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Biology 355 - Evolution > Topic 7 & 8 - Population Genetic Theory (H-W Equilibrium) > Flashcards

Topic 7 & 8 - Population Genetic Theory (H-W Equilibrium) Flashcards

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1
Q

What does the Hardy-Weinberg Equilibrium refer to?

A
  • its the relationship between GENOTYPE FREQUENCIES and ALLELE FREQUENCIES in a population undergoing NO evolutionary change
  • that after one generation of random mating, the genotype frequency will be p2-pq-q2 and will remain as so in succeeding generations
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2
Q

What are allele frequencies?

A

the relative commonness or rarity of an allele (ex the A1 allele or A2 allele)

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3
Q

What are genotype frequencies?

A

the portion of a population that has a certain genotype (ex. A1A1 homozygote, A1A2 heterozygote or A2A2 homozygote)

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4
Q

What is the mathematical basis of H-W Equilibrium?

A

based on the binomial expansion: p2 & pq & q2

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5
Q

What is the basis for H-W Equilibriums? and what 5 assumptions always stand true for them?

A
  • based on NO evolutionary changes occuring
  • Mutation rates are ZERO
  • Populations are INFINITELY large (no random genetic drift)
  • Mating is RANDOM
  • NO gene flow or migration into or out of populations
  • All individuals are equally likely to survive and reproduce (NO natural selection)
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6
Q

Why are H-W Equilibriums useful? - since real populations are NOT static

A
  • gives a starting point to determine what drives evolutionary change - then we can use genetic shifts to infer what processes are acting on a population
  • they explain the abundance of alleles and relative abundance of genes expressed
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7
Q

How do you calculate the “expected number” in H-W equilibrium for the Human MN locus

A
  • multiply the H-W Frequency X sample size

- H-W frequency are the (p2 or pq or q2 decimal number) while sample size is the total number of observed individuals

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8
Q

How do heterozygote proportions change allele frequencies when a population is under H-W equilibrium?

A

The frequency of heterozygotes are the highest when p is equal to q
- p and q being allele frequencies

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9
Q

What is the second assumption of H-W Equilibrium?

A
  • populations are infinitely large (that there is NO genetic drift)
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10
Q

What occurs when a species undergoes genetic drift? give an example.

A
  • random fluctuations (effect of chance) in the frequencies of alleles, since populations are of finite size
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11
Q

In a population, do all genes present among parents get passed on to offspring? (at least some)

A
  • this does not occur, especially in smaller populations
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12
Q

How do allele frequencies change with genetic drift?

A
  • the direction is unpredictable

- magnitude of frequency change depends on population size, smaller populations feel a stronger effect of genetic drift

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13
Q

What are two stages of genetic drift on populations and what would occur?

A
  • short term - random fluctuations

- long term - loss of alleles from populations and genetic divergence among populations

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14
Q

What do population sizes influence in genetic drift?

A
  • if an allele frequency ex. “p” may undergo a pressure which leads the subsequent generations p value to be higher or lower with the equal probability
  • if these random fluctuations continue over time and no stabilizing force returns the allele frequency to normal the allele may either be fixed or lost, become either 0 or 1
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15
Q

What do population sizes influence in genetic drift? (small pop vs large pop)?

A
  • a small population will experience rapid fluctuations of alleles and will be lost or fixed at a higher rate to the point where alleles are lost from the population completely
  • a larger population through fluctuations of the allele frequencies will still maintain a balances allele frequency
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16
Q

How does genetic drift progress over time in populations with the same initial allele frequency?

A

I DON”T KNOW - page 262 textbook

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17
Q

What was Buri’s Drosophila experiment? what did it demonstrate in terms of natural selection?

A
  • he demonstrated random genetic drift
  • of 107 populations all initially starting with 16 heterozygotes, allele frequency began varying over subsequent generations
  • after 19 generations, allele frequencies had become evenly distributed between 0 and 1 where some were lost or fixed in the population
  • there was no effect by natural selection on the small populations
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18
Q

What is a consequence of genetic drift in these small populations?

A
  • that not alleles persist

- ex. all individuals at time t happen to be descended from an ancestor with allele A2

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19
Q

What tends to occur to populations in terms of variance over time? what is the frequency of an allele becoming fixed? what does that mean for rare alleles?

A
  • if they aren’t infinitely large they will gradually lose variation, eventually leading them to become FIXED for a particular allele
  • if an allele is affected by genetic drift, the probability of becoming fixed is related to its INITIAL FREQUENCY in a population, thus rare alleles will tend to disappear
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20
Q

Define Ne, compared to N.

A
  • Ne refers to the effective population size are the ones that contribute genes to the next generation
  • number of individuals in an ideal population (where all individuals reproduce) in which the rate of genetic drift (measured in decline of heterozygosity) would be the same as the actual population
  • N refers to the actual population size
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21
Q

What factors may affect Ne?

A
  • variation in sex ratio (if only 1 male mates with 20 females, the 19 other males cannot contribute to the gene pool)
  • natural selection - variation in number of progeny
  • overlapping generations
  • fluctuations in population size (a small size has a disproportionate effect on the make up of a population)
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22
Q

Define Founder Effect and Genetic Bottlenecks

A
  • bottleneck is a severe, temporary reduction in population size
  • founder effect is the principle that the founders of a new population carry only a FRACTION of the TOTAL genetic variation the source population
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23
Q

What species experienced genetic bottlenecks and are now undergoing a founder effect?

A
  • northern elephant seal

- small effective population and a recent founder effect

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24
Q

How do genetic drift and natural selection contrast each other in their relation to allele frequencies?

A
  • natural selection will lead to HIGHER frequencies of alleles conferring HIGHER fitness
  • genetic drift on the other hand acting on populations will have LESS FIT alleles to almost as likely to increase as decrease in frequency
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25
Q

Explain the case of the Northern elephant seal

A
  • the effective population size is significantly smaller than the census population size because only a few males compete successfully for the smaller females - the winner of this competition will be the father of all the offspring in the entire harem of females
  • hunting reduced numbers to about 20, decreasing the effective population size even more
  • thus genetic drift was responsoble for reducing genetic variation
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26
Q

What to changes does genetic drift lead to in relation to alleles? compared to changes through natural selection

A
  • a random variation in allele frequencies
  • loss of alleles from a population
  • natural selection favours some alleles over others,
  • lower-fitness alleles are more likely to be lost through genetic drift than higher-fitness alleles
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27
Q

Explain the negative effects of genetic bottlenecks in native NZ bird species

A
  • based on the failure rate of hatchlings lead to suffering bottlenecks in population size
28
Q

How do adaptive landscapes work in their graphical representation of how NS shapes allele frequencies? in relation to fitness

A
  • when fitness is constant, NS changes allele frequencies to INCREASE population mean fitness
  • thus the population moves up along the curve
  • thus to measure how allele frequencies will change under selection - simply examine which direction of allele frequencies will increase in
29
Q

What two key terminologies emerged from adaptive fitness landscapes?

A
  • adaptive peaks and adaptive valleys, increase or decrease in fitness of the population
30
Q

How is natural selection affected by fitness interactions among different loci? - when there is NO fitness interaction?

A
  • the relative fitness of different alleles at a locus sometimes depends on which alleles are present at another locus
  • if there is NO fitness interaction - each allele has an independent effect
31
Q

What occurs to the relative fitness if there is a fitness interaction among the different loci?

A
  • relative fitness of genotypes at one locus depends on alleles present at the other locus
32
Q

What is the difference between selection shifts in adaptive fitness landscapes between normal and small populations?

A
  • normal populations with selection push populations TOWARDS ADAPTIVE PEAKS (allele frequencies that give the highest fitness)
  • small populations GENETIC DRIFT will shift the populations AWAY from one adaptive peaks and towards another
33
Q

What is the 3rd assumption of the H-W Equilibrium?

A
  • Mating is random
34
Q

Define Random Mating - what scenario bests suits this practice?

A
  • any individual in a population has an equal likelihood of mating with any other individual (irrespective of their genotype)
  • if populations are spread out spatially
35
Q

Define Random Mating - what scenario bests suits this practice?

A
  • any individual in a population has an equal likelihood of mating with any other individual (irrespective of their genotype)
  • if populations are spread out spatially so individuals are MORE LIKELY to mate with others who are nearby
36
Q

What is one form of NONrandom mating?

A
  • inbreeding, individuals are more likely to mate with relatives than with non relatives OR gene copies uniting games are more likely to be identical by descent than joined by random
37
Q

What occurs with non random mating - inbreeding?

A
  • offspring can inherit MULTIPLE copies of ancestral allele

- this leads to INCREASE frequencies of homozygotes

38
Q

How does inbreeding affect genotype frequencies? but what about allele frequencies?

A
  • since inbreeding will INCREASE homozygotes, the heterozygote frequency will DECREASE in these populations
  • allele frequencies STAY THE SAME, but heterozygote genotypes decrease
39
Q

Explain inbreeding in wild oats - what is another example of inbreeding occuring?

A
  • the genotype frequencies observed at the two loci in the population compared to a population under H-W Equilibrium
  • the inbreeding genotype the heterozygotes are deficient at both loci
  • in zoos with the golden lion tamarin
40
Q

How does inbreeding and genetic drift function together?

A
  • they often occur together in natural populations
  • inbreeding will HETEROZYGOTE GENOTYPE FREQUENCIES but does not affect ALLELE frequencies
  • while genetic drift either leads to the fixation of loss of some alleles over time
41
Q

What is meant by the inbreeding depression?

A
  • there is a loss of viability from homozygotes, which indicates a recessive but deleterious allele
  • thus heterozygotes while being carriers show greater viability
42
Q

What are the 3 examples that demonstrate Gene Flow and Spatial Patterns?

A
  • geographic variation of the Northern Flicker
  • Geographic pattern of genetic variation in the rattle snake
  • geographic variation of Adh alleles in D. melanogaster - with changing latititudes
  • geographic variation in the frequencies of cyanide-producing genes in white clover
43
Q

What is the 4th assumption of the H-W Equilibrium?

A
  • NO gene flow or migration INTO OR OUT OF populations
44
Q

What is gene flow? and what can it change within the population?

A
  • the movement of genes between populations either through migration of individuals or their gametes
  • the genetic structure of the population can change by the ADDITION OF GENES from other populations that differ in their genetic composition
45
Q

So gene flow can be defined as (same as previous definition)

- what two other factors can lead to gene flow?

A
  • the movement of individuals or their genes (or gametes) between populations
  1. recolonization
  2. population extinction
46
Q

What is a tendency of gene flow?

A
  • gene flow tends to counteract divergence among populations (leading to greater homogeneity of populations)
47
Q

What effect does gene flow have on genetic difference among populations?

A
  • gene flow causes populations to CONVERGE their allele frequencies and reach an overall average allele frequency
48
Q

In what two ways is gene flow measured?

A
  1. Measure movements of individuals between BIRTH AND REPRODUCTION
  2. Observe genes moving into populations by comparing parent and offspring allele frequencies
49
Q

How important is gene flow?

A
  • most species will have some form of gene flow between adjacent populations
  • some species have substantial gene flow over LONG distances, ex migratory birds and fishes, wind-pollinated plants, or passively dispersed marine organisms
50
Q

What are two examples of how important gene flow is and any hinderance it has on the organism?

A
  • gene flow in maize, where any gene flow is greatly restricted beyond a certain distance
  • dispersal distances in a texas lizard (measured between birth and reproduction) drops of significantly within a given distance
51
Q

What role does genetic divergence take among populations?

A
  • different populations of species can differ in allele frequencies (this being a reflection in different habitat, geographic location, or population history)
52
Q

How is genetic differentiation measured among populations?

A
  • through Fst, or Gst
  • are measures of observed genetic variation that is AMONG populations
  • Nei’s genetic distance
53
Q

What does Fst stand for, what is its scale and what does this indicate?

A
  • if Fst equals 0, populations will have the SAME alleles in the SAME frequencies
  • if Fst equals 1, populations are FIXED for different alleles
  • Fst is the measure of variation in allele frequency among different populations (FIXATION INDEX)
54
Q

What is Gst?

A
  • for a locus with more than two alleles

- ranges from 0 (no variation among populations) to 1 (populations fixed for different alleles)

55
Q

Examine Gst vs geographic distance?

A
  • there appears to be genetic differentiation among the pitcher-plant mosquitoes
  • Gst measuring allele frequency differences at over 5 loci plotted relationship to distance between populations
  • the red slope indicates southern populations are genetically more different compared to the northern populations
56
Q

Nei’s genetic distance?

A
  • Genetic distance is the degree of genetic difference (genomic difference) between species or populations that is measured by some numerical method.
57
Q

What is the case of isolation-by-distance in the case of human populations?

A
  • divergence between pairs of populations is often correlated with distance between them
58
Q

How are populations affected when different process interact?

A
  • these would fall outside of the 5 H-W Equilibrium
59
Q

What interaction does Gene Flow and Genetic Drift have?

- what does a lack of gene flow look like?

A
  • Gene flow COUNTERACTS GENETIC DRIFT, (it would effectively increase the population size)
  • a lack of gene flow results in ALLELE FREQUENCY DIFFERENCES in adjacent populations
60
Q

What sort of interaction does gene flow have with natural selection?

A
  • NS and GF may REINFORCE one another - if selection favours the same traits in DIFFERENT populations - or COUNTERACT one another if selection is acting in OPPOSITE directions
61
Q

What interaction will occur between GF and NS when they act in opposition?

A
  • NS and GF when they counteract one another, selection is acting in opposite directions
  • it requires LARGE amounts of gene flow to substantially change the effects of selection
62
Q

What occurs during dispersal and evolution of geographic ranges?

A
  • selection can favour high or low dispersal ability (vaires among or within species)
63
Q

What occurs during dispersal and evolution of geographic ranges?

A
  • selection can favour high or low dispersal ability (varies among or within species)
64
Q

What does low dispersal ability affect evolution of geographic ranges and favoured in?

A
  • species in stable environments

- species in isolated patches of favourable habitat (migration/dispersal has a high mortality cost)

65
Q

What does high dispersal ability affect evolution of geographic ranges and favoured in?

A
  • colonizing species
  • species in variable environments (with turnover of habitat patches)
    species subject to directional change in environment (ex. climate change)
66
Q

How can ranges of species expand? and affect dispersal ability?

A
  • intraspecific variation in dispersal ability can lead to range expansion
  • ex cane toads introduced to Australia, have the best dispersers having the longer legs

Biology 355 - Evolution (7 decks)

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