Lecture #26 - Population genetics

Question 1

Define population and gene pool

Answer 1

Population: localised group of individuals of the same species

Gene pool: total aggregate of genes (and their alleles) in the population at one time

POPULATIONS EVOLVE, INDIVIDUALS DON’T!

Question 2

1. What happens to allele frequencies in a population over time?
2. Do recessive alleles eventually get lost from a population

Answer 2

1. Allele frequencies remain constant over time unless acted upon by evolutionary forces
2. NO

Question 3

Assumptions of the Hardy Weinberg Theorem (5)

Answer 3

1. Infinite population size - this is requirement due to the theoratical prediction that only an infinite population will avoid “sampling error” or “sampling effects.” In practise, a very large pop approximates an infinite population mathematically.
2. No mutations - alleles themselves do not change from one generation to the next by mutating to a new form (or forward and backwards mutation rates must be equal)
3. No natural selection - No fitness differences among genotypes (no slection)
4. No migration - no new alleles are introduced or removed from the population
5. Random mating - Males and females find mates at random and no other factors cause mates to be paired

Question 4

What’s the equation for allele frequency?

What’s the equation for genetypic frequency?

What about 3 alleles?

Answer 4

Allele frequency p + q = 1

Genotypic frequency p² + 2pq + q² = 1

p² = AA

2pq = Aa

q² = aa

Three alleles:

Allele frequency p +q +r = 1

Genotypic frequency p² + r² + q² + 2pq + 2pr + 2qr = 1

Question 5

Why is the Hardy thing important?

Answer 5

This enables us to estimate proportions of genotypes and alleles in the population

Question 6

If the proportion of X chromosomes with the allele for haemophilia (a) in a population
is 0.4, how many females will be affected
by the disease? And how many males?

Answer 6

Xa = q = 0.4
Females = XaXa = q2 = 0. 42 = 0.16 or 16%

Males = XaY = q = 0.4 = 40%

Question 7

Allele frequencies change via…..(7)

Answer 7

• Non - random mating

• Assortative mating
• Inbreeding
• Random genetic drift
• Bottleneck effect
• Founder effect
• Natural selection (this is the inly microevolutionary process that is adaptive)
• Gene flow or migration
• Mutation

Question 8

Random Genetic Drift

1. Definition
2. May include what? Most likely to have effect in what?
3. Compare large vs small populations

Answer 8

A random change in allele frequencies due to sampling error over generations

The change in allele frequencies in pops due to chance (not selection) - may include loss of alleles from gene pool. Most likely to have effect in small populations

When populations are large and mating is random, alleles’ frequencies tend to remain stable from generation to generation unless natural selection is operating. However, when populations are small or become small, it is likely that allele frequencies will change from generation to generation by chance and so drift into an increase or decrease (or even complete loss). This has nothing to do with natural selection.

Question 9

The Bottle Neck effect

1. Defintion (likely to be due to…(2))
2. Because pop no. has fallen, it’s likely that…(2)

Answer 9

Huge mortality event thing happened and only a few of the genes are left

Population may be suddenly reduced in number to a smal size. This typically occurs as a result of the following:

• A catastrophic event that indiscriminately removes individuals
• Human action - habitat destruction

Bc pop no. fall, likely that the range of alleles will decrease and the frequencies of alleles will change.

Question 10

Founder effect

1. Defintion
2. Range of frequencis are likely/unlikely to be representaitve because….(2)
3. Bc it’s small, it’s subject to……what occurs at a faster rate?

Answer 10

A founder pop refers to a small group of individuals that colonise a new isolated area such as an island.

Range of frequencies unlikely to be repersentative because it’s small and smaller range of alleles now (some alleles might have been lost) and others less frequent.

Bc founder small - it’s subject to genetic drift and has reduced range and freq of alleles. Evolution likely to occur at a faster rate

Question 11

Stabalising selection

(type #1 of natural selection)

Answer 11

Middle favoured

Question 12

Directional selection

Answer 12

Eg giraffe’s necks

Question 13

Disruptive selection

Answer 13

Question 14

Appreciate

Answer 14

Question 15

Give an example of Disruptive selection

Answer 15

West African black bellied seed crackers display two bill morphologies

Question 16

Sexual selection

Answer 16

Normally females choosing favourable, attractive traits

Question 17

Frequency dependent selection in Perissodus microlepis:

Answer 17

Natural selection maintains an equal proportion of left and right mouthed individuals in the population

If ratio changed - the fish will know which side to look out for

Question 18

Geographic distribution of genetic variation

1. Example
2. What’s a cline?

Answer 18

1. Cyanide production in clover Trifolium repens
2. The gradual change in proportions is termed a cline
   * Selection can vary over space*
   * Like a gradient of variation*

Question 19

Mutation and migration

1. Definition of each
2. 4 things associated with migration

Answer 19

• Mutation: very slow to act and usually disadvantageous, its role is usually of macroevolutionary proportions
• Migration: an individual from another population successfully mates (i.e. contributes gametes) to the gene pool
  • brings new alleles
  • changes proportions of existing alleles
  • changes population size
  • makes two pops more similar

Question 20

What’s a fixed allele? (3)

Answer 20

• A fixed allele is an allele that is the only variant that exists for that gene in all the population.
• A fixed allele is homozygous for all members of the population.
• The term allele normally refers to one variant gene out of several possible for a particular locus in the DNA. When all but one allele go extinct and only one remains, that allele is said to be fixed.

Question 21

What is the cause of genetic drift?

Answer 21

Random forces lead to genetic drift. Sometimes, there can be random fluctuations in the numbers of alleles in a population. These changes in relative allele frequency, called genetic drift, can either increase or decrease by chance over time.

Question 22

What is assortative mating?

Answer 22

Assortative mating is a mating pattern and a form of sexual selection in which individuals with similar genotypes and/or phenotypes mate with one another more frequently than would be expected under a random mating pattern.