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Evolutionary and Population Genetics > Measuring Evolution, Patterns and Models of Sequence Change and The Molecular Clock > Flashcards

Measuring Evolution, Patterns and Models of Sequence Change and The Molecular Clock Flashcards

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Biology 101 flashcards
1
Q

Evolutionary Genetics

A

Broader study of how molecular and population genetics phenomena bring about long-term evolutionary change, including speciation and adaption

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2
Q

Population Genentics

A

Study of the genetic composition of biological populations and changes to this composition

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3
Q

Homology

A

Similarity due to common descent

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4
Q

Problems relying on morphology for reconstructing evolution 3

A
  1. Convergent evolution - independent evolution of shared desired traits that were not present in the LCA
  2. Character reversal - a species may lose the derived trait and revert back to the ancestral form - fleas lost their wings
  3. Erratic rates of morphological evolution
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5
Q

When was DNA identified as a molecule of inheritence?

A

1940-50s

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6
Q

When were tools developed to manipulate and sequence DNA to study genetic variation?

A

1960-70s

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7
Q

What was compared first at a molecular level

A

Proteins then DNA

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8
Q

Molecular homology

A

Molecular similarity due to common descent

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9
Q

How is homology ascertained?

A

Sequence similarity

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10
Q

Hamming distance or degree of divergence

A

The proportion of differences (n/N) for two sequences of length N that differ at n sites

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11
Q

Analogous sequences

A

Sequences that are similar but not homologous, due to chance or due to reoccurring evolutionary processes

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12
Q

All observable differences between homologous sequences are due to

A

Mutation

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13
Q

What is the base-substitution rate across the entire genome in humans?

A

1 mutation every 10^8 base pairs per generation

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14
Q

The Neo-Darwinian Model (Panselectionism) 4 - false

A
  1. selection - strongest force in evolution and drives substitution events
  2. Mutation - ultimate source of genetic variation but only plays a minor role in evolution
  3. Polymorphisms - mainly maintained by balancing selection
  4. Genetic drift - mostly irrelevant
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15
Q

The Neutral theory of evolution 4

A
  1. The majority of new mutations are neutral or deleterious
  2. Neutral alleles have no impact on an organisms fitness and will change in frequency by genetic drift alone
  3. Most substitution events observed occurred by drift, not selection
  4. Negative selection also plays a powerful but silent role, removing deleterious mutations and working to keep the status quo
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16
Q

What does the rate of substitution in a population equal to?

A

The number of new mutations x probability of fixation

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17
Q

What is the rate of substitution for neutral mutations independent of?

A

Population size

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18
Q

Jukes and Cantor’s one-parameter model

A

All substitutions occur with equal probability - there is no bias
3alpha = rate of change of one nucleotide to any other
1-3alpha = probability that nucleotide stays the same

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19
Q

Kimura’s two-parameter model

A

Assumes like-to-like changes are more possible
Transition = pyrimidine to pyrimidine or purine to purine
Transversion = pyrimidine to purine

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20
Q

In vertebrates, how many times more are transitions observed compared to transverions

A

Twice

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21
Q

In mDNA, how many times more are transitions observed than transversions

A

20

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22
Q

What does it probably mean when we observe a high degree of divergence between two sequences

A

Most likely the same nucleotide has undergone multiple substitutions

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23
Q

The simple models of sequences variation assume the probability of substitution is the same across all sites in the sequence of interest. Name two violations to this

A
  1. The probability of mutation occurring can vary across gene or genome regions depending on base pair composition and other factors (chromatin organisation)
  2. The probability of fixation can very across gene or genome regions due to differing strengths of purifying selection
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24
Q

Discuss something that causes variable mutation rates within genomes

A

CpG dinucleotides which are susceptible to methylation - this means they are easily deaminated to give thymine from methylcytosine
The rate of transition substitutions will be higher in sequences with a lot of CG dinucleotides

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25
Q

Discuss the impact of purifying selection

A

Regions under purifying selection evolve slower. Sequences of functional importance are more resistant to substitution
The slowest evolving regions of the genome are protein coding sequences and their regulatory regions

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26
Q

Non-synonymous mutation

A

Nucleotide change that alters the AA sequence

27
Q

Missense mutation

A

Nucleotide substitution that results in an AA change

28
Q

Nonsense mutation

A

Nucleotide substitution that results in a premature stop codon

29
Q

Name two Non-synonymous muutations

A

Missense and nonsense

30
Q

Synonymous mutations

A

Nucleotide change that does not alter amino acid sequence

31
Q

Are nucleotide changes at the first codon position synonymous?

A

Sometimes

32
Q

Are nucleotide changes at the 2nd codon position synonymous

A

Never- they always result AA change, apart from stop codons

33
Q

Are nucleotide changes at the 3rd codon position synonymous?

A

Often

34
Q

Where are substitutions rates lowest?

A

Non-degenerative sites - 2nd codons

35
Q

Where are substitutions rates intermediate?

A

Two-fold degenerative sites - 1st codon

36
Q

Where are substitution rates highest?

A

Fourfold degenerative sites - 3rd codon

37
Q

What is the effect of an AA substition?

A

80% = deleterious
20% = neural and drive molecular clock
~0% = advantageous

38
Q

Whether an AA substitution is deleterious depends on two things:

A
  1. The biochemical properties of the new AA- how similar is it to the AA it replaced
  2. The level of functional constraint - how necessary is this specific AA for the function of the protein
39
Q

What is the biggest AA

A

Tryptophan

40
Q

Are replacements by similar AAs observed more in nature?

A

Yes

41
Q

Example of a protein with functional constraint - Hae

A

Haemoglobin - AA sequence that forms the Haeme pocket is highly conserved, the remainder of the protein only constrained to be hydrophollic

42
Q

Example of a protein with functional constraint -H4

A

Histone 4 - Two copies of H4 required in the histone octamer - almost the whole protein is highly conserved. There are 55 DNA differences in humans and wheat but only two AA differences

43
Q

Example of a protein without functional constrain Fi

A

Cleaved from fibrinogen to activate blood clot formation. Virtually every AA is acceptable at each position as long as it doesn’t hinder cleavage

44
Q

Name one of the fastest evolving proteins

A

Fibrinopeptides

45
Q

Example of a disease that occurs when polymorphism are observed at highly constrains positions

A

Gaucher disease

46
Q

What should we compare when detecting selection?

A

Compare Ka to Ks

Ka = number of non-synonymous substitutions per non-synonymous site
Ks = number of synonymous substitutions per synonymous site

47
Q

Values for detecting selection

A

Ka/Ks < 1 = Non-synonymous substitutions are rare relative to the background mutation rate - suggesting purifying selection
Ka/Ks > 1 = Non-synonymous substitutions are much more frequent than background mutation rate - suggesting positive selection
Ka/Ks ~ 1 = Non-synonymous substitutions occur at the same rate as mutation, possibly neutral selection

48
Q

What is the Ka/Ks for most mammalian genes

A

Ka/Ks < 1

49
Q

What can Ka/Ks >1 indicate

A

Adaptive evolution such as FOXP2 language gene

50
Q

Evolution of non-coding regions

A

Slowest - nondegenerate sites
Intermediate - 5’ flanking regions
Fast - Fourfold degenerate sites, introns and the 3’ flanking region
Fastest - Pseudogenes - best proxy for neutral evolution

51
Q

What is a nondegenerate site

A

any mutation at this position results in amino acid substitution

52
Q

Conserved enhancer region HACNS1

A

HACNS1 increases reporter gene expression in the human forearm, handplate, anterior wrist and thumb and corresponding leg regions
Morphological changes in the hands and feet were vital in human evolution - human gain-of-function

53
Q

Who proposed the Molecular Clock hypothesis?

A

Zuckerkandl and Pauling

54
Q

What is the Molecular Clock Hypothesis?

A

For any given protein, the rate of evolution is constant over time and across all lineages, as long as it retains its original function

55
Q

Name two applications of the Molecular Clock

A
  1. Relative divergence times between species
  2. Evolutionary relationships between species
56
Q

2 ways to find absolute divergence times used to calibrate the molecular clock

A
  1. Radiometric dating methods
  2. Historical dates
57
Q

What is the upper limit of aDNA preservation?

A

2.6 million years - before that the earth would have been too warm for permafrost which is the best environment for preserving very old DNA

58
Q

Calculating the rate of substitution (r)

A

r = K/2T - K is number of subs per site

T= K/2r

59
Q

What is an internal test for evolutionary rate we use to test the molecular clock is correct when there’s an outlier?

A

The relative rate test

60
Q

The relative rate test

A

.A direct internal test of the clock requiring no external data
.Use a third species (C) you know to have branched off earlier than the A and B split.
.Measure the molecular distance from A to C and B to C (number of substitutions)
.Test if the distance from A to ancestor is the same as B to ancestor
.If so, the molecular clock holds and the rate of substitution is the same

61
Q

The molecular clocks holds if (formula)

A

Dac-Dbc=0

62
Q

Causes of variation in the mutation rate

A
  1. Mutagen exposure
  2. Fidelity of DNA repair and replication
  3. Generation times
63
Q

Causes of variation in fixation rate

A
  1. Selection - functional constraints can differ across species
  2. Population size - smaller populations have stronger drift and weaker selection. Larger populations have weaker drift and stronger selection
64
Q

Metabolic Rate Hypothesis

A

smaller-bodied vertebrates generate higher levels of mutagenic oxygen radicals than larger vertebrates - faster in warm-blooded animals

Evolutionary and Population Genetics (7 decks)

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