Population genetics

Question 1

Hardy-Weinburg (H-W) equilibrium

Answer 1

In a population where there is 
 Random mating
 No natural selection  No mutation
 No migration
 No genetic drift
 serves as a model to demonstrate allelic and genotypic frequencies in the absence of evolution in a population

Question 2

Allelic frequencies

Answer 2

are stable at p + q = 1 for two alleles  = copies of one allele / sum of all alleles

Question 3

 Genotypic frequencies

Answer 3

are distributed according to p2 + 2pq + q2 = 1  = number of progeny of one genotype / total number of progeny

Question 4

Population

Answer 4

a group of interbreeding organisms

Question 5

gene pool

Answer 5

is the collection of genes and alleles in this population, distributed into
genotypes
A different population will have a different gene pool

Question 6

What are the assumptions under HW?

Answer 6

1. Population size is infinite (most finite populations still uphold the rest of the assumptions)
2. Random mating occurs in the population (ie no sexual selection/preference)
3. Natural selection does not operate
4. Migration (i.e. gene flow) does not introduce new alleles
5. Mutation does not introduce new alleles
6. Genetic drift is not occurring

Question 7

i

Answer 7

As p decreases, q increases and vice-versa

 Heterozygotes have a max frequency of 50% (p = q = 0.50)

Question 8

Natural selection

Answer 8

is the driving mechanism for evolution over time
Will change allelic frequencies in a population -> ie no longer H-W!

Results from differential reproductive success of individuals in a population
 Ie. No longer random mating!
 Individuals that leave more offspring distribute more copies of their alleles in the next generation
Increases the frequency of certain alleles and decreases the frequency of others

Question 9

Differential reproductive fitness

Answer 9

“favours the most fit”
Traits passed to progeny from more successful reproducers
Traits are not present in individuals with lower fitness
Fitness measured at the individual level

Question 10

Relative fitness w

Answer 10

quantifies the reproductive success of a genotype compared to the most favoured genotype in a population
 Not measured on individuals
 Genotypes with the greatest fitness have w = 1  Genotypes less favoured have w < 1

Question 11

selection coefficient (s)

Answer 11

reduces relative fitness

Question 12

Directional natural selection

Answer 12

shifts the phenotypes in the population to the homozygous genotype
Have higher relative fitness than the other genotypes
 Increases the allelic frequency of the favoured allele, and decreases the
frequency of the other one

Question 13

What conditions does natural selection require?

Answer 13

Varying phenotypes
 Genetic variation is heritable
 More offspring are born than will survive to maturity -> “Struggle for existence”  Some genetic variants produce more offspring that others

Question 14

i

Answer 14

Eventually, directional selection can “fix” an allele (ie p = 1, q = 0)
 Never really gets fixed though since other factors (migration, mutation, etc) can shift allelic frequencies
 The stronger the natural selection pressure, the larger the shift in alleles

Question 15

Directional natural selection against a recessive trait

Answer 15

Dominant allele will increase and
the recessive allele will decrease
 Recessive alleles will reduce more slowly the less there are

Question 16

Balance polymorphism

Answer 16

Alleles reach an equilibrium
 Selective pressure favours maintaining heterozygote but selects against homozygous recessive
Natural selection against bb
 Will lead to a stable equilibrium
Unless natural selection pressures change, allele

Question 17

Heterozygote advantage

Answer 17

Directional natural selection favouring heterozygotes
Ex hemoglobin
 Variants in the beta globin proteins cause sickle-cell disease
ss, ee, cc variants cause anemia and sickle-cell disease
 Heterozygotes for the variants result in some deformed cells, but also some resistance to malaria
Ss, Ee, Cc

Question 18

Mutations

Answer 18

Mutations are slow because they can affect an allele in 2 directions
 Forward mutation rate (μ) creates new A2 alleles by mutating A1
 Reverse mutation rate (v) changes A2 alleles by mutation to A1 Can create a balanced equilibrium in the absence of other factors

Question 19

mutation-selection balance

Answer 19

Natural selection removes the recessive trait, but mutation keeps it in the population -> mutation-selection balance

Question 20

Gene flow

Answer 20

moves alleles into and out of populations
 Introduction of novel alleles can increase allelic frequencies already present
Admixed populations -> addition of new organisms into an existing population
 Individuals moving out can reduce the allelic frequency

Question 21

Island model

Answer 21

> one way flow of genes/individuals
 1-m (m = migrants)
 pN = (1-m)(pI) + mpC

Question 22

Describe the relationship between gene flow and diveregnce

Answer 22

Increase gene flow= low divergence

low gene flow= increased divergence