Mutation
- any heritable change in DNA sequence, or DNA compliment (e.g. # of chromosomes)
- ultimate source of all variation w/in pop’ns
somatic mutations
- mut’ns in somatic cells that are not heritable
- no evolutionary consequence
germline mutations
-occur in gametes or the cells that produce them and can be passed on to offspring
chromosomal mutations
changes in the number or structure of chromosomes
- inversions: goes from one strand to the other strand (and also rotates 180 degrees)
- point mutations: a) transitions b) transversion
inversions
goes from one strand to the other strand (and also rotates 180 degrees)
point mutations
change of a single nucleotide in a DNA sequence
a) Replacement mut’ns (Non-synonymous): mut’ns that change the a.a. in a protein sequence
b) Silent mut’ns (synonymous): do not change the a.a.
transitions
substitution of a purine with a purine or a pyrimidine with a pyrimidine
-much more common than transversions
transversion
substitution of a purine with a pyrimidine and vice versa
Indels
- additions or deletions
- in coding regions: can be highly deleterious –> can disrupt the reading frame
gene duplications
- often, genes or groups of genes can duplicate
- important mode of duplication
- e.g. major histo-compatibility complex (arisen by duplication)
model of recurrent mutation
- A–>a = u (forward mut’n rate)
2. a–>A = v (back mut’n rate)
Infinite alleles model
- assumes each new mutation creates a new unique allele
- by def’n, all copies of any given allele will be IBD
Stepwise mutation model
- length of alleles incrementally increases or decreases one repeat at a time via mutation
- gain or lose repeats
deleterious mutation
mutations which result in lowered fitness
purifying selection
-selection that acts to remove or lower the frequency of a deleterious mutation
3 things which can prevent selection from removing a deleterious mutation
- continued mutation: keeps pumping it back in
- dominance effects: e.g. Aa–>hides in heterozygotes
- genetic drift: when pop’n size is very small
neutral mutations
have no effect of fitness
- e.g. silent mutations which do not change an a.a. are typically neutral
- note: silent mutations are not always neutral
nearly neutral mutations
have very little effect on fitness (so little that they are effectively neutral)
advantageous mutations
- increases the fitness
- they are rare in comparison to deleterious and neutral mutations
- positive selection: sel’n that increases the frequency of advantageous mutations –> reuslts in adaptive evolution
mutational load
- caused by mutation selection balance (ignoring drift)
- results in deleterious effects
- load is equivalent to the reductions in the relative pop’n fitness
- basically deleterious alleles hide in heterozygotes
Effect of a mutation on H-W eq’m
- does not affect H-W eq’m
- next gen, you have H-W eq’m..but based on the new allele freq’s
