Lecture 03: Natural Selection

Question 1

Modes of Selection

Answer 1

1) Directional selection -> favours one allele or value of a quantitative trait

2) Stabilizing selection -> reduces variation by favouring intermediate traits (Birth weight in humans)

3) Disruptive selection -> increases variation by favouring extreme values (Black bellied seedcracker- Birds with intermediate size bills have lowest survival)

4) Truncation selection -> only ones with highest trait value are reproducing

5) Balancing selection -> maintains polymorphism and includes frequency-dependent selection when rare alleles are favoured

Truncation: Abschneiden

Question 2

Negative selection

Answer 2

mutation or trait is counterselected (disadvantages traits decreases in frequency over time)

Question 3

Positive selection

Answer 3

mutation or trait is favoured, because it results in a fitness advantage

Question 4

Soft selection vs. hard selection

Answer 4

soft selection:
everybody survives, some are favored

hard selection:
only survival of the fittest

Hard selection
When natural selection is actually substituting one gene for another, it does so by means of extra mortality, on top of the background mortality which exists anyway. This is called hard selection. If selection requires extra mortality, it will tend to depress the population size. The rate of evolution, therefore, has an upper limit: if selection is too strong it will depress the population size to zero.

Soft selection
Under soft selection, the selective deaths are substituted for non-selective background mortality. If all the mortality suffered by a population was selective, evolution could proceed at an immense speed.

Hard and soft selection are extremes of a continuum and in practice, natural selection in different cases could be any mixture of hard and soft selection.

Question 5

Sexual and artificial selection in Asian Elephant

Answer 5

-Artificial selection favours phenotypes selected against by sexual selection
-Artificial selection dramatically changes the operational sex ratio

*Artificial selection is the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations.

Question 6

What mode of selection acts on the major histocompatibility complex class II (MHC-II) genes and why is that?

Answer 6

polymorphism is extremely high in the antigen binding sides of MHC class II proteins as they play an important part in the T-cell mediated immune response against extracellular pathogens
- variation on the MHC-II genes translates into variability in the antigen binding sites
- this variation is maintained by balancing selection through sexual selection

Question 7

Explain the negative frequency dependent selection hypothesis on mimicry

Answer 7

• mimetic polymorphism -> the abundance of mimics is limited by that of the models
• if the model is relatively more abundant, the defensive benefit of mimicry is thought to increase
• this benefit decreases as the abundance of mimics increases relative to that of the models
• eventually, the mimicry ratio approaches an equilibrium, at which the fitness of mimic and non-mimic types are equal
• this equilibrium corresponds to the local abundance of the model