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+ BIOL 121 > Evolution - Week 5 > Flashcards

Evolution - Week 5 Flashcards

1
Q

What is evolution?

A

change in heritable characteristics (traits) of biological populations (or species) over many generations

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2
Q

What does evolution act upon?

A

over entire populations, NOT individuals

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3
Q

What can mutations be caused by?

A
  • DNA replication errors
  • DNA repair errors
  • non-disjunction
  • base change in DNA that is copied during replication (it is a permanent change in DNA sequence)
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4
Q

What happens in nondisjunction?

A

before: meiosis I starts normally - tetrads line up in middle of cell

nondisjunction occurs with one set of homologs

after: meiosis II occurs normally
result: aneuploidy - all gametes have an abnormal number of chromosomes, either too many or too few

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5
Q

What are alleles?

A

genetic variants

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6
Q

What is a mutation?

A

random (unpredictable) permanent change in nucleotide sequence, and the source of completely new alleles

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7
Q

What are neutral mutations?

A

most mutations are expected to be (relatively) neutral with respect to fitness, because most mutations will have little to no impact on phenotype

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8
Q

What are lethal mutations?

A

we rarely se them because they die before they are observed

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9
Q

What are deleterious mutations?

A

harmful mutations

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10
Q

What are beneficial mutations?

A

result in individual having a higher fitness (more likely to reproduce)

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11
Q

Describe actual mutations vs. observed mutations.

A

our observations of mutation rates may not reflect what actually happened in evolutionary history of a population

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12
Q

What is adaptive evolution?

A

called adaptive: because allele frequencies are increasing or decreasing based on how they affect the organism’s fitness

called evolution: because allele frequencies in the population are changing between generations

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13
Q

What do mutations introduce?

A

new alleles that may be lethal, detrimental, or beneficial in nature

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14
Q

Why is a mutation not a significant mechanisms of evolutionary change on its own?

A

it is comparatively infrequent

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15
Q

What is the Hardy-Weinberg equilibrium used for?

A

to compare predicted vs. observed genotype frequencies

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16
Q

What is the Hardy-Weinberg equilibrium equation?

A

p^2 +2pq + q^2 = 1

where p = allele #1
where q = allele #2
where p + q = 1

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17
Q

What assumptions do we make when we predict offspring genotype frequencies using the H-W equilibrium?

A
  • random fusion of gametes (random mating)
  • infinitely large population (no change in allele frequencies due to sampling bias
  • no other changes in allele frequencies between generations (no evolution through evolutionary mechanisms)
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18
Q

How do we test whether or not a population is in H-W equilibrium?

A

we need to compared predicted genotype frequencies to observed genotype frequencies

ie. p + q = 1 does not mean the population is in H-W equilibrium
ie. p2 + 2pq + q2 = 1 does not mean the population is in H-W equilibrium

these tell you your math is correct, but doesn’t tell you if the population is undergoing non-random mating or evolution

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19
Q

How do you calculate allele frequency?

A

(# copies of particular allele) / (total # of alleles)

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20
Q

How do you calculate genotype frequency?

A
  • multiply the allele frequencies

- frequencies should add up to 1

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21
Q

What does a population that is not in H-W equilibrium tell you?

A
  • the population is not necessarily evolving

- one of the H-W model assumptions has been violated

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22
Q

What is a null hypothesis?

A

hypothesis that there is no significant difference between specified populations, any observed difference being due to sampling or experimental error

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23
Q

There is no evolution if there is no…

A
  • mutation
  • migration
  • selection
  • genetic drift
24
Q

Null Hypothesis: H-W Equilibrium

What assumptions do we make?

A

random mating (but non-random mating could be happening)

evolution through one more of the 4 mechanisms is occurring (mutation, migration, selection, genetic drift)

25
Q

What is non-random mating?

A

predictable mating, meaning no evolution

26
Q

What is inbreeding?

A

mating with closely related individuals

27
Q

What is positive assortative mating?

A

like mates with like: mating occurs between individuals that are alike

28
Q

What is negative assortative mating?

A

mating occurs between individuals that are unlike

29
Q

What is sexual selection?

A

everyone wants to mate with the most attractive male/female

30
Q

What is gene flow?

A

migration of alleles between gene pools

31
Q

How do we describe the movement of alleles in gene flow?

A

from the perspective of the population that we are studying

32
Q

What is genetic drift?

A

change due to random sampling

33
Q

What happens in genetic drift?

A

random sample of individuals survive/reproduce

these individuals are non-representative of previous generation (for unpredictable reasons)

34
Q

In which populations is genetic drift more likely to occur?

A

small populations

35
Q

In the context of population genetics, why can we think of each generation as a genetic sample of the previous generation?

A
  • if every single individual in the population passed on all of their alleles in equal proportions (through offspring), we would have effectively “sampled” the entire population in creating the next generation
  • however, this is unlikely to be the case; it’s much more likely that only some individuals will survive and successfully reproduce, and that those individuals who do survive and reproduce will only pass on some of their alleles
  • if survival and reproduction are not influenced by the alleles we are focusing on, then next generation should be a random sample of those particular alleles
  • if we take a random sample of something, we should expect that sample to be representative – assuming that the sample is large enough
  • the smaller our sample is, the less likely it is that our sample will be representative of the original
  • this is the case even if the original population itself was small
  • consider an extreme example, where we only have four copies of gene A in our population: A1, A1, A2, and A3
  • in generation 1, the allele frequencies are freq(A1) = 0.5, freq(A2) = 0.25, and freq(A3) = 0.25
  • even if 75% of the alleles successfully get passed on to generation 2, there will still be a considerable change in allele frequencies between generations, because each allele represents such a large portion of the total population
36
Q

What is the cause of genetic drift?

A

natural disasters

  • it does not matter if the event was random
  • who survives/reproduces is random
  • we cannot predict who will survive/reproduce
37
Q

When students are asked to describe an example of genetic drift, a common response is: “A volcano randomly erupts and kills half of the alleles in the population.”

What is a better way of phrasing this?

A

“a volcano erupts, and randomly kills half of the alleles in the population”

  • it’s entirely possible that a random event like a volcano could act as an agent of natural selection – think about what would happen if some individuals carried an allele that allowed them to tolerate the effects of toxic volcanic ash, and others didn’t
  • we would expect the outcome of the volcano to be natural selection for the beneficial allele
38
Q

What is important when it comes to genetic drift?

A

it doesn’t really matter whether we think the event itself was random; the important point is whether the effect on allele frequencies was random or not

39
Q

Give an example in which a particular event might have non-random consequences for some alleles, but random consequences for others.

A

predator might selectively eat all of the yellow individuals in a population, which would eliminate the yellow allele

alleles that aren’t linked with the yellow allele, and that have no effect on colour might unpredictably increase or decrease in frequency due to the loss of the yellow individuals

in other words, the decrease in population size due to natural selection against yellow individuals might also lead to genetic drift of other alleles

40
Q

What does the bottleneck effect do?

A
  • restricts # alleles (and diversity of alleles) that can be passed on to the next generation
41
Q

What happens after a bottleneck event?

A

population may eventually grow back to its previous size, however, the allele frequencies in the future population will probably be very different from allele frequencies in original population

42
Q

What is a population bottleneck?

A

sudden decrease in number of individuals in population

43
Q

What is a genetic bottleneck?

A

sudden decrease in number of alleles in population

44
Q

What is the bottleneck event?

A
  • individuals have not moved to a new habitat
  • existing population suddenly decreases in size
  • allele frequencies in remaining population differ from those in the previous population
45
Q

What is the founder effect?

A
  • small number of individuals have moved to a new habitat, establishing a complete new population
  • brand new population is founded
  • allele frequencies in new population differ from those in the parent population
46
Q

What are the two mechanisms by which genetic drift can occur?

A

population/genetic bottlenecks

founder effect

47
Q

In the bottleneck event and founder effect, what population are we focussing on?

A

current population we are focusing on represents a small sample of the original population

48
Q

Are the small samples we focus on in the bottleneck event and founder effect random?

A

yes

it’s possible that the deaths that resulted in the population bottleneck, or the movement of some individuals that resulted in the founder event, were influenced by genotypes for a small number of genes, which would mean these samples would not be random with respect to those alleles

survived or who moved, and we would consider the samples to be random samples of alleles for those genes

49
Q

Are the small, random samples we focus on in the bottleneck event and founder effect non-representative of the previous population?

A

yes

this is an essential requisite in order for genetic drift to occur: if a small, random sample is representative of allele frequencies in the previous generation, then allele frequencies will not have changed between generations, and genetic drift will not have occurred

however, the smaller the sample, the more likely it is to be non-representative

keeping in mind that there are many different genes in the population, it is extremely unlikely that a small sample will be representative for all genes/alleles

50
Q

Is genetic drift predictable?

A

no, there is no way of predicting outcomes of these events as they are random

51
Q

What is natural selection?

A

process by which individuals with certain inheritable traits tend to produce more surviving offspring than do individuals without those traits, often leading to a change in the genetic makeup of the population

52
Q

Which organisms have higher fitness?

A

organisms better adapted to their environment will more likely survive and reproduce, compared to those without those adaptations

53
Q

What is the difference between genetic drift and natural selection?

A

genetic drift:

  • change due to random sampling
  • random (unpredictable) sample of individuals survive/reproduce
  • these individuals are non-representative of previous generation

natural selection:

  • specific sampling = selection
  • can predict which individuals will survive/reproduce
  • can predict what the current population is based on the previous population (if the environment hasn’t changed)
54
Q

What are the 4 evolutionary mechanisms?

A

mutation: the only source of novel genetic variation, random and rare

gene flow (= migration of alleles): introduces “new” alleles to population, homogenizes allele frequencies between populations

genetic drift: which alleles survive/reproduce is unpredictable

natural selection:
specific alleles survive/reproduce for predictable reasons

55
Q

Why do reductions in allele frequencies and increases in allele frequencies go hand-in-hand?

A

because frequencies are relative measures, when one allele decreases in frequency, another must consequently increase (and vice versa)

+ BIOL 121 (11 decks)

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