Lecture 38 Flashcards
What is the basis of the molecular archive?
Biological systems have inscribed in their organization information of their history
Resulting in the evolutionary history of an organism, especially its sequence of divergences can be inferred through comparison of homologous sequences
What information is included in the molecular archive?
The approximate time of existence of a molecular ancestor common to the chains that are being compared
The probable amino acid sequence of the ancestral chain
The lines of descent along which given changes in the amino-acid sequence occurred
How can the approximate time of existence of a molecular ancestor common to the chains that are being compared be determined?
Assessment of the overall differences between homologous peptide chains
How can the probable amino acid sequence of the ancestral chain and lines of descent along which given changes in the amino-acid sequence occurred be determined?
Through a comparison of individual amino-acid residues as found at homologous molecular sites
What is the pan-selectionist view of the molecular archive and clock?
Belief that the speed and direction of evolution are almost completely determined by natural selection with mutation playing only a small role and a belief that neutral mutations were very rare so random genetic drift also played no role
This view was challenged by the molecular archive when new techniques such as electrophoresis, immunoprecipitation, DNA hybridization, protein sequencing emerged suggesting that there was much more genetic variation than phenotypic variation which could not be explained by this theory
How does the theory of natural selection relate to allele frequencies?
A significant proportion of genetic change is caused by the fixation of selectively neutral alleles
With rate of replacement being equal to the mutation rate
There is a roughly constant rate of neutral substitutions over time
What is the probability of a newly arisen mutation replacing the old allele if it is selectively neutral?
Rate that the neutral allele will displace the previous allele by random chance: 1/(2population size)
Rate of appearance of new neutral mutations:
2population*rate of appearance of neutral mutations
Simplified: Rate of Neutral replacement= rate of appearance
What occurs if we plot the number of substitution mutations between two species against time of divergence?
There is a straight line with a slope equal to the rate of mutation
How can adaptive evolution be recognized in the molecular archive?
The theory of neutral evolution predicts that there is an equal ratio of synonymous to non-synonymous mutations
If there has been a period of adaptive evolution then there will be an excess of non-synonymous mutations
What is the molecular clock?
The rate of mutation per year per protein should be relatively constant among diverse lineages
The pattern of this mutation is random over long times
Often overall mutation rates are high with approx one substitution per genome every two years for mammals
This relatively constant rate of mutation allows us to use the clock to infer dates not evident in the fossil record (provided the mutations used are neutral)
What are the complications of the molecular clock?
The relative rate is not constant between all species
Clock is affected by selection
Calculation of the rate is absolutely dependent upon a correct phylogeny
