lecture 9

Question 1

measuring selection

Answer 1

• selection differential (S): change in mean of trait after selection. ex: S = 10.1 - 9.4 = +0.7Tmax
• response to selection (R): change in mean of trait from one generation to the next. ex: R = 9.7 - 9.4 = +0.3Tmax

Question 2

The Breeder’s Equation R = S x h^2

Answer 2

• an equation to predict the evolutionary response to selection
• observed S = 0.7Tmax
• observed R = 0.3Tmax
• estimated heritability (h^2) = midparent/offspring regression = 0.77
• predicted R = S x h^2 = 0.7Tmax x 0.77 = 0.5TMax
• can also used observed values to calculate realized heritability (h^2) = R/S = 0.3Tmax/0.7Tmax = 0.43

Question 3

Null model for no change in allele frequency across generations

Answer 3

• what happens to the frequencies of two alleles at a single locus when the evolutionary forces are not acting on a population, and where mating is random?
• if allele frequencies are the same between a parental and offspring generation, the no evolution has occurred at that locus.
• serves as a null hypothesis in evolutionary biology and population genetics.

Question 4

Mechanisms of evolutionary change

Answer 4

• evolutionary mechanisms that change allele frequencies in populations:
• selection, mutation, gene flow (migration), genetic drift, non-random mating

Question 5

Hardy-Weinberg Equilibrium

Answer 5

p^2 + 2pq + q&2 = 1.0

• selection violates HWE because it can cause allele frequencies to change generation to generation
• persistent selection can produce substantial changes in allele frequencies over time

Question 6

directional selection

Answer 6

• favors variants of one extreme
• tends to reduce genetic diversity in a population over time.
• changes the average value of a trait in the population

Question 7

stabilizing selection

Answer 7

• favors intermediates; selects against extremes
• reduces population genetic diversity over time.
• the average value of a trait in the population remains the same.

Question 8

disruptive selection

Answer 8

• favors variants of opposite extremes; selects against intermediates.
• overall genetic diversity is maintained over time.
• extreme phenotypes are favored.

Question 9

balancing selection

Answer 9

• no single allele has an advantage; instead there is a balancing among several alleles, with each having advantages in certain situations.
• heterozygote advantage
• selection in variable environments
• frequency-dependent selection

Question 10

heterozygote advantage

Answer 10

• describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype.

Question 11

selection in variable environments

Answer 11

• certain alleles are favored by natural selection at different times or in different places

Question 12

frequency-dependent selection

Answer 12

• certain alleles are favored when they are rare, but not when they are common

Question 13

mutation

Answer 13

• over many generations, mutation can produce changes in allele frequency; violates HWE

Question 14

why do some deleterious alleles persist in a population?

Answer 14

• mutation-selection balance: when the rate at which copies of a deleterious allele are being eliminated by selection is exactly equal to the rate at which new copies are being created by mutation

Question 15

low q hat

Answer 15

= sqrt(u/s)

• equilibrium frequency is low
• mutation rate u is low
• selection coefficient is high

Question 16

high q hat

Answer 16

= sqrt(u/s)

• equilibrium frequency is high
• mutation rate is high
• selection coefficient is low