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05_Selection

05_Selection - BI SC 002 LECTURE 5SELECTION Draft November...

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BI SC 002 LECTURE 5—SELECTION Draft: November 8, 2010 Thanks to favorable traits, some individuals survive better than others. Those that survive better than others produce more offspring with those same traits, thus increasing their chances of survival. Other traits, of course, are eliminated because those individuals are unable to survive and reproduce. Over time, the overall makeup of the population (ratios of each phenotype) will shift towards the more beneficial phenotypes. For instance, what if we selected against a recessive gene? If we go back to our blue-eyed example with p 2 + 2pq + q 2 being .25 + .5 + .25: If, for instance all blue-eyed individuals failed to survive/failed to reproduce, that will make .25 + .5 =.75, so p 2 is really .25/.75 = .33 and 2pq is really.5/.75 = .67 (since only 75% of the population survives). So, for this generation, p = .57 and q =.43. For the next generation: p 2 + 2pq + q 2 is .33 + .49 + .18 (using p and q from the previous generation). But, that .18 (blue eyes) fails to reproduce, so p 2 is really .33/.82 or .40, and 2pq is really .49/.82 or .60. So, that now shifts p = .63 and q = .37. So, for the next generation: p 2 + 2pq + q 2 is .40 + .46 + .14 (using p and q from above). But, again, the blue eyed individuals (.14) are eliminated from the population (fail to reproduce). So, p 2 is really .40/.86 or .47, and 2pq is really .46/.86 or .53. So, p is now .69 and q is now .31. One more generation: p 2 + 2pq + q 2 is now .47 + .43 + .10 So, over the course of just 3 generations, because the blue-eyed individuals are unable to reproduce, the population shifts from 75% brown/25% blue to 90% brown/10% blue. If we were to continue, the percentage of blue eyed individuals will get smaller and smaller and smaller. It “should” never go extinct. Why is that? The heterozygotes are the ones carrying the blue-eyed allele, so they are really the ones producing blue-eyed individuals. There “should” always be heterozygotes in the population that can interbreed with each other. What would be making the individuals be unable to reproduce? This is selection at work. But, what is really doing the selecting? What factors allow certain individuals to reproduce while others are unable to do so? Two sets of factors play into selection: 1) the biotic (living) factors —these are influences on an organism by other living organisms (mainly competition and predation), and 2) the abiotic (non-living) factors —the environment’s effect on the organism (mostly climate and habitat). Remember, fitness is REALLY the number of offspring that an individual contributes to the next generation—that is really what shows the success of one genotype to another. If an individual does not reproduce, then from an evolutionary process, that individual is “unfit”. Natural selection is a culling process—helping to determine if an individual is “fit”.

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