1: Patterns within species diversity Flashcards
Compare polymorphism vs divergence
Polymorphism → Differences between individuals of the same species
Divergence → Differences between individuals of different species
Describe the process of molecular evolution
Molecular evolution = change in allele frequency over time
- Chromosome carries 1 possible allele at any given locus
- Mutation generates a new allele → can be inherited by descendants
- Each new allele starts as a mutation in a single individual
- Frequency of the allele can increase / decrease in each gen (due to selection, genetic drift etc.)
Describe the relationship between polymorphism and divergence
Polymorphism and divergence are 2 interconnected phases of evolution
→Need polymorphism before fixation
What does the neutral theory (Motoo Kimura,1968) hypothesise?
Hypothesises that the fate of most mutations contributing to molecular diversity is determined by drift rather than selection
= Mutations are neutral or sufficiently weakly selected that they behave like neutral variants
Describe briefly how genetic drift decreases variation
Change in allele frequency in smaller pops. → causing alleles to be lost (decreasing variation)
What does the neutral hypothesis provide us with?
Null hypothesis to test data on molecular variation and diversity against
What are SNPs?
Sites where the nucleotide differs between some members of the pop.
Define Nucleotide diversity (π)
= the av. no. of nucleotide differences per site between two randomly chosen DNA sequences in a population
A proportion
If nucleotide diversity (π) = 0, what does this mean?
If π = 0, that means the nucleotide sequences are exactly the same
If nucleotide diversity (π) = 1, what does this mean?
If π = 1, that means that every nucleotide position is different
When we calculate π, we use data from neutral sites in the genome, why?
- To compare between species
= So we know what the normal variation is when there’s just genetic drift (not selection) - Neutral sites = synonymous mutations / non-coding regions → not under selection
The Wright-Fisher model describes alleles in a pop. with … ?
No selection
No mutation
No migration
Non-overlapping gens
Random mating between hermaphrodites
Define effective pop. size (Ne)
no. of individuals that a Wright-Fisher pop. would need to have to produce the same amount of genetic drift as a real pop.
Ne not the same as the Consensus pop size (actual pop. size) → Most of the time its smaller
How does Ne affect diversity levels?
- Loss of genetic variation by drift is faster with smaller Ne
- Pops. with smaller Ne tend to be less polymorphic
- Important for conservation applications
Species with larger Ne and/or higher mutation rate should be more
polymorphic
The Wright-Fisher model predicts that the expected level of diversity at neutral sites is:
(Equation)
