Lecture 8:
Directional selection and adaptation:
- 3.4 the evolutionary genetics of adaptation
- Insights into mutation frequencies and the size of their effects
- Does adaptation proceed by fixation of many genes of small effect….
- ….Or a few genes with large effect?
Evolution by natural selection:
- Engine: natural selection (drift)
- Fuel: variation; beneficial mutations
- Beneficial mutation are the ultimate source of variation
Fisher’s geometric model of adaptation:
- Fisher’s model
o Two dimensional phenotypic space
o A single fitness maximum
o
- Fisher’s model, three main points
1. Movement towards the optimum unlikely to involve large steps (i.e. macromutations)
a. Black dot is the optimum, and mostly only small steps are made to get to the
optimum.
b. If you take big steps you fall outside the optimum track (overshoot the fitness
maxima).
c.
,2. Kimura, the chance of a mutation fixing in a population is related to its selective
advantage ( Qfix ≈ 2hs
a. Intermediate effect-size mutations
b.
3. In an entire sequence of successive steps, the distribution of effect sizes is approximately
exponential in form
a. Many small-effect mutations
b. A few large ones the tend to be fixed early on
i. And can help in the beginning to get on track to the optimum.
c. The mean effect size decreases with the number of dimensions in phenotype: it
becomes less likely to move to the optimum
i. More traits less degrees of freedom. This also shows that you have many
small steps and a few large.
ii. More traits to cater for; remember developmental constraints
iii.
, Beneficial mutations are very rare; the model aspergillus nidulans
- A. nidulans was placed on a plate with anti fungal
MGR = Milecium growth rate
Directional selection and adaptation:
- 3.4 the evolutionary genetics of adaptation
- Insights into mutation frequencies and the size of their effects
- Does adaptation proceed by fixation of many genes of small effect….
- ….Or a few genes with large effect?
Evolution by natural selection:
- Engine: natural selection (drift)
- Fuel: variation; beneficial mutations
- Beneficial mutation are the ultimate source of variation
Fisher’s geometric model of adaptation:
- Fisher’s model
o Two dimensional phenotypic space
o A single fitness maximum
o
- Fisher’s model, three main points
1. Movement towards the optimum unlikely to involve large steps (i.e. macromutations)
a. Black dot is the optimum, and mostly only small steps are made to get to the
optimum.
b. If you take big steps you fall outside the optimum track (overshoot the fitness
maxima).
c.
,2. Kimura, the chance of a mutation fixing in a population is related to its selective
advantage ( Qfix ≈ 2hs
a. Intermediate effect-size mutations
b.
3. In an entire sequence of successive steps, the distribution of effect sizes is approximately
exponential in form
a. Many small-effect mutations
b. A few large ones the tend to be fixed early on
i. And can help in the beginning to get on track to the optimum.
c. The mean effect size decreases with the number of dimensions in phenotype: it
becomes less likely to move to the optimum
i. More traits less degrees of freedom. This also shows that you have many
small steps and a few large.
ii. More traits to cater for; remember developmental constraints
iii.
, Beneficial mutations are very rare; the model aspergillus nidulans
- A. nidulans was placed on a plate with anti fungal
MGR = Milecium growth rate
