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Lecture 14

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Biology 1001A

Lecture 14  Selection on quantitative traits:  o stabilizing, directional, disruptive o stabilizing is the most common, the extreme phenotype gets selected against o directional, the population at one extreme end will have better survival rates o  why hasn't directional selection already removed all genetic variation for traits related to fitness?  Selection pressure varies over time, and across habitats  o selective advantage for a population at one environment than other environments o that's why there are many variations in the wild o adaptations (traits that increase bearer's relative fitness) are environment-specific  Disruptive selection  o least common out of the three o favours both extremes of the traits, o e.g. bill size in birds, small bills have high fitness, and individual with big bill have high fitness. the individuals with intermediate bills will not survive.  Frequency-dependent selection  o negative frequency-dependence o  e.g. predators form search images of prey  these predators will remember the image of the prey which ones taste good, they only select against this one type of prey  preferentially hunt common forms   when one type becomes very little (rare), the predator will change their preference and start selecting against the more common prey. so that with this the allele frequency will stay 50 - 50 ratio  maintain multiple alleles, indefinitely, selection cannot remove a type of allele  Frequency-dependent selection  o Positive frequency-dependence o  e.g. warning colouration  tells the predator that these prey taste bad or has poison, with different colours  if the predator had a previous bad experience by eating one of these coloured prey before, then it'll remember never it that again  so if the whole group has the one colour, the predator will not eat it and they'll have a selection advantage and they'll live  positive frequency-dependence will result in that the favoured allele will be fixated and the bad allele will be diminished  Think on:  o why aren't all living things perfectly adapted to their environment? o  environment kept changing   environment changes, the organisms will adapt to the new environment, by the time the organism is adapted to the environment the environment changed again  time lag  natural selection does not predict the future  selection does not choose the best allele, it choose the best fitted allele   selection is limited by mutation  trade off:   provides protection against one disease or thing, will be more prone to another disease   e.g. CCR5-delta 32 protect against HIV but more prone t
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