Population Genetics I: Natural Selection Flashcards
Hardy-Weinberg Equation
(p + q)^2 = p^2 + 2pq + q^2 = 1
within a population, do allele frequencies change from generation to generation?
no, allele frequencies will not change
what happens if the random mating assumption is violated?
genotype frequencies will change but allele frequencies will not
evolutionary processes
- infinite population size
- no migration
- no mutation
- no natural selection
- mating is random
how many rounds of random mating is required to reestablish HW equilibrium?
one round of random mating
how to find the frequency of the A allele
p^2 + (1/2) 2pq
how to calculate the expected number of individuals with a genotype given allele frequencies
p^2
2pq
q^2
then total the answers
steps to a statistical test
- have a hypothesis
- define the expectation under the null hypothesis of no effect
- collect data
- compute a test statistic
- find the probability of your test statistic
- interpret your results
chi squared test
X^2 = sum (observed - expected)^2 / expected
critical value of X^2 distribution
3.84
absolute fitness
the average lifetime reproductive success of individuals with that phenotype
fitness component
an individual score on a measure of performance expected to correlate strongly with the genetic contribution to the next generation
examples of fitness components
- survival
- fecundity
natural selection
a consequence of differences in fitness between individuals that differ in phenotype
symbol for relative fitness
W
relative fitness
the fitness of a phenotype compared with others in the population
what is the maximum relative fitness typically equal to
1
how to calculate relative fitness
absolute fitness / maximum absolute fitness
how to calculate the allele frequencies at time t0
p = AA / total t0 + 0.5 (Aa / total t0)
q = aa / total t0 + 0.5 (Aa / total t0)
how to calculate the expected # of individuals at time t1
AA = p^2 + total t1
Aa = 2pq + total t1
aa = q + total t1
add all together
how to calculate absolute fitness
observed at time t1 / # expected at time t1
- done for all genotypes
how to calculate relative fitness
all fitnesses / maximum absolute fitness
how to see there was evolution?
delta q = q1 - q0
fitness is a _________, not an _________
description, explanation
how did Herbert Spenser define natural selection?
- he defined natural selection as survival of the fittest
- this is not an accurate definition
how does an advantageous trait contribute to an organisms?
the organisms survival and reproductive success
what does the phenotype of an organism in relation to the environment it inhabits determine?
how fit that organism is
fitness is…
context dependent
the general selection model equation
delta q = pq [q (W3 - W2) + P (W2 - W1)] / W
how to calculate average effect of an allele substitution
[q (W3 - W2) + p (W2 - W1)]
what does the general selection model quanify?
the relationship between allele frequencies and fitness
the greater the difference between the relative fitnesses…
the greater the rate of evolution
S
selection coefficient
AA
W1 = 1
Aa
W2 = 1a
aa
W3 = 1 - S
how to calculate selection with dominance
delta q = -spq^2 / 1 - sq^2
directional selection
a mode of natural selection in which a phenotype is consistently favored, causing allele frequencies to shift over time in the direction of that phenotype
the rate of change in allele frequencies
a function of the allele frequencies, the relative fitness values, and the mean population fitness in the population
does the directional selection always increase mean population fitness?
yes
how to calculate the selection with dominance when the fitness of the homozygous recessive = 0
qn = q0 / 1 + nq0
eugenics
the study and practice of control over the evolution of human populations in an effort to decrease the frequency of undesirable traits
cystic fibrosis
cell surface protein expressed in the mucus lining of the intestines and lungs
what does CTFR enable?
cells to ingest and destroy the bacterium
how does cystic fibrosis form?
homozygotes for loss-of-function mutations suffer chronic infections with the bacterium
how to calculate heterozygote advantage
p = S2 / S1 + S2
q = S1 / S1 + S2
balancing selection
forms of natural selection that maintain variation within a population
Hb^A
normal allele in hemoglobin
Hb^S
sickle cell allele in hemoglobin
what are homozygotes for the Hb^A allele susceptible to?
malaria in West Africa
what is malaria caused by?
protozoan Alveolata
symptoms and effects of malaria and sickle cell anemia
- invades red blood cells & eats the hemoglobin
- malaria causes severe fevers
- infected blood cells clump up and cause dangerous clots
Hb^S allele anemia
- the red blood cells do not carry oxygen well and form a sickle shape when they do not have oxygen
- the blocks blood vessels and eventually leads to death
- when sickle cells pass through the spleen they are detected by white blood cells as defective and destroyed
Hb^A Hb^S heterozygotes
resistant to malaria but suffer mild anemia
is S positive or negative in underdominance?
negative
mutation
- fundamental to evolutionary theory
- the raw material of the evolutionary process
mutation includes
- point mutations
- indels
- gene duplication
- transposable elements
- chromosomal mutations
- genome level mutations
is mutation planned or random?
mutation has causes, but is random with respect to adaptive needs
the environment doe snot induce adaptive mutations
no “inheritance of acquired characters”
what prevents mutations?
developmental constraints prevent many mutations from ever being expressed at the phenotypic level
purifying selection
selection against deleterious mutation
how is purifying selection removed?
removed by selection if dominant or codominant
u = A –> a
forward mutation rate
v = a –> A
backward mutation rate
how does mutation change the frequency of allele a?
if deltaqm is the per generation change in q, due to mutation, then
detlaqm = pu -qv
mutation without selection will…
increase the frequency of the a allele
what happens when there is a selection against the aa genotype?
the frequency of the a allele declines
selection without mutation will…
decrease the frequency of the a allele
what happens at mutation selection balance?
a allele will achieve an equilibrium
what kind of disease is spinal muscular atropy?
neurodegenerative disease
neurodegenerative disease
degeneration of the muscles that control voluntary movement
what causes neurodegenerative disease?
deletions at one locus
what maintains neurodegenerative disease?
mutation/selection balance
