Chapter 25 - Population Genetics

Question 1

What is genotypic frequency and how do you determine it?

Must know equation

Answer 1

The proportion of a particular genotype within a population

Add number of individuals with a specific genotype and divide it by the total number of individuals in a population (N)
f(AA) = # of AA individuals/N
f(Aa) = # of Aa individuals/N
f(aa) = # of aa individuals/N

f(AA) + f(Aa) + f(aa) = 1

Question 2

What is allelic frequency and how is it determined?

Must know these equations

Answer 2

The proportion of a particular allele within a population
- Always fewer alleles than there are genotypes
- Number and types of alleles have more continuity from one generation to the next
- Alleles not necessarily changing but combinations of alleles to express a phenotype changes

Calculate in 2 ways:
Number of genotypes in population
- p = f(A) = 2nAA + nAa/2N
- q = f(a) = 2naa + nAa/2N
- p + q = 1

Frequencies of genotypes in a population
- p = f(A) = f(AA) + 1/2 f(Aa)
- q = f(a) = f(aa) + 1/2 f(Aa)
- p + q = 1

Question 3

What is the Hardy-Weinberg Law?

Answer 3

A way to segregate the alleles in gamete formation and combine alleles in fertilization to determine how it influences the gene pool, in a theoretical manner

Assumption: the population is large, randomly mating, not affected by mutation, migration, or natural selection

Prediction 1: the allelic frequencies of the population do not change
- Allele frequencies determine the frequencies of the genotype

Prediction 2: the genotypic frequencies will stabilize after one generation

Question 4

What is Hardy-Weinberg equilibrium?

Answer 4

When assumptions are met, reproduction alone does not alter allelic or genotypic frequencies, and the allelic frequencies determine the frequencies of genotypes

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

Question 5

What are the assumptions of the HW law?

Answer 5

HW assumes that the population is infinitely large at the theoretical level
- In practice, many large populations have genotypes that are predicted within HW
- Deviations from HW tend to occur in small populations

Members of the population randomly mate with respect to genotype; therefore, each genotype mates relative to its frequency
- If the frequency of AA is 0.6, AA mating to AA will occur at a frequency of 0.6
- 0.6 x 0.6 = 0.36
- There is a 36% chance that an AA will mate with an AA

Allelic frequencies of the population are not affected by natural selection, migration, or mutation (microevolution forces)
- Rate of mutation is so low that it tends not to affect HW
- While natural selection and migration are regular in populations, the HW law only examines the effect of reproduction in the gene pool
- When that effect is known, other factors like selection and migration are examined
- HW can be used as a baseline/control to see why populations are out of HW equilibrium

HW law applies only to a single locus
- Nearly all populations don’t mate randomly for all loci and natural selection affects nearly all populations at some loci

Question 6

What is microevolution?

Answer 6

Changes in an allele frequency in a population over time

Question 7

What are the implications of HW law (6)?

Answer 7

Within HW equilibrium, if allele frequencies are stable, a population cannot evolve

When a population is in HW equilibrium, the genotypic frequencies are determined by the allelic frequencies

The frequency of the heterozygote is at a maximum when allelic frequencies are equal (at 0.5)

When the frequency of one allele is low, the homozygotes for that allele will be rare
- Most of the copies of that allele will be present in the heterozygote

When the frequency of one allele is high, most of the individuals are homozygotes

When a population is not in HW equilibrium, one of the assumptions has not been met

Question 8

What are the proportions of genotypic frequencies of X-linked female genotypes?

Answer 8

p^2

2pq

q^2

Question 9

What are the proportions of genotypic frequencies in X-linked male genotypes?

Answer 9

p and q

Question 10

What are the genotypic frequencies for genes with 3 alleles?

Answer 10

p^2

2pq

q^2

2pr

2qr

r^2

Question 11

How can you estimate allelic frequencies with the HW law?

Answer 11

Take square root of a given frequency for homozygote (p^2 or q^2)

Subtract that from 1 to determine the other frequency

Calculate other frequencies as above

Question 12

What is the effect of nonrandom mating?

Answer 12

Affects the way in which alleles combine to form genotypes and alters the genotypic frequencies of a population

Does not affect frequency of alleles

Question 13

What are the 2 types of nonrandom mating?

Answer 13

Positive assortative mating – tendency for like individuals to mate

Negative assortative mating – tendency for unlike individuals to mate

Question 14

What is inbreeding and what is its effect out of HW equilibrium?

Answer 14

Preferential mating between two related individuals

Positive assortative mating for relatedness

Affects all of the genes in an individual

Effect: tends to lead to an increase in homozygotes in a population and a decrease in heterozygotes

Question 15

What is outcrossing?

Answer 15

Preferential mating between two unrelated individuals

Question 16

What is the inbreeding coefficient?

Answer 16

Measures the probability that two alleles are identical by descent

F = 0 - mating is occurring randomly
F = 1 - all alleles are identical by descent

Decreases proportion of heterozygotes by 2Fqp in each subsequent generation
- f(AA) = p^2 + Fqp
- f(Aa) = 2pq - 2Fqp
- f(aa) = q^2 + Fqp

Question 17

What are the implications of inbreeding?

Answer 17

Inbreeding depression - harmful to outcrossing species because it increases homozygotes and frequency of lethal or deleterious alleles in a population

Favorable - if inbreeding for many generations, all individuals will eventually be homozygous and all deleterious alleles will be weeded out, leaving the population homozygous for only beneficial alleles

Co-adapted gene complexes - at molecular level, can help preserve groups of genes that can exhibit different gene interactions that work well together in different environments

Question 18

What are the 4 processes that change allelic frequencies (microevolutionary forces)?

Answer 18

Mutation, migration, genetic drift, and natural selection

Question 19

How does mutation affect allelic frequencies?

Answer 19

Affects the rate at which one genetic variant increases at the expense of another

Amount of change dependent on mutation rate and amount of first allele in the population
- Amount of first allele is large = large amount of change
- As allele frequency of first allele decreases as a result of mutation, the change in frequency due to mutation will also decrease

Reverse mutations take 2nd allele back to first allele – occurs at a different rate than forward mutations
- Allele frequency of 2nd allele decreases and frequency of 1st allele increases

The effect of typical mutation rates on HW equilibrium is negligible and requires many generations
- If mutation is the only force acting on a population for long periods of time, mutation rates will eventually determine allele frequency (normally isn’t the only force)

Question 20

What is migration?

Answer 20

AKA: Gene flow

Movement of genes from one population to another

Changes allele frequencies of a population

Question 21

How does migration affect genetic variation?

Answer 21

Decreases genetic differences between populations

Increases genetic variation within populations

Question 22

What is genetic drift?

Answer 22

Change in allelic frequencies in a population from generation to generation due to random chance/events

Question 23

What is sampling error?

Answer 23

The deviation from an expected ratio due to low sample size

Arises when gametes unite to produce progeny, especially when population size is small

Chance influences which alleles are present in the next generation

Direction is unpredictable but the magnitude is predictable

Question 24

How is the magnitude of genetic drift determined?

Answer 24

By examining the change in allelic frequencies within a single population or by examining the magnitude of genetic difference that accumulate among populations

Within a single population, the frequency of an allele can increase or decrease by random chance

Between multiple populations, the allele frequencies will become different between the populations over time due to random chance

Estimated from the variance in allelic frequency (s^2)
- Drift is maximal when p and q are equal
- Drift is maximal when population size is small
- s^2 = pq/2N

Question 25

What is population size?

Answer 25

The number of “breeding” individuals within a population

AKA: Effective population size

Question 26

What are breeding individuals?

Answer 26

Those that contribute genes to the next generation

Question 27

What are the 3 causes of genetic drift?

Answer 27

Population reduction

Founder effect

Genetic bottleneck

Question 28

What is population reduction?

Answer 28

Decrease in population size due to limited food, space, or other resources in the population’s environment

Those who survive can have a greater impact on the allele frequency of the population

Question 29

What is the Founder effect?

Answer 29

Establishment of a population by a smaller number of individuals

If a few individuals break off from the main population and start a new population, it will be random what their allele frequencies will be in the new populations

While it can eventually grow large, the genes carried by all of its members are derived from very few genes present in the founders

Random chance plays a role in which genes are found in founders of the new populations

Question 30

What is a genetic bottleneck?

Answer 30

Develops after a population undergoes a drastic reduction in population size

It’s by chance which individuals are left in the population that survive bottleneck event

Question 31

What are the effects of genetic drift (3)?

Answer 31

Change in allelic frequencies within a population
- Because random, frequency is just as likely to increase or decrease
- Over time, allele frequency will wander

Reduction in genetic variation within populations (fixation of allele)
- Over time, allele frequency may be 1 or 0 (homozygous for one allele or the other)
- 0 = allele no longer exists in population
- 1 = all individuals in population have the allele

Population divergence between populations
- Because drift is random, allele frequencies don’t change the same in each population, which allows populations to individually acquire genetic differences over time

All of these occur simultaneously in a population - result of sampling error

Question 32

What is natural selection and when does it occur?

Answer 32

The differential reproduction of genotypes

Occurs if individuals with adaptive traits produce a greater number of offspring than individuals without the trait
- If adaptive advantage has some sort of genetic basis, it will be inherited by offspring
- When inherited, they’re going to appear with greater frequency in the next generation

Question 33

What is fitness (W)?

Answer 33

Reproductive success of a genotype relative to that of other genotypes in a population

Question 34

How is fitness calculated?

Must know this equation

Answer 34

W = mean number of offspring produced by genotype / mean number of offspring produced by the most prolific genotype

Question 35

What is the selection coefficient (s) and how is it determined?

Answer 35

Relative intensity of selection against a genotype

s = 1 - W

Question 36

How does fitness impact allelic frequencies?

Answer 36

Differential fitness among genotypes leads to changes in the frequencies of genotypes over time, which in turn leads to changes in allelic frequencies that make up the genotypes

Question 37

What is the general selection model used for and what are the required components?

Answer 37

Can determine changes in allele frequency due to natural selection

Requires: knowledge of initial allele frequencies of population and the fitness values of the genotypes

Assumes mating is random and that only force is natural selection

Can be used to calculate allele frequency after any type of selection is acting on a population

Can also determine if change in allele frequency after selection is against recessive, dominant, or codominant traits, and traits in which heterozygotes have a larger fitness

Question 38

What are the 3 types of selection?

Answer 38

Directional selection - one allele/trait is favored over another
- Expressed on graph as peak moving to the left or right of the bell curve
- Selection can be against recessive, dominant, or codominant traits

Overdominance - heterozygote has a higher fitness than either homozygote (stabilizing selection)
- Expressed as peak in the middle of bell curve
- Called heterozygous advantage

Underdominance - heterozygote has a lower fitness than either homozygote (disruptive selection)
- Expressed as 2 peaks - one on left and one on right with a dip in the middle

Question 39

What are the long term effects of each microevolutionary force?

Answer 39

Mutation - equilibrium reached between forward and reverse mutations

Migration - equilibrium reached when allelic frequencies of source and recipient population are equal

Genetic drift - fixation of one allele

Natural selection
- Directional selection - fixation of one allele
- Overdominant selection - equilibrium reached
- Underdominant selection - unstable equilibrium