L10 Patterns of Selection and Side Effects

Balancing selection

occurs when no single allele has a distinct advantage

consequence: selecting on different alleles is balanced

ex:

• Negative frequency dependent selection

• heterozygote advantage (overdominance)

balancing selection does what to genetic diversity

maintains genetic diversity

Negative frequency dependent selection

occurs when the fitness of alleles change depending on their own frequencies - rare alleles are the most fit

selects for the least frequent - it is most fit but eventually after generations of selection it becomes less beneficial and the other allele that is now the rare one is the most beneficial

• increases in frequency and then is no longer rare - the other allele is now most frequent

Heterozygote advantage or overdominance

occurs when the heterozygote has higher fitness than both homozygote

when overdominance alleles reach

a polymorphic equilibrium

polymorphic equilibrium

both alleles are maintained an equilibrium

how does overdominance maintain genetic variation

heterozygote disadvantage - underdominance

occurs when heterozygotes are the least fit of all genotypes

with underdominance if the beneficial allele starts below certain threshold

they will be lost

with underdominance if the beneficial allele starts above certain threshold

they will fix

artificial selection

a selection by humans of a specific trait in another organism (sometimes called selective breeding)

this is different from natural selection in that survival and reproduction (fitness) is dependent on human preference for a chosen trait

• the trait may not be beneficial in the wild

genetic correlations

occur when two traits tend to be inherited together

genetic correlation occur through

• pleiotropy

• evolutionary trade-offs

• genetic hitchiking

direct response

selecting for

indirect response

selection of - side effect

pleiotropy

a single mutation affects multiple traits

• generates a genetic correlation between traits

epistasis

interactions of multiple loci to produce a single phenotype

Allele A1 is pleiotropic. Individuals with the A1 allele have pink fur and purple eyes. Purple eyes are advantageous, they allow individuals to see better at night. Pink fur is neither beneficial nor deleterious. As the A1 allele spreads through the pop

a. purple eyes only will fix

b. pink fur only will fix

c. purple eyes and pink fur will fix

d. pink fur will increase but not fix, purple eyes will fix

purple eyes and pink fur will fix

evolutionary trade-off

occurs when one trait has fitness benefits and fitness costs

• ex. frogs with the loudest mating call attract the most mates and the most predators

evolutionary trade-off can occur as a side effect of pleiotropy

a single allele increases fitness through one trait and the same allele decreases fitness through another trait

Bighorn sheep with the dominant Ho+ allele have increased mating success. However, sheep with the recessive Hop allele have increased survival. Use this information and the information in the figure to determine if genetic variation at this locus will

a. decrease over time

b. increase over time

c. be maintained over time

be maintained over time

• polymorphic equilibrium - overdominance

hitchhiking

occurs when an allele at one locus spreads by natural selection acting on a linked allele at a second locus

hitchhiking is a consequence

of linkage disequilibrium

• when a beneficial allele spreads to fixation it can eliminate polymorphism nearby

• this is a signature of selective sweep