Exam 2:Evolutionary Mechanisms Flashcards
What are the 3 kinds of variation?
genetic (G), environmental (E), genotype x environment interaction
What does selection act on?
selection acts on individual phenotypes but really affects underlying allele frequency; genetic variation is raw material for evolution
What is genetic variation? (G)
mendelian: patterns of inheritance of sexual reproduction, alleles, discrete trait
continuous: example human height and weight; governed by presence of many genes
What is environmental variation? (E)
plasticity (sometimes adaptive); non-genetic influences which alter the phenotype ex:if 2 plants put into dif. environments
What is interaction of genotype and environment? (G x E)
more flexible/pliable to environment=greater adaptive potential
ex: when tanning, some people burn others tan, etc.
What is reaction norm?
spectrum of phenotypic variation produced when individuals of similar genotypes exposed to varying environmental conditions; ability of genotype to respond to environment
What is a point mutation?
change of nucleotide
What is frameshift mutation?
insertion: adding base
deletion: removing base
What is a gene mutation?
point mutation, one base substituted for another, results in 3 possible consequences:
Nonsence Mutation: code for stop, resulting in unfinished protein
Missence Mutation: code for dif. amino acid
Silent Mutation: code for same amino acid
What is a gene duplication?
source of (almost) all genes; perfect duplicates are usually redundant, usually 2 copy eventually lost by drift; sometimes persist and acquire new functions
What is neofunctionalization?
process which gene acquires new function after duplication event
What is subfunctionalization?
following gene duplication, paralogues undergo divisions of labor
What is nonfunctionalization?
duplicate gene non-functional, usually lost in genetic drift
What is chromosomal mutation?
usually lethal; inversion, deletions, duplication, translocation, non-disjunction
What is inversion?
segment of genes flip on chromosome
What is non-disjunction?
fails to separate @ meiosis
What is polyploidazation?
polyploidy result of WHOLE genome duplication
What is autoploidy?
individual chromosomes doubled (duplicate copy of own/identical genome)
What is allopolyploidy?
chromosomes from individuals for hybrid so those chromosomes doubled, keep both copies of parent 1 and parent 2 100% of each parent
What are the mutation effects?
detrimental (common), neutral (common), beneficial (rare), mutations tend to erode adaptation, only when trait selected for will it persist for long
What is Hardy Weinburg equilibrium?
Provides the framework for understanding how populations evolve
Laws:
1. There is no mutation or migration
2. Mating is random w/ respect to the alleles
3. Alleles have equal fitness
4. Population is infinitely large
How can Evolution Happen?
One of the Hardy-Weinberg assumptions MUST be false.
- no mutation (no new alleles added to gene pool)
- no migration/gene flow
- mating is random with respect to the alleles (no sexual selection)
- alleles have equal fitness=selection
- the population is infinitely large=genetic drift
When is HW used?
Used to predict genotype frequencies in a population, used ONLY for traits in simple dominant-recessive systems (2 alleles)
What is the principle of HW?
allele and genotype frequencies of population will remain constant from generation to generation=equilibrium UNLESS acted upon by selection
- by using HW, can calculate expected phenotypes, genotypes, alleles
- genotype result of probability of inquiring allele
- phenotype dependent on genotype
What are the 4 strategies of solving HW problems?
- If given genotypes (AA,Aa,aa) calculate p and q by adding up total number of A and a alleles
- If given phenotypes, use recessive genotype “aa” to find q^2 and then q and p
- Use p^2+2pq+q^2 to find genotype frequencies
- If p and q are NOT constant from gen to gen, then population is evolving
When does selection occur?
when individual phenotypes (genotypes) have greater net fitness
Can selection occur without evolution?
YES, fitness may NOT change allele frequency over time
What is average excess of fitness?
the difference between the average fitness of individuals bearing the allele and the average fitness of the population as a whole, when 1 allele consistently enhances fitness relative to other alleles
A=[p(w11-wbar)]+[q(w12-wbar)]
p= probability of an A1 allele finding itself in a A1A1 homozygote q= probability of an A1 allele finding itself in an A1A2 heterozygote
What is survival rate?
overall survival rate is % of individuals born that survive to reproductive age. Often measure % that survive over period of time
What is reproductive rate?
for any given genotype or phenotype, the average # of offspring born per individual
What is relative fitness (w)?
is survival and/or reproductive rate of genotype (or phenotype) relative to maximum survival and/or reproductive rate of other genotypes in populatio
What is dominance and relevance to natural selection?
Consequences of natural selection depend on dominance of fitness, i.e. whether fit phenotype is due to dominant or recessive allele; usually use selection coefficient (s)
For a dominant allele: faster/more pronounced when jumping to fixation
For a recessive allele: less pronounced jump to fixation, recessivee alleles can “hide” @ lo frequency and not affected by selection, when p
What is the selection coefficient (s) measuring?
measure of relative strength of selection acting against a genotype, compares 2 phenotypes and provides a measure of proportional amount that given phenotype is less fit
- s=0 means genotype is NOT selected against
- means that s=1 (lethal, selection against genotype is total and makes NO contributions to the next generation)
- S usually used to measure strength of selection against genotypes containing dominant allele relative to homozygous recessive (but selection in favor of genotypes containing dominant allele can be accommodated by using NEGATIVE S)
How do we model evolution when we only care for relative fitness?
when there is dominance, often use selection coefficient (s) the difference between mutant homozygotes fitness and wildtype fitness, when latter is set to 1
What is directional selection?
one allele consistently enhances fitness or makes it stay the same(never reduce), results in the favored allele to increase in frequency, causes rapid fixation in a population, deleterious mutations driven extinct and kept @ lo frequency, acts on single locus in diploid; REMOVES variations (like drift but quicker) but can be replenished by mutations, frequency of alleles in infinite populations depend on balance between mutation+selection
What does fitness differences in genotypes result in?
result in differences in average excess of alleles when:
- genotypes differ in fitness
- current allele frequency are not in equilibrium
What are 2 scenarios for adaptive evolution?
- Environment changes causes relative fitness to change, pre-existing mutants favored by selection
- environment remains constant, new mutations arise that enhance fitness
(REMEMBER MUTATION RATE IS ALWAYS CONSTANT)
What are the characteristics of dominance?
Often when an allele is is dominant can have difference in starting+jumping to fixation (lot more steeper when compared to a recessive allele), recessive alleles can “hide” @ lo frequency, if p
Explain why recessive genetic diseases still prevalent in population
recessive disease alleles @ lo frequeency in almost all populations, as recessive alleles hide and are not affected by selection, dominant disease alleles @ even lower frequency, frequencies governed by selection-mutation balance
What allele is favored in directional selection?
either recessive or dominant, 1 allele is favored, maintain alleles in population even one that are deleterious in one of the homozygous conditions; usually beneficial mutations go to fixation (positive selection) and deeleterious mutations tend to bee kept @ lo freq (negative selection)
What allele/genotype favored in balancing selection?
heterozygotes most fit, tends to maintain variation in population bc it keeps both alleles, when A1 is rare tends to increase in freq, when common tends to decrease in freq
w11w22 (overdominance)
What allele/genotype favored in disruptive selection?
heterozygotes least fit, tends to disfavor whichever allele rarer and favor whichever allele is common; prevents evolution ofo potentially beneficial alleles generated by mutation
w11>w12
What is genetic drift?
- random fluctuation of allele variants in a populations
- happens ALWAYS in finite populations
- decreases genetic variation
- rate of heterozygosity lost by genetic drift dictated by pop size (larger pop=longer time to get to fixation)
- drift increases as pop gets smaller
- acts on all LOCI compared to directional selection which only acts on 1
- jaggedness on graph
How long until fixation happens?
- higher selection=faster fixation
- when no directional selection ONLY drift, population size determines the rate because small populations more EASILY lose variation faster (think about probability, more likely to get equal numbers of heads and tails in 100 rounds vs only 10)
-important to remember that directional selection overcomes genetic drift in LARGE populations and when favored allele has high initial frequency
Which allele will be “fixed” under drift?
initial frequency is the chance of an allele to go to fixation
ex: if the starting frequency of allele A is 0.5 then it has a 50% of going to fixation or a 50% chance of getting lost given that there is NO selection
How long until fixation happens?
- higher selection=faster fixation
- when no directional selection ONLY drift, population size determines the rate because small populations more EASILY lose variation faster (think about probability, more likely to get equal numbers of heads and tails in 100 rounds vs only 10)
-important to remember that directional selection overcomes genetic drift in LARGE populations and when favored allele has high initial frequency
What is overdominant selection?
heterozygotes most fit, tends toward equilibrium allele frequency, tends to keep allelic variation, w11w22; frequency of allele likely to gravitate toward a stable equilibrium freq
ex:codominace and incomplete dominance
What is underdominant selection?
heterozygotes least fit, unstable equilibrium, favors whichever allele has a high frequency, rare alleles lost common alleles usually go to fixatio, w11>w12; one allele likely go to fixation other lost depennds on initial allele freq.
ex:mimicry
If genetic drift AND directional selection are present in a population which will win?
Directional selection will overcome G.D. in large populations + when favored allele is already @ high frequency, ineffective in small populations (i.e. favored allele may not get fixed in smaller pops)
Directional selection INEFFECTIVE for rare alleles in small pops
What are the ways that selection can act on traits/phenotypes?
- Directional: one extreme favored
- Stabilizing: Average values are favored
- Disruptive: both extremes are favored
Is stabilizing selection the same as overdominant selection?
NO!! Overdominant selection favors heterozygous GENOTYPES and tends to MAINTAIN allelic variation at ONE locus, while stabilizing selection favors PHENOTYPES near the mean and tends to REMOVE allelic variation @ MANY LOCI
What is disruptive selection?
favors phenotype near both extremes, increase variance/standard deviation of trait, tends to MAINTAIN allelic variation
What is stabilizing selection?
favors phenotypes near the mean, tends to remove allelic variation @ many loci, variance decreases, powerful agent of conservation
What is directional selection-phenotypic?
one extreme favored, population mean moves, variance decreases, favored allele fixed
What is selection-mutation balance?
equilibrium of number of deleterious alleles created by a mutation in a population occurs at the same rate @ which deleterious alleles eliminated by selection
What is frequency-dependent selection? What is positive? What is negative?
fitness of phenotype depends on how “common” it is in the pop
positive= common phenotypes favored; fitness of phenotype/genotype increases as it becomes more common ex:pathogen-host coevolution
negative=rare phenotypes favored, as allelic fitness decreases with increasing frequency in the protection of rare alleles
ex: self-incompatability allele
What is mutation rate?
u=d/2t
t=divergence
u= rate of allele substitution of generation
What does the probability of new non-neutral mutations depend on?
- Impact of mutation on fitness, s
- effectiveness of selection vs drift (chance) determined by population size Ne
- So when Ne*s is very small (either Ne or s) then allele will have about same probability of fixation as neutral allele
when 2Ne*s<1 an allele behaves as though it was neutral, neutral alleles have probability of fixation of 1/2N
-expect to see higher mutation rates in smaller populations
At what rate does evolution happen?
- when no selection, depends on mutation rate
- when selection active, rate depends on rate of mutation + strength of selection + population size
How to determine if selection or genetic drift is occurring via NeS?
- Selection occurs when Ne*s >1
- Drift occurs when Ne*s<1
- -if not too negative, could happen that mildly deleterious allele goes to fixation
What are smaller populations more susceptible to?
sperm (haploid) sensitive to genetic load- build up of deleterious alleles, mutational meltdown
What are the 2 processes that influence genetic drift and diversity of a population?
- Bottleneck Effect
- sudden reduction in pop size due to change in environment or fragmentation, new gene pool may not. reflect original, if pop increases genetic diversity may remain lo - Founder Effect
- few individuals become isolated from large pop, allele freq in small founder pop will be dif from those in larger parent pop
What are some conservation implications? (3)
small pops are more @ risk for extinction
- lower heterozygosity=less adaptable
- greater pairwise similarity=shared susceptibility to pathogens
- deleterious alleles @ hi freq=hi genetic load
When does evolution stop?
- when p=0 or q=0
- when population is at an intermediate equilibrium
Only happens when either
-heterozygotes are fittest=overdominace
-heterozygotes least fit=underdominance
