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Natural selection was proposed by? (2)
- Darwin
- Alfred Russel Wallace
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Def Natural Selection
- Differential reproduction and survival of genetically determined forms
- higher fitness genes are selected for
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Adaptation
Feature of organism that improves ability to survive and reproduce within natural habitat
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Natural selection important points
- not synonymous with evolution
- operates through the "agents" in the enviornment (disease, parasitism, predation, competition)
- selection operates to better adapt pop to their enviornment
- Phenotype is the target of natural selection
- Other genetic forces do not improve adaptiveness, only NS improves overall fitness of pop
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Adaptive vs Neutral variation
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Fitness
- A concept related to NS
- Fitness - repoductive contribution of a genotype to future generations
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Selection coefficient and what does it mean?
- (s) the intensity of selection = 1-w
- (w) = relative fitness
- if w = 1 then s = 0 and you are perfectly fit for env, selection is not working for or against you
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Components of Fitness
multidimensional = overall fitness is made of different components
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Fitness Components (3)
- viability (zygotic) selection =
- Sexual - some genotypes mate better than others usually when competition between sexes is involved
- less common phenotypes have more chance of mating because different
- Fertility - some genotypes are more fertile than others
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How to calculate fitness value - convert absolute fitness to relative fitness value (w)
divide each absolute frequency value by the largest absolute genotype frequency
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Points about fitness values(3) + note about trend
- Enviornment specific
- Rarely constant
- Difficult to measure
As high fitness geneotype becomes more prevelent/high frequency, fitness declines
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Howt is HW model modified to account for Natural Selection Model? How do you calculate frequencies?
- zygotes form by random mating according to (p+q)2 over time (t)
- selection occurs before zygotes are adults
- t+1: ratios of genotype after selection are p2wAA etc
- convert ratios to frequencies (average pop fitness = Wbar) = w(p+q)2
- Frequency = w(p+q)2/(wbar)
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How do you calculate q'?
q' = q(pwAa + qwaa)/(Wbar)
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Types of Selection
- select for dominant
- select for recessive
- For heterozygote/overdominance/Heterozygote advantage/selection against homozygose
- partial dominance
selects for dominant, and partialy hetero - recessive selected againstunderdominant - rapidly one or the other allele will be fixed depending on which way it is pushing (heterozygote is selected against)
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Things to know about selection against a recessive
- steepness of curve is determined by s
- q --> 0 asymptotically: recessive alleles hidden in heterozygote
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Things to know about selection for Dominant
- Steepness of curve determined by s
- Curves do not approach q --> asymptotically
- Dominant lethal will be removed from population in 1 generation
- System has trivial equilibria
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things to know about selection for heterozygote
- qeq determined by selection coefficients
- stable equilibrium mechanism for maintaining both alleles in the population
- polymorphism and genetic load are arguments agianst any advantage to selecting for heterozygotes
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Most famous example of overdominance is ________?
Sickle-cell anemia
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Things to know about Partial dominance
Equilibrium is reached faster than in full dominance
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Underdominance
- selecting against heterozygote
- three possible equilibrium:
- pop becomes fixed for A1
- pop becomes fixed for A2
- both alleles in pop - unstable
- not a good way to maintain polymorphism in a population
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Fitness increase over time
- Average pop fitness will increase over time as less fit genotypes become more rare unless;
- there is no variation
- the fene pool is at equilibrium
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Mutation selectin model
- Model considers the joint effects of selectionand mutation.
- The model:
- 1. Dominant allele A present in frequency p.
- 2. Recessive allele a present in frequency q.
- 3. Selection against aa (waa= 1 - saa.(4. Forward mutation of A to a at rate µ
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Mutation-Selection model
Mutation and selection operating in opposite directions
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