![Page 1: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/1.jpg)
Selection and Genetic Variation1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are homozygous consider Dawson’s flour beetles: started with population of all heterozygotes, + / l
l / l is lethal, but + / l is same as wildtype +/+
![Page 2: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/2.jpg)
Selection and Genetic Variation1) selection against recessive alleles
Although selection initially removed the l allele from population at a rapid rate, with each generation the frequency of l declined more slowly
![Page 3: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/3.jpg)
Selection and Genetic Variation
2) selection against homozygotes This population was started with 100% heterozygotes for a viable allele V, and an allele L that is lethal when homozygous
although selection rapidly caused the V allele to increase in frequency, the L allele never disappeared
in fact, the frequency of L stabilized at 0.21
![Page 4: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/4.jpg)
Selection and Genetic Variation
2) selection against homozygotes 1/5th of the population carried the lethal allele at equilibrium (the point where the population ceased to evolve)
Why?
![Page 5: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/5.jpg)
Selection and Genetic Variation3) selection against heterozygotes
consider the case of flies with compound chromosomes
normal pair of homologouschromosomes
compound chromosomes: arms swapped - one ends up with both left halves - other ends up with both right halves
when these flies make sperm/eggs, meiosis gets screwed up... they make 4 kinds of gametes
![Page 6: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/6.jpg)
Selection and Genetic Variation3) selection against heterozygotes
- Flies can be homozygous for C (compound) or N (normal) allele
- two N/N flies can reproduce; all zygotes are viable (fitness =1)
- two C/C flies can reproduce; 1/4th of zygotes viable (fitness = 0.25)
- C/N flies don’t exist; they never develop (fitness = 0)
C and N flies can’t make viable zygotes together
![Page 7: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/7.jpg)
Selection and Genetic Variation3) selection against heterozygotes one or the other allele quickly becomes fixed in a mixed population
![Page 8: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/8.jpg)
Selection and Genetic Variation3) selection against heterozygotes one or the other allele quickly becomes fixed in a mixed population
- why? if there are few N/N flies, the odds of 2 mating are low- most N/N flies will not produce viable offspring- the allele will vanish
- if there are many N/N flies, they quickly out-breed C/C flies, due to their 4-fold advantage in producing viable offspring
this is underdominance:
![Page 9: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/9.jpg)
Models of heterozygote superiority and inferiority
- in overdominance (heterozygote fitness > homozygote fitness),
population fitness is maximized at its stable internal equilibrium,
the point to which the population naturally returns
![Page 10: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/10.jpg)
Models of heterozygote superiority and inferiority
- in underdominance (homozygote fitness > heterozygote fitness),
the population fitness is minimized at the unstable internal
equilibrium, the point from which the population naturally diverges
![Page 11: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/11.jpg)
Frequency-dependent selection
scale-eating fish of Lake Tanganyika
Attack other fish by sneaking up, rushing them, biting off a mouthful of scales - Those with mouths that curve to the right attack the left side of victims, and vice-versa - Handedness of mouth is determined by a single locus with 2 alleles (simplest case!) - Right-handedness is dominant
![Page 12: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/12.jpg)
Frequency-dependent selection
- victims come to expect attacks from the direction that the majority of the scale-eaters attack from, at that particular time
- when right-handed fish are more common, victims pay less attention to their right side (where few attacks come from); this gives left-handed fish the edge
- as left-handers get more food, they survive and reproduce better
- then, when left-handed offspring are the majority, the situation reverses
![Page 13: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/13.jpg)
Frequency-dependent selection
- squares = proportion of successful breeding adults
pro
po
rtio
n o
fle
ft-h
an
de
rs
![Page 14: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/14.jpg)
Frequency-dependent selectionp
rop
ort
ion
of
left-
ha
nd
ers
![Page 15: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/15.jpg)
Frequency-dependent selection
The equilibrium point should be 50/50 of each phenotype…
…so what are the expected allele & genotype frequencies? Alleles: R LAllele frequencies 0.3 0.7 Possible genotypes: RR RL LL Hardy-Weinberg predicts: R2 + 2RL + L2
Genotype frequencies: 0.09 0.42 0.49
![Page 16: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/16.jpg)
Frequency-dependent selection 2Another case: pea aphid Acyrthosiphon pisum occurs in green and red color morphs
- what maintains polymorphism, the occurrence of both phenotypes in the population?
Differential vulnerability to predation versus parasitism, depending on color
- green aphids are more parasitized by wasps that lay their eggs inside aphids
- red aphids get eaten more by ladybugs (they’re more obvious sitting there on green plants)
![Page 17: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/17.jpg)
Mutation as an evolutionary force
Mutation is ultimately responsible for creating new alleles and genes, but.. - can mutation also represent an evolutionary force, by changing allele frequencies?
- can mutation affect the predictions of Hardy-Weinberg equilibrium?
![Page 18: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/18.jpg)
Mutation as an evolutionary force
Consider a population where allele frequencies are:
A a (a recessive, loss-of-function allele) 0.9 0.1
In the ordinary Hardy-Weinberg state, adult genotypes will be:
AA Aa aa0.81 0.18 0.01
![Page 19: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/19.jpg)
Mutation as an evolutionary force
Now assume A mutates to a at a rate of 1 per 10,000 genes each generation due to mutation, the allelic makeup of gametes will be:
A a 0.9 – (0.9)(0.0001) 0.1 + (0.9)(0.0001)= 0.899991 = 0.10009
![Page 20: Selection and Genetic Variation 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are](https://reader035.vdocuments.net/reader035/viewer/2022070415/56649e685503460f94b64bf2/html5/thumbnails/20.jpg)
Mutation as an evolutionary forceWhen gametes randomly fuse to form zygotes, the genotype frequencies will be:
AA Aa aa0.80998 0.18016 0.01002
Hardly any change; mutation had little effect over one generation Over thousands of generations, mutation can affect allele frequencies