance (heterozygote disadvantage)
- Frequency dependent selection
Common modes of natural selection
purifying and directional (selection acts on phenotypes regardless of genetic basis)
Why isn’t purifying selection perfectly effective at removing deleterious alleles
o Not all phenotypic variation has a simple genetic basis
o Deleterious recessive alleles can be sheltered in heterozygotes
Because paired with other allele that is in some way more beneficial
o Mutation continually introduces genetic variation into population
If new mutation is codominant and advantageous
codominant genotype will be selected for at a slow rate, causing a gradual rise in f(B). However, once BB homozygotes are present in the population from the mating of ABxAB, there will be selection for BB even over the codominant case. Thus, allele B will eventually reach fixation, since genotype BB is being selected for above all else; will take more time before initial slope because evolution proceeds more slowly since advantage is small
If new mutation is dominant and advantageous
like codominant but initial slope occurs before; may not become fixed or takes longer because A2 alleles will persist because sheltered in heterozygotes but selection will remove it
If new mutation is recessive and advantageous
evolution is delayed by selection because everyone has same fitness; allele stays very rare and frequency determined by drift
natural selection is strong…..
in large populations
if N x s is less than 1 ….
drift is more powerful force, where N is population size and s is strength of selection on selected allele
if N x s is greater than 1…
selection is more powerful
Heritability (the proportion of trait variance that is genetic) gets small when
environmental contributions to the trait get large
Heritability predicts how well population responds to
directional selections
R
response to selection; difference between the trait mean in the progeny and the trait mean in parents
large S means
strong selection
