BIOL 2040 Lecture Notes - Lecture 8: Synonymous Substitution, Alcohol Dehydrogenase, Neutral Mutation
27 Apr 2018
School
Department
Course
Professor
Lecture 8 - Evolutionary change at the DNA level (end),
Evolution of Mating Systems
April 8, 2018
3:27 PM
Synonymous vs. Non-synonymous
• Synonymous - mutations that doesn't change the current phenotype
o Assumed to be neutral mutations when comparing to base-line data
• Non-synonymous - mutations that do change the phenotype
o If phenotype is different compared to the base-line data, it is not a neutral mutation
If the mutation is harmful, more preference for the synonymous mutation (ds) would be seen. If the
mutation is advantageous, preference for the non-synonymous mutation (dN) would be seen.
Advantageous mutations tend to go to fixation - natural selection pushes it to that direction.
If mutations are positive, polymorphism should not be present as much.
Polymorphism - having more than 1 allele in a population
MacDonald Kreitman Test
• Are new mutations ever favoured by selection?
• Neutral mutations can drift to fixation (sometimes)
o Substitution and polymorphism occurs
o Multiple alleles are still in one population but substitution occurs (allele becomes present in
all individuals within a population)
• Advantageous mutations that go to fixation by selection:
o Substitution without very much polymorphism
o A decrease in multiple alleles within one population
• Alcohol dehydrogenase:
Alcohol de-hydrogenase
Polymorphic
Fixed
Ratio
Synonymous
41
17
17:41 = 0.41
Non-synonymous
3
7
7:3 = 2.3
o Non-synonymous mutations have more substitutions and fewer polymorphic alleles relative
to synonymous mutations
o Lack of polymorphism implies that those alleles that went to fixation may have been
mutations
o If the ratio of fixed-to-polymorphic is much larger in non-synonymous mutations than
synonymous mutations, it implies that those mutations that did go to fixation went there
"faster" and without drifting for a long time in the population - suggests selection
• Creates evidence that some mutations are advantageous and are driven by positive
selection
Lots of polymorphism generally implies that there are very little mutations that are driven to fixation by
positive selection. You would compare the results of non-synonymous to synonymous - if they are very
different, something changed.
Evolution of Mating Systems
• Question: should you mate with someone similar to you (inbreeding) or someone dissimilar
(outbreeding)?
o If similar: your genes must be good, why would you change or possibly mess it up with
someone else's genes?
o If dissimilar: you may have flaws, someone else's genes may make your offspring better
• There are traits that tend to produce one type of mating over another
o Hermaphrodites vs. separate sexes
o Pollen dispersal distance
o Migration rate
o Timing of reproduction
o Mate choice
• Some mating types produce higher fitness progeny than others depending on the circumstances
o Alleles that produce those mating types will be present in high fitness progeny, and those
alleles will spread
Inbreeding: Self-fertilization
• A1A1 individuals can only produce A1A1 offspring
• A2A2 individuals can only produce A2A2 offspring
• A1A2 individuals, if they continuously produce with itself or with others, it will increase the
expected heterozygosity
Inbreeding coefficient within populations
• Inbreeding coefficient is the proportion reduction in observed heterozygosity as a proportion of
that expected under Hardy Weinberg
o AKA looking at the expected under Hardy Weinberg, what proportion of it are not
heterozygotes in the observed
• Inbreeding reduces the observed heterozygosity
o We observe less heterozygosity than we expect
• We may expected A1A1 to only breed with themselves (inbreeding), A2A2 to only
breed with themselves (inbreeding), and A1A2 to only breed with themselves
(inbreeding) but in reality, A1A2 individuals breed with both A1A1 and A2A2,
producing more homozygotes and less heterozygotes
o Missing portion of heterozygote offspring is hidden in the homozygotes - result of
inbreeding
• Inbreeding decreases heterozygosity - makes genotypes homozygotes
• Ex.
P = 0.5
Expected
Observed
AA
0.25
0.35
Aa
0.5
0.3
Aa
0.25
0.35
o Expected heterozygosity = 0.5 (2pq)
o Observed heterozygosity = 0.3
o 0.3 (observed)/ 0.5 (expected) = 60% of the expected is present, 40% is missing
o F = 0.4 (the missing 40%)
Document Summary
Lecture 8 - evolutionary change at the dna level (end), Synonymous - mutations that doesn"t change the current phenotype: assumed to be neutral mutations when comparing to base-line data, non-synonymous - mutations that do change the phenotype. If phenotype is different compared to the base-line data, it is not a neutral mutation. If the mutation is harmful, more preference for the synonymous mutation (ds) would be seen. If the mutation is advantageous, preference for the non-synonymous mutation (dn) would be seen. Advantageous mutations tend to go to fixation - natural selection pushes it to that direction. If mutations are positive, polymorphism should not be present as much. Polymorphism - having more than 1 allele in a population. 7:3 = 2. 3: non-synonymous mutations have more substitutions and fewer polymorphic alleles relative to synonymous mutations, lack of polymorphism implies that those alleles that went to fixation may have been mutations.
