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Lecture 20

Lecture 20.docx

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Department
Biology
Course Code
Biology 1002B
Professor
Denis Maxwell

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Lecture 20: Molecular Convergence 1. Synonymous vs nonsynonymous mutations  Synonymous –mutational change in nucleotide but no effect (neutral) on amino acid 2. Characteristics of the neutral theory of molecular evolution  Selection theory o Mutations could be deleterious advantageous but most are not  Neutral hypothesis o Lots of mutations don’t affect phenotype (synonymous) o Very few are advantageous o Neutral mutations have no selective advantage or disadvantage 3. Relationship between frequency of amino acid substitutions in given proteins vs. time since common ancestor  Use molecular sequence data to see how one species change to another species o The number of differences between protein sequences of different species are proportional to the time since the species diverge 4. Relative rates of accumulation of synonymous vs. non-synonymous mutations and molecular clock  Number of amino acid substitutions per 100 residues vs millions of years since divergence o Fewer amino acids differences if organisms diverge more recently o Straight line rate as a change in function of time  Constant rate = molecular clock  Selection is hard to use as a justification for this constant rate  Useful to learn more about two unknown sequences o Substitution – switching one amino acid for another o Find times of divergence  Synonymous occur faster than nonsynonymous 5. Variables that affect the rate of evolution of a particular protein  Rate of mutation of histone H4 is very slow  Lots of mutation over evolution time for α globin  Why is one mutation rate faster than another? o Histone is more sensitive to change – constraint o You can’t change the protein that much without changing the structure and function  Fibrinopeptide can change a lot since their function does not depend on their 3D shape o Weak constraint since they are function to clot blood 6. Deduce time of divergence given number of amino acid changes in particular protein  Fewer amino acid differences if organisms diverge more recently 7. Characteristics of the "molecular" clock  Genetic equidistance has often been used to infer equal time of separation of different sister species from an outgroup  Number of residue differences between cytochrome C of any two species is mostly conditioned by the time elapsed since the lines of evolution leading to these two species originally diverged o Cytochrome c of all mammals should be equally different from the cytochrome c of all birds o Since fish diverge from main stem of vertebrate evolution earlier than either birds or mammals, cytochrome c of both mammals and birds should be equally different from cytochrome c of fish 8. Regions of two unrelated proteins that would be expected to be similar if they were the products of convergent evolution  Molecular
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