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Biology 355 - Evolution > Topic 6 - Natural Selection (Genetic Perspective) > Flashcards

Topic 6 - Natural Selection (Genetic Perspective) Flashcards

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1
Q

What is the definition of Natural Selection using a genetic perspective.

A
  • it is the change of allele frequencies within a population resulting from fitness differences among individuals
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2
Q

What is one way of assessing natural selections.

A
  • look at how it affects fitness
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3
Q

What are the 5 main components of fitness from which it could be measured from. - two stand outs as most important?

A
  • survival ability
  • mating success (sexual selection of a partner)
  • Fecundity (producing enough offspring)
  • Quality of Gametes
  • Time to maturity

Survival and Fecundity

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4
Q

What does R stand for?

A

the absolute fitness of a genotype

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5
Q

What does R - absolute fitness mean? what is a good R value?

A
  • the absolute fitness of a genotype
  • meaning R is the MEAN number of progeny per parent (or per capita growth rate)
  • if R equals 1, then that genotype is replacing exactly itself, and remains at constant numbers
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6
Q

What does w stand for?

A
  • the relative fitness of a genotype
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7
Q

What does w - relative fitness mean? what does it vary between?

A
  • the relative fitness is defined as “its absolute fitness RELATIVE to the maximum absolute fitness”
  • varies between 0 and 1
  • so w will indicate the increase or decrease in a population, relative to other genotypes
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8
Q

Define fitness (via the textbook)

A
  • is the average per capita lifetime contribution of individuals of that genotype to the population after one or more genereation
  • reproductive success
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9
Q

How do genetic models typically describe evolution? how does this answer relate to fitness levels?

A
  • through allele frequencies, their abundance in a population
  • higher fitness genotypes in a population contribute a greater proportion of their alleles to the next generation
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10
Q

What do allele frequencies and these population genetic models not tell us, how do we solve this through absolute and relative fitness?

A
  • they give no information about population increases or decreases
  • this is because the genotype with the highest relative fitness (w) in the population can still have an absolute fitness (R) below 0 thus have a declining population in overall numbers
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11
Q

How does natural selection affect populations? (2 ways)

A
  • it will change relative frequencies of alleles (changing the TRAIT MEANS in a population)
  • it will change allelic composition of genotypes (changes in VARIANCE of trait means)
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12
Q

What are the 3 modes of natural selection - in SINGLE-LOCUS traits?

A
  • directional selection (which increases an advantageous allele)
  • overdominance
  • underdominance
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13
Q

Explain the homozygote advantage.

A
  • occurs in directional selection

- the homozygote has the highest fitness

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14
Q

Explain the heterozygote advantage.

A
  • this occurs in over-dominance selection

- when the heterozygote has the highest fitness (and the homozygote may be lethal)

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15
Q

Explain the heterozygote disadvantage. what could this lead to in extreme cases?

A
  • this occurs in under dominance selection
  • when the heterozygote has the lowest fitness
  • may lead to two populations not mating with one another
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16
Q

What are the 3 modes of natural selection - in QUANTITATIVE, continuously varying traits?

A
  • Directional selection (which increases an advantageous allele)
  • stabilizing selection
  • diversifying selection
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17
Q

Define each of the modes of selection when there is a quantitative continuously varying trait.

A
  • directional selection will INCREASE a proportion of a genotype with the more extreme value of a favoured trait
  • stabilizing selection does NOT alter the mean, but will REDUCE Variance
  • Diversifying selection will shift the mean
18
Q

What was Darwins basis for selection? and how did he describe new traits?

A
  • he used his observations and writings focused on DIRECTIONAL selection
  • through the appearance, spreading, and gradual change in populations and species over time
19
Q

What does “s” denote? and what type of selection does it denote?

A
  • ”s” is denoted as the selection coefficient

- this measures the rate of spread of an advantageous allele over time (a form of directional selection)

20
Q

When “s” is high, what would you expect to observe? vs a low “s”

A
  • there would be strong selection towards the favoured allele, thus those without would expect faster disappearance of individuals lacking this allele
  • a low selection value will tend to keep unfavourable alleles around longer as more time is required to allow for disappearance
21
Q

What rate of allele frequency do dominant, co-dominant, and recessive alleles take?

A
  • dominant alleles would be expressing in both heterozygous and homozygous conditions
  • recessive alleles would only be expressed under homozygous conditions
22
Q

Why do deleterious alleles tend to hang around in populations at low frequencies?

A
  • if the trait is recessive, it may take a while to become extremely prevalent in the population
23
Q

What don’t stabilizing and diversifying selections NOT do? what do they do instead?

A
  • DO NOT change the mean value of a trait in a population

- they will INCREASE OR DECREASE the amount of variation in that trait

24
Q

What type of selection maintains a polymorphism, an example?

A
  • the sickle-cell hemoglobin mutation
  • the heterozygote advantage (aka overdominance)
  • these heterozygotes (carrier) have the highest fitness while the homozygotes (carriers of the disease) still persist in the population at LOW frequency
25
Q

What is the African finch an example of?

A
  • birds with intermediary sized beaks are less likely to survive to adulthood than birds with wide or narrow beaks (diversifying selection)
  • superior fitness of difference genotypes on different resources to maintain polymorphism
26
Q

What are two forms that dictate selective pressures? what does this statement mean as a whole in the direction of selection.

A
  • selection pressures can vary over space and time - there may be multiple variables acting applying selection pressures, however there will be a net effect pushing the population in one direction
  • temporal variation
  • spatial variation
27
Q

Temporal variation can present its selective pressure in two ways? what are they?

A
  • short-term

- long-term

28
Q

Define short-term temporal variation in selection pressures, what about their predictability?

A
  • INDIVIDUAL GENOTYPES are exposed to different selection pressures in their lifetime
  • where individuals are able to favour the conditions the parents were under, ex fur colours with changing seasons
  • temporal variations from selection pressures may be predictable (in observed PHENOTYPIC PLASTICITY) or unpredictable
29
Q

Define long-term temporal variation selection pressures

A
  • UNIFORM selection pressures on individual genotypes over a SINGLE LIFESPAN - but we observe CHANGE over MULTIPLE generations
  • so the population will show genetic change over TIME
  • long term changes in the environment may dictate changes in the genetic make-up of the population and multiple generations due to LONG TERM CHANGES
30
Q

What are the two types of selection pressures in spatial variation - and define “grain”

A
  • fine-grained
  • course-grained
  • grain refers to how large the habitat patch is in relation to the organism
31
Q

Explain fine-grained selection pressures during spatial variation

A
  • INDIVIDUAL GENOTYPES encounter DIFFERENT patches of habitat
  • in each DIFFERENT habitat they are exposed to different selection pressures
  • thus the fittest genotype will be the generalist (more prevalent in the habitat favouring it)
32
Q

Explain coarse-grained selection pressures during spatial variation

A
  • INDIVIDUAL GENOTYPES more likely to stay in a SINGLE PATCH of habitat
  • local adaptation is favoured (fittest genotypes are more likely to be specialized)
33
Q

What does the blue mussel case tell use about selection pressures

A
  • there is a spatial gradient (fine or coarse grained) where the blue mussel demonstrate changing allele frequencies of the aminopeptidase I based on changing salinity levels
  • changing environment favoured brackish waters to open ocean
34
Q

What is the third mechanism for variable selection pressures? and what two forms arise?

A
  • Frequency-dependent selection - the frequency of selection is dependent on how prevalent a species is (as a result of competition between individuals)
  • INVERSE frequency-dependent selection
  • POSITIVE frequency-dependent selection
35
Q

What are the variables for selection pressures, and their acting mechanisms.

A
  • Temporal: short and long term
  • Spatial: fine and coarse grained
  • Frequency dependent: inverse or positive
36
Q

Define the two forms of Frequency-Dependent selection pressures.

A
  • Inverse frequency dependent selection: RARER genotype has the HIGHER fitness
  • Positive frequency-dependent selection: COMMON genotype has the HIGHER fitness
37
Q

What example orchestrates INVERSE frequency dependent selection - explain how these selection works and what form of natural balance is struck?

A
  • the colour morph of the elderflower orchid
  • the rarer morph of the flower colour will have higher reproductive success because of more pollinator attention
  • THUS THE RARER GENOTYPE HAS HIGHER FITNESS
  • an equilibrium will be reached where the RARE morph will increase in abundance leading to the once abundant morph becoming rare and bees favouring this morph now
38
Q

What are 3 other examples of INVERSE FREQUENCY DEPENDENT selection

A
  • Mating Strains: individuals cannot mate with individuals with identical set of alleles thus mating favours the RARER genotype
  • 1 to 1 Sex ratios???
  • or competition for food by similar genotypes
39
Q

What form of selection does Mullerian mimicry in butterflies fall under?

A
  • this is a form POSITIVE frequency-dependent selection
  • the more common colour morph has a higher fitness since predators associate the common colour with unpalatability and toxicity
40
Q

Explain further how the two species of Heliconius butterflies

A
  • the butterflies eat a passion flower vine containing alkaloids
  • they have bright aposematic (warning) colouring within multiple races of the species
  • 2 differing species of this butterfly have corresponding pairs of colour races which occur together in the same geographic region
  • thus with the convergence of colour patterns (and prevalence of one colour pattern over another) is driven by bird predation
41
Q

What MOLECULAR signatures are linked with natural selection, what acts in its place if NS does not?

A
  • Natural selection CAN act at the molecular level, in its absence EVOLUTION is driven by MUTATION and GENETIC DRIFT
42
Q

What are 3 examples of molecular signatures on DNA differences

A
  • NS on DNA differences can cause a reduction of genetic variation (Background & Positive Selection) OR an increase in genetic variation (Balancing selection)
    1. Background (purifying) - decreases deleterious mutations (mutation has a negative effect on the organisms)
    2. Positive (directional) - increases favourable mutations
    3. Balancing - favours heterozygotes for a given locus (the combination of an allele is more favourable than a single allele on its own)

Biology 355 - Evolution (7 decks)

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