Random Events Flashcards
why is large population size important for the Hardy-Weinberg theorem?
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
what is genetic drift?
RANDOM change in allele frequencies between generations
no tendency for A or a to be favored
If genetic drift is the only microevolutionary force acting on a population, what are the eventual frequencies of alleles in the population?
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
what did the genetic drift computer simulation show?
each panel showed ten simulations beginning at p=q=.5
drift eventually results in fixation, even in “large” populations
what does genetic drift lead to?
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
what is heterozygosity?
a standard measure of genetic variation per locus in a population
also called “gene diversity”
what is the formula for heterozygosity?
H = 1 - (p^2 + q^2)
p is the frequency of allele 1
1 is the frequency of allele 1
what is H?
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!!
what is homozygosity?
(f)
the chance of drawing 2 identical alleles
what are the two formulas for homozygosity?
f= (p^2+q^2)
OR
f= 1-H
what are the effects of drift and mutation on heterozygosity?
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
what is the founder effect?
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
what are some outcomes of the founder effect
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
what is porphyria variegata?
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
what is Huntington’s disease? How does it demonstrate the founder effect?
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
what is Polydactyly? How does it demonstrate the founder effect?
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
What is the chance of drawing one homozygote randomly?
p^2 + q^2
What is the chance of drawing two homozygotes randomly?
(p^2)^2 plus (q^2)^2
what is the Total chance of homozygosity?
[ (p^2)^N + (q^2)^N ]
N = size of founding population
Are founder events likely to produce homozygosity?
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
what are population bottlenecks?
Environmental (or other) catastrophe decimates a large percentage of the population
Population rebounds, but genetic diversity is a subset of what existed before catastrophe
when does a population bottleneck occur?
a bottleneck occurs when population size decreases, often resulting in a loss of genetic variation
founder effect is one kind of bottleneck
how is a past bottleneck identified?
A past bottleneck is often inferred from low levels of present genetic diversity
importance of randomness in shaping population allele frequencies
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
what does drift do to genetic variation?
drift reduces genetic variation
given enough time drift will end in fixation for neutral alleles
what are neutral alleles?
neural alleles are those with no selective advantage/disadvantage
rate of evolution and N relationship
rate of evolution via drift is independent of N
small populations are more sensitive to random events
what is the chance that a given neutral allele becomes fixed?
1/2N
if you have a small population, the drift component is going to be a larger number
what is the rate that neutral alleles are formed by mutation
2N(u)
u = rate of neutral mutations
what is the rate of neutral evolution?
(1/2N) x 2N(u) = u
what is u?
rate of neutral mutations
what does the rate of neutral evolution depend on?
the rate of neutral evolution depends entirely on mutation rate
what does it mean when 1/2N is a big number? how does that effect 2N(u)
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***
what does mutation do to HW?
mutation violates hardy-weinberg by introducing new alleles into the population
neutral mutation and polymorphism
neutral mutation maintains polymorphism
if drift occurs shouldn’t populations be homozygous?
NO: mutation balances drift to produce an equilibrial heterozygosity (H*)
what is polymorphism?
the occurrence of different forms among the members of a population or colony, or in the life cycle of an individual organism.
what is H*?
equilibrial heterozygosity
mutation balances drift to produce an equilibrial heterozygosity
when is H* lower?
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
purifying selection
even if drift is important, most mutations may have negative consequences
natural selection weeds the highly negative ones out right away through purifying selection
how common is mutation in asexual vs sexual organisms?
mutation is not very common in sexual organisms
high mutation rate may be advantageous in an uncertain world
sex ratio of gonochoric population
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
asexual reproduction
all/most members of the population can reproduce
reproductive output of the population is higher because more individuals reproduce
what evidence is there that sex is actually good from an evolutionary standpoint?
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
why do organisms have sex?
sex is advantageous because recombination is advantageous in the long term if not in the short term
sex and more variability
- 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
sex and fewer lethals
sex may help to hide or purge deleterious alleles (or even those that just have low fitness) from the general population
what are benefits of sex?
- more variability
- few lethals
what is Muller’s ratchet?
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
consequences of Muller’s Ratchet
- 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
how does migration violate HW?
by introducing or removing alleles from the population
migration can maintain genetic diversity in the face of natural selection or genetic drift
mutations through generations
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
mutation meltdown and sex
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
what happens when there’s gene flow from a large population to a small population?
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
is evolution random?
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
what are the 4 mechanisms for microevolutionary change
1) mutation
2) genetic drift
3) migration/gene flow
4) natural selection
these cause change in gene frequencies across generations
