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Evolution > Natural Selection > Flashcards

Natural Selection Flashcards

1
Q

What is natural selection?

A

Any consistent difference in fitness among different classes of organisms

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2
Q

What is fitness?

A

The number of offspring an individual leaves in the next generation (also reproductive success; includes survival and reproduction)

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3
Q

What is required for evolution by natural selection?

A
  1. A phenotypic variation must exist in the population (NS needs something to act on)
  2. The variation must cause fitness differences between individuals who have it and those that do not
  3. Variation must be heritable (the phenotype is connected to the genotype)
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4
Q

What’s the difference between evolution and natural selection?

A

Evolution is the variation between and within species whereas natural selection is one of the contributing forces driving evolution

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5
Q

Describe the concepts of alleles and genotype frequencies

A

Allele frequencies are the proportions of all alleles of a gene type
Ex. A and a

Genotype frequencies are the proportion of the different allele combinations in a population
Ex. AA Aa aa

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6
Q

T or F: alleles and genotype frequencies are true for an entire species

A

False! they are for a population

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7
Q

Can evolution occur without natural selection?

A

Yes, there are other forces contributing to evolution (ex. Genetic drift, mutations, gene flow)

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8
Q

Can natural selection occur without evolution?

A

Yes, there can be differences in phenotypic fitness that might not be heritable and may not be passed on through generations

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9
Q

Why is industrial melanism in Boston betularia (peppered) moths a textbook example of rapid evolution by natural selection in nature?

A

The evolution of Melanic alleles in these moths occurred so rapidly with the increase of pollution and then decreased rapidly with the decline in pollution that a lot of studies have been done on these moths

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10
Q

Describe the trend of melanism in moths during the industrial revolution and then decrease in pollution

A

Nonmelanic moths became less frequent with higher pollution rates as melanic phenotype became more frequent = camouflage against darkened, soot covered trees

As pollution declined, melanic moth frequency decreased and non-melanic increased for camouflage to white trees

Changes within the population occurred rapidly

This was seen in other parts of the world too and with other species of moths

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11
Q

T or F: there is a strong connection between the phenotype and genotype of the peppered moths

A

True

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12
Q

Was melanic allele in peppered moths dominant or recessive?

A

Dominant

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13
Q

How did the recessive, non-melanic allele persist in the populations when the melanic phenotype was most prevalent?

A

Non-melanic allele couldn’t be lost because it still existed in the homozygotes Aa

Also possible influence of gene flow, migration

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14
Q

How did Majerus demonstrate fitness differences and natural selection in the peppered moths? What were his major results/interpretations?

A

Majerus studied the selective bird predation on the peppered moths by creating an experiment that best mimicked a natural situation - released moths slowly over 7 years and observed disappearance of these moths on trees

Found a consistent difference between the speckled (nonmelanic) and melanic where non-melanic had higher rates of survival (at the time, no soot on trees) and loss of melanic was due to bird predation

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15
Q

What was the average survival of melanic moths and nonmelanic? What does this mean for genotype fitness?

A

Non-melanic: typicals available - typicals eaten / total typicals
= 4522 - 963 / 4522 = 0.79 —> 79% survived

Melanic: melanics available - melanics eaten / total melanics
= 342 - 100 / 342 = 0.71 —> 71% survived

Non-melanic had higher survival rate than melanic

AA fitness = 0.71
Aa fitness = 0.71
aa fitness = 0.79

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16
Q

What was the relative survival of non-melanic to melanic moths?

A

Non-melanic = 0.79 —> 1
Melanic = 0.71 —> 0.9

0.71/0.79 = 0.9

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17
Q

Has the selection for or against the melanic allele changed over time? If so, how?

A

Yes

When there was a lot of pollution and melanic moths camouflaged against trees, there was stronger selection for the melanic allele (ie., fitness of A allele increased and frequency increased)

When pollution decreased and melanic moths were no longer camouflaged, there was stronger selection against the melanic allele (ie., fitness of A allele and frequency decreased)

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18
Q

How might you study the evolution of melanism in popG?

A
  1. Decide whether melanic allele is A or a
  2. Set starting frequency of allele
  3. Set relative fitness of genotypes
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19
Q

What did Amy Eacock et al., discover about colour change and background matching in peppered moth caterpillars?

A

Caterpillars change colour to match their background and can do so without using their eyes - - they have light and colour sensing genes under their skin

Caterpillars can also select their background to match their current colouration rather than changing colours

Example of phenotypic plasticity

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20
Q

How does colour variation differ between peppered moth caterpillars and adults?

A

In adults, colour variation is due to genetic polymorphism - it does not change over an individuals life (individuals cannot change colours throughout their life, they’re either melanic or nonmelanic)

Whereas, caterpillars have phenotypic plasticity and colour variation changes throughout their life stage

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21
Q

What is the general selection model?

A

The theoretical framework for studying how natural selection influences allele and genotype frequencies

It looks at the effects of natural selection after only one generation

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22
Q

What do we need to know to apply the general selection model?

A

The relative fitness of the 3 genotypes (AA, Aa, aa)
Initial frequency of the genotypes

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23
Q

How do we determine the frequency of the 3 genotypes in the next generation using the general selection model?

A

Relative fitness x initial frequency / total

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24
Q

What do we do after calculating the genotype frequencies in the next generation (general selection model)?

A

Calculate allele frequencies for the next generation and compare to first generation (before selection)

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25
Q

What does the general selection model tell us?

A

How the different types and strengths of selection are expected to affect genetic variation and allele and genotype frequencies

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26
Q

What happens on PopG if dominant allele (A) has low initial frequency and AA and Aa have lower fitness than aa?

A

The dominant allele will eventually be lost

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27
Q

What are two important factors to consider when determining relative fitness?

A

Dominance coefficient (what the heterozygote looks like compared to the other genotypes)

Selection coefficient (selective advantage)

28
Q

How do we determine the dominant allele?

A

It will be the one associated with the heterozygous phenotype

29
Q

What are the symbols for dominance and selection coefficients in the general selection model?

A

Dominance coefficient = h
Selection coefficients = s

30
Q

T or f: both s and h will always be between 0 and 1

A

True

31
Q

What does it mean when s = 0? s = 1?

A

s = 0 —> there is no selective advantage
s = 1 —> aa is lethal or sterile

32
Q

What does it mean when h = 0? h = 1?

A

h = 0 —> heterozygote is completely recessive
h = 1 —> heterozygote is completely dominant

33
Q

What is the equation for the relative fitness of AA genotype using the general selection model?

A

WAA = 1

34
Q

What is the equation for the relative fitness of Aa genotype using the general selection model?

A

WAa = 1-hs

35
Q

What is the equation for the relative fitness of aa genotype using the general selection model?

A

Waa = 1-s

36
Q

When is the frequency of an allele changed the most between one generation to the next according to the general selection model?

A

When s is large (selection coefficient) and pq (heterozygote frequency) is large

37
Q

What determines the fitness of the heterozygote Aa?

A

on the phenotype

if Aa looks exactly like AA homozygote, then the fitness of Aa = AA

if Aa looks like aa homozygote, then the fitness of Aa = aa

38
Q

What is the dominance coefficient (h) mean?

A

describes the fitness of the heterozygote

39
Q

How do we determine the fitness of the heterozygote in the general selection model?

A

1-hs

40
Q

How do we calculate h in the general selection model?

A

first calculate s using the first equation: 1-s

then plug s into the second equation: 1-hs

41
Q

T or F: the change in allele frequency due to selection is proportional to the strength of selection (s) and to the allele frequencies (p and q)

A

true

42
Q

what does it mean for allele frequencies if s is small and if p or q is small (ie., A or a is rare)?

A

change in allele frequencies will be slow

43
Q

What does it mean for allele frequencies if there’s a completely recessive mutation (h=0)?

A

the change in allele frequency is a result of selection being proportional to strength of selection (s) and to p*q^2 = allele frequency change is VERY slow is q^2 is small (a is rare)

44
Q

T or F: genetic variation can either be decreased or increased due to natural selection

A

true

45
Q

What are the two major types of natural selection?

A

negative (purifying) selection
positive selection

46
Q

What is the range of natural selection between negative and positive?

A

deleterious - slightly deleterious - neutral - beneficial

47
Q

Describe purifying/negative selection

A

selection that removes deleterious mutations –> decreases variation

48
Q

Does negative/purifying selection increase or decrease variation?

A

decrease

49
Q

Describe positive selection

A

selection that favours beneficial mutations –> can increase or decrease depending on the type of positive selection

50
Q

Does positive selection increase or decrease variation?

A

it can do either

ex. decrease: introduction of advantageous mutation (ex. non-melanic) replaces melanic

51
Q

What are the 2 types of positive selection?

A

directional selection
balancing selection

52
Q

How does directional selection affect genetic variation?

A

it reduces variation as it favours a dominant allele over a recessive allele

53
Q

How does balancing selection affect genetic variation?

A

it maintains polymorphism (variation) it keeps both the dominant and recessive allele types in the population

increase in heterozygosity and variation

54
Q

Which is more common type of selection, negative or positive? why?

A

negative/purifying because most new mutations are deleterious and negative selection will act to remove these

55
Q

Which type of positive selection is most common?

A

directional (ex. melanic moth allele replacing nonmelanic)

56
Q

What are 3 reasons for balancing selection?

A

heterozygote advantage

frequency-dependent selection

fluctuating selection in space/time

57
Q

Describe the heterozygote advantage - what type of selection is this? give an example

A

When the heterozygote has higher fitness than AA and aa

both A and a will be maintained in the population (balancing selection)

ex. sickle cell anemia

58
Q

Describe how sickle cell anemia is an example of heterozygote advantage

A

aa has very reduced fitness (1-s2)

Aa has higher fitness than wildtype, AA, when malaria is present (1)

AA (1-s1)

59
Q

how would you study the evolution of heterozygote advantage in popG?

A

set initial frequency of A to 0.95 cause most common initially and introducing new allele, a

set fitness of Aa to 1

set fitness of AA and aa to less than 1

60
Q

What happens in popG when looking at heterozygote advantage if fitness of AA and aa are both really low?

A

the frequency of A allele is reduced quickly over time but does not go extinct - both A and a are ~ equally frequent in population

61
Q

What happens in popG when looking at heterozygote advantage if fitness of AA and aa are both moderate?

A

the frequency of A is reduced a bit more slowly over time and reaches about 50% frequency

62
Q

How did the frequency of the different orchid phenotypes, genotypes and alleles change over time?

A

initially the darkest colour (dom allele) had highest frequency but rapid change to frequency over time as lighter red (Aa) and white phenotypes (recessive allele) become more frequent

63
Q

What was the relative fitness of the different orchid phenotypes and genotypes?

A

the heterozygote had highest fitness, but both the homozygotes had relatively high fitnesses too

64
Q

Why did orchid heterozygotes have highest fitness?

A

caused by pollinator preference

flies and bees both preferred the Aa flowers enough that it maintained both

flies significantly preferred white and bees not much, but probably levelled out to maintain the recessive allele

65
Q

How would you model the fitness of the orchids in popg?

A

set population size to 10,000

set A (dark colour) to 0.95 (high initial frequency, new a allele introduced)

set fitness of Aa to 1

set fitness of AA and aa to less than one, but still relatively high

66
Q

What genes and mutations were involved in the orchid experiment?

A

the colour differences are a result of differing concentrations in anthocyanidin pigment

dark red has very high concentration of pigment

Aa has some

aa has very little, but not totally absent

67
Q

What mutations were involved in the orchid experiment?

A

the differing concentrations of the pigment responsible for the dark red colour is due to a mutation in a transcription factor (GrMYB1) which controls expression of the pigment

Evolution (11 decks)

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