Lecture 5 - Quantitative variation 3 and Mutation Flashcards
What is artificial selection?
The process by which humans breed plants and animals to select for certain traits
What are the features of natural selection?
- reponse to selection can lead to phenotypes beyond natural variation
- there are limits to the response (no dog has six legs)
What was Johannsen’s unsuccessful selection experiment?
1926
- started with an inbred line of seeds
- bred heavy seeds and light seeds
- there was no response to selection
- later repeated the experiment with a mixture of seeds which led to a response
What does a response to selection require?
- genetic variability of the trait
i. e. the trait needs to be heritable
What factors will influence the strength of selection ?
- the greater the heritability, the stronger the response to selection
- the strength of selection will also affect (the difference in fitness between individuals in the parental population)
How is the response to selection limited and by what?
Often plataeus -no further genetic variation -traits may be linked: on the chromosome (physical linkage) through negative pleiotropic effects
What is pleiotropy?
when one gene influences multiple, seemingly unrelated phenotypic traits
What do response to selection graphs help to visualise?
the different factors that determine the magnitude of the response to selection
What is the formula for the s (selection differential)?
s = Ms - M
What is the formula for the response to selection?
R = M’ - M
How does the mean in the offspring generation often differ from the mean of the selected parents?
The mean in the offspring generation is usually smaller than the mean of the selected parents
-this means that the response is less strong than selection
Why is the reponse to selective breeding in the offspring generation often less strong than selection?
- not all phenotypic variation within the parental generation is due to genetic variation i.e. h2 <1
- heritability is therefore useful for predicting the response to selection
How can heritability help to predict the response to selection?
- not all phenotypic variation within the parental generation is due to genetic variation i.e. h2 <1
- through the breeders equation
What is the breeders equation?
R = h^2*s
How does a response to selection graph look if h2 = 0?
Use breeders equation
R=0
M’ and M are the same
How does a response to selection graph look if h2 = 1?
Use breeders equation
R=s
M’ = Ms
How was heritability and the response to selection used to predict selection on beak size in the Galapagous finches?
Finches hatched in 1976, before the drought
-mean bill depth - (M) = 9.0mm
Birds that survived the drought (parents)
-(Ms) = 10.2mm
Next generation
-(M’) = 9.7mm
Selection differential
Mean selected parents - population mean before drought
s = Ms - M
=10.2mm - 9mm = 1.2m
Predicted response to selection
R = h^2s
R = 0.81.2 = 0.96mm
Predicted mean bill depth next generation, M + R
= 9mm +0.96 = 9.96
Actual measurement 9.7mm
Actual response
-New mean - previous mean = 9.7 - 9 = 0.7
What are the features of mutations?
- help create variation
- understanding the mutation process helps understanding of nucleotide composition of genomes and the difference in it among species (e.g. elevated TA or GC content)
- occur at very low frequencies (although highly variable)
What are the different types of mutation?
Point mutations Insertions and deletions -gene duplication -copy number variation -repeat sequences i.e. microsatellites Inversions Lateral gene transfer
Why use point mutations?
- they are very common
- their mutation process is very well understood
- their effect is reasonably well understood
How much variation is there in SNPs in the human population?
-Human mutation rate = ~ 1 X 10^-9 muations/position/generation
-Diploid genome is ~2X3.2X10^9 nucleotides
-~6.4 mutations/diploid genome/generation
-7 billion people
-7 billion X 6.5 mutations/generation/position
= 44.8 X 10^9 mutations per generation
Large genomes and population sizes mean that there are a lot of mutations in a population
What is the human mutation rate?
-Human mutation rate = ~ 1 X 10^-9 muations/position/generation
How many nucleotides are there in a diplod human genome?
-Diploid genome is ~2X3.2X10^9 nucleotides
What are the two types of effects of mutations on allele frequencies?
Mutations can be
- irreversible
- reversible
What is the result of an irreversible mutation in the absence of other forces?
eventually leads to the loss of the initial allele
What is the formula for the frequency of A (original allele in an irreversible mutation) after time t?
Pt = Po (1 - μ (mutation rate of A to a/generation))^t
What are the results of allele frequencies with a reversible mutation?
A: initial allele with frequency p
Reversible mutation to a (has an initial frequency of q)
-results in an equilibrium of allele frequencies
Can the equilibrium allele frequences of a reversible mutation be calculated?
Mutation rate A to a: μ Mutation rate a to A: v Frequency of A after t generations: pt = pt-1(1- μ) + (1-pt-1)*v At the equilibrium, p=pt=pt-1
P=(v/( μ+v))
