Random Events

Question 1

why is large population size important for the Hardy-Weinberg theorem?

Answer 1

a large population will follow HW more closely than a small one because alleles A and a are in a 1:1 ratio, so just like flipping a coin, and the larger the population, the closer the actual ratio of alleles passed on will be to 1:1

In a smaller population on 10 heterozygotes you might get 6 organisms that pass on A and 4 that pass on a and that would not be surprising but then the allele frequency has randomly shifted from p=.5 to p=.6

In a larger population of 1000 heterozygotes any random shift in p should be very small

Question 2

what is genetic drift?

Answer 2

RANDOM change in allele frequencies between generations

no tendency for A or a to be favored

Question 3

If genetic drift is the only microevolutionary force acting on a population, what are the eventual frequencies of alleles in the population?

Answer 3

All alleles except one will always be lost from the population 

the speed at which this will happen is dependent on the size of the population

drift is the march to homozygocity, it will erode variation to the point where there’s only one allele IF it’s the only thing acting upon a population

Question 4

what did the genetic drift computer simulation show?

Answer 4

each panel showed ten simulations beginning at p=q=.5

drift eventually results in fixation, even in “large” populations

Question 5

what does genetic drift lead to?

Answer 5

it is the “march to homozygosity”

if alleles are selectively neutral, population will eventually “drift” to homozygosity (fixation)

evolution has occurred by a random process

Question 6

what is heterozygosity?

Answer 6

a standard measure of genetic variation per locus in a population

also called “gene diversity”

Question 7

what is the formula for heterozygosity?

Answer 7

H = 1 - (p^2 + q^2)

p is the frequency of allele 1
1 is the frequency of allele 1

Question 8

what is H?

Answer 8

heterozygosity: the chance of drawing 2 different alleles

in HW equilibrium, H is the proportion of heterozygotes

even a population with no heterozygotes can have heterozygosity...
- frequency of AA = .5
- frequency of Aa = 0
- frequency of aa = .5
BUT H = .5!!

Question 9

what is homozygosity?

Answer 9

(f)

the chance of drawing 2 identical alleles

Question 10

what are the two formulas for homozygosity?

Answer 10

f= (p^2+q^2)

OR

f= 1-H

Question 11

what are the effects of drift and mutation on heterozygosity?

Answer 11

drift tends to reduct heterozygosity

mutation increases heterozygosity

Genetic polymorphism is thus a function of rate of drift and rate of mutation

selection can act before, during, or after, in concert with or in opposition to these forces

Question 12

what is the founder effect?

Answer 12

establishment of a new population by a few “founder” which carry only a small fraction of the total genetic variation of the parental population –> not representative of the whole population –> non-random mating

if the founder event occurs but genetic variation of the new population is the same, then there is no genetic drift via founder effect

ex. original population has yellow, blue, and red
1) founder effect - yellow lost, blue more frequent
2) founder event, but not strong founder effect because there’s still a mix of all alleles
3) founder effect, nothing lost, but red much more frequent

Question 13

what are some outcomes of the founder effect

Answer 13

composition of founder population is not purposefully selected to include particular genotypes

genes in low frequency in original population become common in founder population

well studied for genetic diseases in human populations

Question 14

what is porphyria variegata?

Answer 14

abnormal heme molecules cause intermittent skin disorders, circulatory and respiratory problems

1 in 300 South African Afrikaaners

the mutant gene can be traced to a single couple who settled in South Africa in the 17th century

Question 15

what is Huntington’s disease? How does it demonstrate the founder effect?

Answer 15

prevalence among Afrikaners can be traced to a single Dutch founder

432 carriers in Australia can be traced to a single English founder with 13 children

prevalence in Lake Maracaibo can be traced back 200 yrs to 1 founder with 10 children

Question 16

what is Polydactyly? How does it demonstrate the founder effect?

Answer 16

more common among Amish than the American population at large

small number of founders

most marriages within population

gene can be traced to a single pair of founders in 1744

Question 17

What is the chance of drawing one homozygote randomly?

Answer 17

p^2 + q^2

Question 18

What is the chance of drawing two homozygotes randomly?

Answer 18

(p^2)^2 plus (q^2)^2

Question 19

what is the Total chance of homozygosity?

Answer 19

[ (p^2)^N + (q^2)^N ]

N = size of founding population

Question 20

Are founder events likely to produce homozygosity?

Answer 20

not likely to produce homozygosity

However:

• rare alleles may be lost
• new allele frequencies might be very different
• rare alleles might be relatively more common
• small founding populations vulnerable to subsequent drift

Question 21

what are population bottlenecks?

Answer 21

Environmental (or other) catastrophe decimates a large percentage of the population

Population rebounds, but genetic diversity is a subset of what existed before catastrophe

Question 22

when does a population bottleneck occur?

Answer 22

a bottleneck occurs when population size decreases, often resulting in a loss of genetic variation

founder effect is one kind of bottleneck

Question 23

how is a past bottleneck identified?

Answer 23

A past bottleneck is often inferred from low levels of present genetic diversity

Question 24

importance of randomness in shaping population allele frequencies

Answer 24

genetic drift causes changes in allele frequency because a non-representative sample of the population produces more gametes –> resulting offspring don’t mirror the original population in terms of allele frequency (but because of chance, not fitness differences)

small populations may be especially vulnerable to random events because it is “easier” to draw a non-representative sample from them

Question 25

what does drift do to genetic variation?

Answer 25

drift reduces genetic variation given enough time drift will end in fixation for neutral alleles

Question 26

what are neutral alleles?

Answer 26

neural alleles are those with no selective advantage/disadvantage

Question 27

rate of evolution and N relationship

Answer 27

rate of evolution via drift is independent of N small populations are more sensitive to random events

Question 28

what is the chance that a given neutral allele becomes fixed?

Answer 28

1/2N if you have a small population, the drift component is going to be a larger number

Question 29

what is the rate that neutral alleles are formed by mutation

Answer 29

2N(u) u = rate of neutral mutations

Question 30

what is the rate of neutral evolution?

Answer 30

(1/2N) x 2N(u) = u

Question 31

what is u?

Answer 31

rate of neutral mutations

Question 32

what does the rate of neutral evolution depend on?

Answer 32

the rate of neutral evolution depends entirely on mutation rate

Question 33

what does it mean when 1/2N is a big number? how does that effect 2N(u)

Answer 33

there's a good chance of a mutation becoming fixed in the population however, 2N(u) is a smaller number and there's a small chance of that mutation actually occurring though --> less alleles, less chance of the mutation is actually going to occur these two forces oppose each other***

Question 34

what does mutation do to HW?

Answer 34

mutation violates hardy-weinberg by introducing new alleles into the population

Question 35

neutral mutation and polymorphism

Answer 35

neutral mutation maintains polymorphism if drift occurs shouldn't populations be homozygous? NO: mutation balances drift to produce an equilibrial heterozygosity (H*)

Question 36

what is polymorphism?

Answer 36

the occurrence of different forms among the members of a population or colony, or in the life cycle of an individual organism.

Question 37

what is H*?

Answer 37

equilibrial heterozygosity mutation balances drift to produce an equilibrial heterozygosity

Question 38

when is H* lower?

Answer 38

1) N is small (small population) 2) u is small (mutations are infrequent) under these conditions, drift is more powerful H is dependent on Nu...the larger Nu, the larger H is

Question 39

purifying selection

Answer 39

even if drift is important, most mutations may have negative consequences natural selection weeds the highly negative ones out right away through purifying selection

Question 40

how common is mutation in asexual vs sexual organisms?

Answer 40

mutation is not very common in sexual organisms high mutation rate may be advantageous in an uncertain world

Question 41

sex ratio of gonochoric population

Answer 41

Most gonochoric (dioecious) populations have a sex ratio ~ 50/50 Females determine the reproductive output of a population Any individual female typically has higher mating success than any individual male

Question 42

asexual reproduction

Answer 42

all/most members of the population can reproduce reproductive output of the population is higher because more individuals reproduce

Question 43

what evidence is there that sex is actually good from an evolutionary standpoint?

Answer 43

1) ubiquity - seen in most eukaryotes - other modes of recombination prevalent in prokaryotes 2) longevity of sexual lineages compared to asexual lineages - most exclusively or primarily asexual organisms relatively recently derived from a sexual ancestor

Question 44

why do organisms have sex?

Answer 44

sex is advantageous because recombination is advantageous in the long term if not in the short term

Question 45

sex and more variability

Answer 45

- sex facilitates the generation of novel combinations of alleles more readily than mutation alone - sex disrupts linkage disequilibrium through crossing over and thus facilitates novel allele combinations more than mutation alone - variability hypotheses are associated with variable or fluctuating environments

Question 46

sex and fewer lethals

Answer 46

sex may help to hide or purge deleterious alleles (or even those that just have low fitness) from the general population

Question 47

what are benefits of sex?

Answer 47

- more variability | - few lethals

Question 48

what is Muller's ratchet?

Answer 48

deleterious alleles will accumulate in an asexual population over time if mutations from "bad" back to "good" or "neutral" are rare the number of individuals with 0 low-fitness mutations will decrease from generation to generation as mutation load increases

Question 49

consequences of Muller's Ratchet

Answer 49

- over time, the proportion of the population comprised of low-fitness genotypes increases - this will decrease the size of the population as genotypes slide off the end of the scale

Question 50

how does migration violate HW?

Answer 50

by introducing or removing alleles from the population migration can maintain genetic diversity in the face of natural selection or genetic drift

Question 51

mutations through generations

Answer 51

in the first generation there are a few individuals with no deleterious mutations over time, all subsets of the population acquire mutations so the "zero" class gets eliminated the the "1" class becomes the strain with the smallest mutation load over even more time, all subsets of the population acquire mutations so that the "1" class gets eliminated and the "2" class becomes the strain with the smallest mutation load

Question 52

mutation meltdown and sex

Answer 52

the "speed" of the ratchet increases with decreasing population size combined effect of Muller's ratchet and drift leads to "mutation meltdown" and extinction sex "reconstitutes" low-load genotypes through recombination and independent assortment

Question 53

what happens when there's gene flow from a large population to a small population?

Answer 53

when there's gene flow/migration from the large "source" population to a small "sink" population: 1) gene flow from source creates genetic variation 2) selection or drift erodes genetic variation 3) gene flow restores genetic variation

Question 54

is evolution random?

Answer 54

some aspects of evolution are random 1) mutation: ultimate source of all genetic variation 2) genetic drift: random change in gene frequencies between generation 3) migration/gene flow: can result in random gene flow in/out of population natural selection is not random 1) differential non-random survival and reproduction but only natural selection can explain adaptive evolution

Question 55

what are the 4 mechanisms for microevolutionary change

Answer 55

1) mutation 2) genetic drift 3) migration/gene flow 4) natural selection these cause change in gene frequencies across generations