Microevolution: The effects of selection

Know the uses and limitations of mathematical models

They can help us understand how different modes of selection will cause allele frequencies to change but are simplified approximations of reality.

Be able to calculate the fitness and selection coefficients of various genotypes in a population

Relative fitness = survival rate compared to most fit genotype = w

1 – w = s = selection coefficient (in other words, the relative amount a genotype is being selected AGAINST)

Define directional selection

Selection against one homozygote

Describe how directional selection affects allele frequencies over time

1. total elimination of recessive

as q approaches 0, (delta)q slows considerably, even if fitness is 0

(note that you can read graph in handout as “reverse of how p is changing frequency”)

2. partial elimination of recessives

when q is 1/3 eliminated, rate of loss slows considerably

in both cases, allele selected against eventually becomes extinct, allele selected for eventually becomes fixed

Know the meaning of the following examples

fruit flies in alcoholic environment (text)

selection causes change from Hardy Weinberg expectations

CCR5-D32 allele (text)

We can use mathematical models to predict rate of evolution

flour beetles (text)

Model predictions are accurate

compulsory sterilization programs (text)

Models teach us surprising things about selection

Define stabilizing selection

Selection for heterozygote (against both homozygotes)

Describe how stabilizing selection affects allele frequencies over time

Tends to keep genetic polymorphism in population

Sickle cell anemia, Tay-sachs alleles (lecture)

fruit flies with V and L alleles (text)

Define disruptive selection

Selection against the heterozygote

Describe how disruptive selection affects allele frequencies over time

Tends to drive one allele to fixation (each can be given a probability of extinction based on relative fitness of genotypes)

Limpet population (lecture)

Fruit flies with chromosomal difference (text)

Be able to apply models to calculate allele frequencies in the next generation of a population experiencing selection. Be able to calculate equilibrium allele frequencies where appropriate.

When given formulae. In other words, you should be able to determine from raw data on survival or reproduction which type of selection is occurring and you should know which numbers you calculate correspond to which symbols in an equation. You are not expected to memorize the equations.

Define frequency dependent selection

Selection for the less common phenotype

Describe how frequency dependent selection affects allele frequency

Color polymorphism in Elderflower orchid (text)