1: Interspecific divergence and selection Flashcards
Describe Fixation probability without selection
Under Wright-Fisher model:
- The probability of fixation of a new allele is equal to its initial frequency in the population
- This is WITHOUT selection
What is the fixation probability of a neutral mutation?
1/(2N)
What is the fixation probability of a beneficial mutation?
> 1/(2N) → the more beneficial the bigger the increase
What is the fixation probability of a deleterious mutation?
<1/(2N) → more deleterious bigger the decrease
In a pop with N (no. of diploid individuals), the expected no. of new mutations arising each gen =
2Nu
(2 because 2 chromosomes)
What is the fundamental equation of neutral molecular evolution
λ = 2Nu X 1/(2N) = u
λ = rate of substitution
These things basically cancel each other out: so the bigger our pop. is the more mutations there will be, but the lower the chance of these being fixed due to being at low frequency. Drift stronger in smaller pop.
What does rate of accumulation of substitutions dependent on and what is it independent of?
Dependent on neutral mutation rate
Independent of population size
Neutral theory predicts a —— relationship between T and K
linear
K is proportional to the mutation rate (the more genetically distant they are, the more genetic differences they will have accumulated)
Divergence between species
E(K) = 2Tλ = 2Tu
What does K stand for?
K = expected no. of substitutions per site between 2 homologous DNA sequences from 2 species
E(K) = 2Tλ = 2Tu
What does T stand for?
T = Time in generations
How do we measure divergence (K)?
compare 2 homologous sequences and calculate the proportion of nucleotide sites that are different
Describe molecular clock
used to estimate the times of divergence of taxa when palaeontological data are absent
T = K/(2u)
