W8 Molecular + W9 sex dimophisms Flashcards
What is the neutral theory?
- Evolutionary changes and variability are the results of random genetic drift operating on mutant alleles that are selectively neutral, or nearly so
- The frequency of alleles is determined largely by stochastic rules rather than pos selection
Which mutations are most likely to be neutral?
- Synonymous Mutations- Where the change has no effect in the amino acid coded for- These mutations are invisible to selection
- Non-Synonymous Mutations- Mutation that does cause a change in the amino acid coded for- Visible to selection
- The third position in a codon is most flexible often synonymous
- Germ-line mutations are inherited- Affected by selection
What is the wright-fisher model in simplifying assumptions ?
- Population of randomly-mating hermaphrodites
- Non-overlapping generations
- No change in population (N adults in each generation)
- Diploid inheritance (2N gene copies in each generation)
- Two types of alleles (A and a): o p is the proportion of genes with allele A o q = 1 – p is the proportion of genes with allele a
- No natural selection
- Poisson distributed offspring numbers
Why don’t most populations conform to the wright-fischer model?
- Population size fluctuates
- Non-random mating
- Non-Poisson offspring numbers
- Other forms of inheritance (e.g., sex-linkage)
- Different numbers of breeding females and males
- Census versus effective population size
What is a molecular clock?
- Postulates that changes in DNA and proteins accrue at approximately constant rates over geological time
This means that the number of ticks in DNA and proteins is roughly the same per generation for any given species
How is a molecular clock calibrated?
Calibrating a molecular clock;
- Take two present day species and their common ancestor
- Measure the number of genetic differences between them / twice the divergence time of A and B = Rate of molecular clock
- For some genes and in some species, this works out Amino acid differences vs. time for pairs of species
What are dating events?
-Significant divergent events in evolution have been dated
Why isnt the molecular clock perfect?
- Hot blooded species and those with small bodies or high metabolic rates tick faster
- Male germlines tick faster
- Ultrafast evolution viruses (no repair function for mutations)
Fixation of Mutations by Random Drift
- The rate of synonymous mutations (ds) is the background/null model for genetic drift
- This can be used to test for when non-synonymous mutations (dn) are affected by selection
How to test for selection?
dn/ds Ratios
•dn and ds are estimates of the number of synonymous and non-synonymous events per a set of compared sequences
•Used for coding regions only
How do chromosomes affect sex ratios?
- The different types of chromosomes spend different amounts of time in the bodies of the sexes
- Autosomes – equal time in both sexes – 1:1
- X Chromosomes – skewed towards females but also present in males – 2:1
- Y Chromosomes – only present in males – 0:1
sex biased mutation
- 3- to 4-fold rate of mutation transmission (human) fathers compared to mothers
- large proportion of mutations are passed on through males
- bias in transmitting mutation
sex biased recombination
General pattern- For some species the heterogametic sex does not undergo recombination
- The rate is generally different between the two sexes
- in mammals, females undergo recombination more
Sex differences in selection
Asymmetric Inheritance
Rates of adaptation on the X chromosome and autosomes
Sex conflict
- Alleles that benefit one sex but, harm the other
- E.g. height, developmental time
- Conflicting selection can maintain genetic variation
- Mutation very beneficial for males/females – becomes fixed within that sex
