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Selection and Genetic Variation PowerPoint Presentation
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Selection and Genetic Variation

Selection and Genetic Variation

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Selection and Genetic Variation

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  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 homozygous consider Dawson’s flour beetles: started with population of all heterozygotes, + / l l/ lis lethal, but + / lis same as wildtype +/+

  2. Selection and Genetic Variation 1) selection against recessive alleles Although selection initially removed the lallele from population at a rapid rate, with each generation the frequency of l declined more slowly

  3. 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

  4. 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?

  5. Selection and Genetic Variation 3) selection against heterozygotes   consider the case of flies with compound chromosomes normal pair of homologous chromosomes compound chromosomes: arms swapped - one ends up with bothleft halves - other ends up with bothright halves when these flies make sperm/eggs, meiosis gets screwed up... they make 4 kinds of gametes

  6. Selection and Genetic Variation 3) 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

  7. Selection and Genetic Variation 3) selection against heterozygotes one or the other allele quickly becomes fixed in a mixed population

  8. Selection and Genetic Variation 3) 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:

  9. 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

  10. 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

  11. Frequency-dependent selection 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 scale-eating fish of Lake Tanganyika

  12. 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

  13. Frequency-dependent selection proportion of left-handers - squares = proportion of successful breeding adults

  14. Frequency-dependent selection proportion of left-handers

  15. Frequency-dependent selection The equilibrium point should be 50/50 of each phenotype… …so what are the expected allele & genotype frequencies? Alleles: RL Allele frequencies 0.3 0.7 Possible genotypes: RRRLLL Hardy-Weinberg predicts: R2 + 2RL + L2 Genotype frequencies: 0.09 0.42 0.49

  16. Frequency-dependent selection 2 Another case: pea aphid Acyrthosiphon pisum occurs in greenand redcolor 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)

  17. 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?

  18. Mutation as an evolutionary force Consider a population where allele frequencies are: Aa (a recessive, loss-of-function allele) 0.9 0.1 In the ordinary Hardy-Weinberg state, adult genotypes will be: AA Aa aa 0.81 0.18 0.01

  19. 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: Aa 0.9 – (0.9)(0.0001) 0.1 + (0.9)(0.0001) = 0.899991 = 0.10009

  20. Mutation as an evolutionary force When gametes randomly fuse to form zygotes, the genotype frequencies will be: AA Aa aa 0.80998 0.18016 0.01002 Hardly any change; mutation had little effect over one generation Over thousands of generations, mutation can affect allele frequencies 

  21. Mutation as an evolutionary force Alleles may be kept in a population through a balance between mutation (creating deleterious alleles) and selection (removing them) - in mutation-selection balance, the frequency with which new alleles are created by mutation equals the rate at which they are eliminated by selection When the frequency of a harmful allele (say, cystic fibrosis) is higher in a population than you’d expect from the mutation rate of that gene, then you have reason to suspect some other force (i.e., selection) may be keeping that allele around

  22. Mutation as an evolutionary force Why does the F508 allele, which causes cystic fibrosis, occur at a high frequency (0.02) in populations of European descent? - selection against homozygotes is strong - mutation rate is too low to explain high allele frequency

  23. Mutation as an evolutionary force More importantly, mutation promotes evolutionary change by genetic innovation - once a rare beneficial allele is created by mutation, it can rapidly become fixed in the population through selective sweeps bacteria evolved in a series of jumps:

  24. Migration Migration is the movement of alleles between populations Migration can rapidly change allele frequencies, especially for small populations - individuals leaving a continent make little difference to the allele frequencies on that continent - those arriving on an island with a small population can make a huge difference to allele frequencies on the island

  25. Migration Example: banded vs unbanded water snakes

  26. Migration Example: banded vs unbanded water snakes - one gene w/ 2 alleles determines banded, unbanded or intermediate morph - natural selection favors banded snakes on mainland, where they are cryptic (hidden from predators) - selection favors unbanded snakes on islands, where bands stand out when snakes sun themselves on rocks to warm up

  27. distribution of banded vs unbanded snakes

  28. Migration Why doesn’t selection fix the unbanded allele on islands? (drive it to a frequency of 100%) - migrants from mainland continually introduce banded allele into island population - about 13 snakes per year move to islands, which have ~1300 snakes (roughly 1% migration per year) Migration acts as a homogenizing force: - equalizes allele frequencies among populations; makes them more similar than they would otherwise be

  29. Genetic Drift A sampling process (flipping a coin, drawing beans from a bag) may produce results different from theoretical expectations - flip a coin four times, and you may get 4 heads When the actual results differ from theory, this is sampling error Sampling error depends largely on the number of samples drawn - flip a coin 40 times, and you are very unlikely to get 40 heads - will probably get ~20 heads, give or take a few

  30. Genetic Drift Sampling error in production of offspring in a population is genetic drift Initial frequencies are always heavily skewed during random sampling - ie, drawing alleles one at a time from a big “batch” (= gene pool)

  31. Genetic Drift Sampling error is very sensitive to population size - as population increases, effects of genetic drift diminish - odds of getting the expected allele frequencies when you make 10 zygotes by drawing alleles at random

  32. Random fixation of alleles pop. size = 4 Given enough time, any allele will eventually become fixed or disappear if genetic drift is the only mechanism at work - when one allele is fixed, all others have a frequency of zero - the odds that any given allele will be the one that goes to fixation is the initial frequency of that allele 40 400

  33. Genetic Drift (1) Every population follows a unique evolutionary trajectory, because sampling error affects allele frequencies at random - if selection were at work, different populations would evolve along similar trajectories (2) drift works faster and stronger in small populations - allele frequencies change more dramatically if population size is small (3) even in large populations, drift can cause substantial evolution over long times - geographic isolation results in differentiated populations

  34. Genetic Drift (1) Every population follows a unique evolutionary trajectory, because sampling error affects allele frequencies at random - if selection were at work, different populations would evolve along similar trajectories (2) drift works faster and stronger in small populations - allele frequencies change more dramatically if population size is small Question to ponder: What forces prevent drift from fixing alleles in natural populations?

  35. Random fixation and loss of heterozygosity Frequency of heterozygotes decreases over time, as alleles drift towards fixation or extinction - all else being equal (no selection, etc), the frequency of heterozygotes should fall in every generation - given by the relationship Hg+1 = Hg 1 - 1 where N is population size 2N You are trying to maintain a group of 50 endangered llamas despite your efforts to arrange random matings...

  36. Random fixation and loss of heterozygosity Heterozygosity decreases in every generation, but more slowly in large populations - the faster an allele disappears due to drift, the more quickly you lose heterozygosity What can reverse this effect?

  37. Random fixation and loss of heterozygosity tested experimentally by Buri with fruit flies - started 107 replicate populations each with 8 boy + 8 girl flies - all flies were initially heterozygotes for a brown eye color allele (bw/bw-75) - each generation, out of all offspring, 16 were chosen to start the next generation - monitored for 19 generations

  38. Random fixation and loss of heterozygosity Expected result: - no selective advantage, so bw-75 allele should drift to fixation 50% of the time and be lost 50% of the time - heterozygosity should decrease over time Results after 19 generations: - in 30 populations, bw-75 allele was lost - in 28, it was fixed

  39. Random fixation and loss of heterozygosity Heterozygosity could also be directly scored by eye color - decreased every generation, as predicted by theory - actually decreased faster than expected, as though N = 9 flies (not 16)

  40. Founder Effect When a population is founded by a few initial colonizers, their genetic make-up will largely determine the allele frequencies as the young population grows A small group of founders will not carry all the alleles present in the larger population they came from (reduced genetic diversity) - if founders carry rare alleles, these alleles will be over- represented in the new population relative to the original large population example: Pennsylvania Amish carry allele for a rare form of dwarfism, at a frequency of 7% - only present at 0.1% in most populations - one of original 200 founders had the recessive allele - consequence: way more Amish dwarves than you’d expect

  41. Genetic drift and Elephant tusks 98% of 174 female elephants in the Addo National Park lack tusks - population was reduced to 11 individuals by hunting, until protected in 1931 - at that point, 50% of females lacked tusks - near loss of female tusks is likely a result of genetic drift, following population bottleneck proportion of females w/ tusks population size

  42. Genetic drift and Elephant tusks 98% of 174 female elephants in the Addo National Park lack tusks - Alternative hypothesis: ivory hunters imposed strong selection against tusks - if tusklessness were a recessive trait, what would you expect to happen to the frequency of tusklessness since the population was protected? Why? proportion of females w/ tusks population size