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9 Cards in this Set
- Front
- Back
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How does selection affect genetic variation?
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Only selection leads to adaptive genetic change: by increasing the frequency of beneficial alleles
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Name two major life history traits that cause selective differences among genotypes
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1. Viability: probability of survival to reproductive age
2. Fertility: average number of offspring per individual that survive to reproductive maturity |
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directional selection
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● One allele is always at a selective advantage
● Advantageous allele will increase in frequency and become fixed ● 2 types, overdominance and underdominance |
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Heterozygous Advantage
(overdominance) w11< w12> w22 |
•Maintains both alleles in the population
•Produces a stable polymorphism e.g. p = 0.5 if the fitnesses of both homozygotes are the same |
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Heterozygous Disadvantage
(underdominance) w11> w12< w22 |
Heterozygote is the least fit
•Will not maintain genetic variation •Produces an unstable equilibrium e.g. heterozygotes for chromosomal rearrangements often have reduced fertility •Populations went to fixation for whichever chromosomal type became most frequent in early generations because of drift |
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If a locus shows no departure from HW equilibrium, it still may be affected by selection.
Why? |
● differences in fecundity will not affect HW proportions
● strong differences in survival may not cause departures if w11 X w22 = √w11w22 ● Only very strong differences in survival can be detected using HW |
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Give an example of frequency dependent selection
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example: self-incompatibility locus (S) in plants
● Pollen grains can only fertilize plants that don’t have the same S allele ● Homozygotes cannot be produced ● A new allele produced by a mutation would have a great selective advantage because it would be able to fertilize every plant in the population and would increase in frequency (unless lost due to drift) Orchid Dactylorhiza sambucina’s rare color morphs attracted more pollinators, selective advantage |
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frequency dependent selection
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● Fitnesses can change with different allele frequencies
○ e.g. a genotype can become less fit as it becomes more common ● This type of selection can maintain many alleles in a population ● Causes: Sexual selection, predation & disease, ecological competition |
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Give four reasons why selection is important in the context of conservation
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1. Hunting & fishing
–Evolution of less preferred phenotypes, as the desirable ones are the ones hunted. 2. Local adaption –difficult to detect since only detectable during extreme conditions (fire, drought) –Implications for translocations 3. Loss of variation –Allelic diversity more affected by bottlenecks than heterozygosity 4. Adaptation to captive conditions |
