Lecture 7 - Inbreeding, Molecular Evolution and the Neutral Theory Flashcards Preview

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Flashcards in Lecture 7 - Inbreeding, Molecular Evolution and the Neutral Theory Deck (29)
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1
Q

What are the main importance’s of the bottleneck/founder effect?

A

Reduction in heterozygosity (H) influences fitness immediately after bottleneck
-reduced heterosis, exposure of deleterious alleles
-small initial impact
-influenced by subsequent rate of increase
Reduction in allelic diversity influences ability to respond to long-term selection
-reduced ability to respond to stochastic events
-larger inital impact (on rare alleles)
-less influenced by subsequent rate of increase

2
Q

What is inbreeding?

A

Mating between two related individuals

3
Q

What is inbreeding depression?

A

Loss of fitness due to loss of genetic diversity

4
Q

What are the features of inbreeding

A
  • inbreeding depression
  • homozygosity of 2 alleles by desend
  • increase in homozygosity in a population
  • genetic subdivision of a population
5
Q

Where is inbreeding inevitable and why?

A

Inbreeding is inevitable in small populations
-every individual becomes related quickly becuase it is impossible to find a completely unrelated partner
-

6
Q

What is the formula for the increase in inbreeding in a certain population size?

A

Increase in inbreeding in a random mating population of size Ne is proportional to 1/(2Ne) per generation

7
Q

How does genetic drift affect homozygosity?

A

Increased homozygosity through loss of alleles

8
Q

How does inbreeding affect homozygosity?

A

Increased homozygosity due to change in genotype frequencies

-no change in allele frequencies

9
Q

What two events are likely in a small population?

A
  • Genetic drift

- Inbreeding

10
Q

What two explanations are there for inbreeding depression (loss of fitness due to loss of genetic diversity)?

A

Dominance hypothesis

Overdominance hypothesis

11
Q

What is the dominance hypothesis of inbreeding depression?

A
  • Assumes (partially) recessive deleterious alleles

- Increased number of homozygotes lead to more expressed deleterious alleles

12
Q

What is the Overdominance hypothesis of inbreeding depression?

A
  • Assumes heterozygote advantage/heterosis

- Reduced number of heterozygotes leads to reduced fitness (hybrid vigour)

13
Q

How might inbreeding depression change over time under the dominance hypothesis?

A

Purging the genetic load

  • deleterious recessive alleles will be expressed
  • selection can act
  • this leads to allele frequency changes and removes deleterious alleles from the population
  • increased fitness after successive generations of inbred matings
14
Q

What evidence is there for purging?

A

Mice
-continued inbreeding in mice shows fitness improvement consistent with the dominance hypothesis
Drosophila
-Base population 1250
-30 lineages derived from base population
-maintained a n=20 for 19 generations
-6 purged populations, each maintained n =250 for 3 generations
-Each of the 6 had 15 full-sibling lines
-inbreeding depression decreased in the purged population compared to the base population

15
Q

What are the three formulae involved in loss and fixation due to drift?

A

Frequency of a new mutation: (1/(2Ne))
Chnace of getting fixed = frequency: (1/(2Ne))
Chance of getting lose: 1-(1/(2Ne))

16
Q

What is a mutation (in terms of fixation)?

A

A change that may or may not go to fixation

Mutation rate = μ

17
Q

What is a substitution (in terms of fixation)?

A

A mutation that has gone to fixation and replaced all other alleles at that locus
Substituation rate = k

18
Q

What is the formula for rate of substitution?

A

μ
(Probability of new mutations in a population X probability of fixation of a new mutation)
=> Substituation rate = mutation rate and does not depend on population size

19
Q

What are the features of DNA substitution rates?

A
  • Rates are variable among organisms
  • Rates are variable among chromosomes (and regions within chromosomes) within species (e.g humans and chimpanzees, Y chromosome especially different)
  • rates are variable depending on the type of substitution
20
Q

What are the features of the rate of evolution of the α-Globin gene?

A
  • present in sharks, cats, newt, chicken, echnidna, kangeroo, dog, human etc. (very wide spread)
  • the amino acid substituations over time are a good fit to a straight line
  • this indicates that the rate of α-globin evolution has been relatively constant over the past 450 million years
  • k = 0.9 X 10^-9 amino acid replacements per year
21
Q

What is the molecular clock?

A

When the rate of substitution k = constant

22
Q

What can the molecular clock be used for?

A

-to estimate divergence events, if some divergence events can be dated independently (e.g. fossil record)

23
Q

Give an example of the use of the molecular clock

A

Speciation and the molecular clock on Hawaii

  • Hawaiian islands have been formed in succession
  • Kauai (5.1 Mya), Oahu (3.7Mya), Maui-Nui (1.9/1.6 Mya), Hawaii (0.43 Mya)
  • Mean Cytd distance between the different islands birds’ are a good fit to a straight line against the time since separation
  • Mean Yp1 distance between the different islands drosophila are a good fit to a straight line against the time since separation
24
Q

What are the features of evolutionary rates in different proteins?

A

Data taken from Fibrinopeptides, Hemoglobin, Cytochrome c proteins across species with different divergence times

  • linear lines siggest that rates have not changed over time
  • rates constant among species but differ among proteins
25
Q

Why are the evolutionary rates in different proteins constant among species but differ among proteins?

A

Selection as the main driving force
-more important molecules evolve more rapidly as they are being constantly fine tuned
Neutral theory
-Less important molecules evolve more rapidly because they have fewer selection constraints

26
Q

What expectations are there from the genetic code about the evolution of sites in chromosomes?

A
  • most synonymous changes are in third position, some in first
  • more substituations per nucleotide over a long divergence time at synonymous sites
  • less substitutions per nucleotide over a long divergence time at nonsynonymous sites
  • sites with fewer selection constraints evolve faster
27
Q

What is the neutral theory?

A

The neutral theory argues that the most genetic variation at the molecular level is selectively neutral and is therefore maintained by mutation and genetic drift

28
Q

What type of model is the neutral theory and how?

A

A null model

  • does not deny that all deleterious alleles are selected against
  • does not suggest that all morphological, behavioural or physiological differences are the result of drift
29
Q

What is the likely fate of new mutations?

A

Lost from the population