Population Genetics

Question 1

Why is Hardy-Weinberg equilibrium important? What
does it tell us about allele frequencies in a population?

Answer 1

p^2 + 2pq + q^2 = 1

When a population is in HW equilibrium, it is not evolving, and allele frequencies will stay the same across generations

Question 2

Derive the HW equation using the “bucket full of gametes” logic, where gametes are pulled at random out of the bucket

Answer 2

5-gallon bucket full of gametes (egg and sperm). the population here is so large we can imagine it is infinite- thus, when we remove a few gametes it won’t alter the frequencies of alleles in the bucket

-if you pull out two A alleles, the genotype is AA. This will be drawn with frequency p^2
-Aa will be obtained with frequency 2pq (you could draw either an A or a allele first)
-genotype aa will be obtained with frequency q^2

Question 3

What are the 4 assumptions of HW equilibrium that, if violated, result in evolutionary change? These are the 4 microevolutionary processes that change allele frequencies.

Answer 3

Infinite population size
No migration
No mutation
No natural selection

Question 4

Which process changes the frequency of genotypes, but does not directly change the
frequencies of alleles?

Answer 4

non-random mating

Question 5

How many generations of random mating are required to re-establish HW equilibrium?

Answer 5

One generation

Question 6

If a population is in HW equilibrium, and all the assumptions are met, will the frequencies of alleles shift between generations?

Answer 6

No

Question 7

In general, what is a critical value? Be able to
describe “statistically significant” in words. What P value is typically taken as “significant”?

Answer 7

df=1, X^2>(or equal to) 3.841
When P <(or equal to) 0.5, the results are significant

Question 8

What are fitness components?

Answer 8

an individual’s score on a measure of performance is expected to correlate strongly with the genetic contribution to the next generation (e.g., survival, fecundity, etc.)

zygotes –> viability selection –> reproductive adults –> mating success –> fecundity selection

Question 9

Is fitness a description or an explanation?
Can the causes of differences in fitness be studied? What is the relationship between ecology and fitness?

Answer 9

A description
Yes
Fitness is directly determined by the ecological conditions an organism faces.

Question 10

Can natural selection act on a mutation that is not expressed in the phenotype?

Answer 10

No, natural selection cannot directly act on a mutation that is not expressed in the phenotype, as natural selection only operates on observable traits, which are the phenotypic expressions of an organism’s genotype;

Question 11

True/false: Mutation is random with respect to future needs.

Answer 11

True

Question 12

True/false: The environment induces adaptive mutations

Answer 12

False

Question 13

What is a tautology? Why is fitness not a tautology? Don’t confuse tautology with teleology.

Answer 13

Who survives? the fittest. Who are the the fittest? Those that survive.

Because fitness is not intended to be an explanation
-fitness differences can be studied

Question 14

How does Natural Selection impact mean population fitness?

Answer 14

Natural selection consistently increases the mean population fitness over time by favoring individuals with traits that enhance their survival and reproduction

Question 15

What is directional selection?

Answer 15

A mode of natural selection in which a phenotype is consistently favored, causing allele frequencies to shift over time in the direction of that phenotype

The rate of change in allele frequency is a function of the allele frequencies, the relative fitness values of the phenotypes, and the mean population fitness in the population

Question 16

What is eugenetics? From a strictly population genetic standpoint (setting aside for a moment the horrors of implimentation), why is it that eugenics would be
ineffective?

Answer 16

the study and practice of control over the evolution of human populations in an effort to decrease the frequency of undesirable traits

Because it is not impacting fitness

Question 17

Compare and contrast overdominance and underdominance. Be able to calculate the
equilibrium frequencies. Understand the idea of stable vs. unstable equilibria and how this applies to overdominace and underdominance.

Answer 17

Over dominance is simply selection favoring the heterozygote, like with sickle cell anemia and malaria. Underdominance would be harder to maintain because it would require that the recessive and dominant homozygote are selected against equally. If this were the case, there would be no need to distinguish against dominant and recessive alleles.

Question 18

Is underdomance an example of balancing selection?

Answer 18

No, overdominance is

Question 19

How does Dominance vs. Incomplete Dominance influence the amount of time required to fix a selected allele?
Why?

Answer 19

A dominant allele is expressed even in heterozygotes, leading to a quicker increase in its frequency within the population, while incomplete dominance results in a blended phenotype where the advantageous allele is only partially expressed in heterozygotes, slowing down the fixation process

Question 20

Why is mutation important for evolution?

Answer 20

they are the “raw material” of the evolutioanry process
-fundamental to evolutionary theory

Question 21

What is thought to be more common: Mutations that are (1) selectively advantageous, (2) selectively neutral or nearly neutral, or (3) extremely deleterious?

Answer 21

2

Question 22

What is purifying selection?

Answer 22

selection against deleterious mutations
-removed by selection if dominant
-if recessive, mutation/selection balance

Question 23

Describe in words mutation-selection balance. Does mutation-selection balance maintain variation in a population? How does it account for the existance of
genetic diseases?

Answer 23

an equilibrium in the number of deleterious alleles in a population that occurs when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by selection
-the a allele will achieve an equilibrium

mutation without selection will increase the frequency of the a allele

selection without mutation will decrease the frequency of the a allele