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a201-11f-09-PopulationGeneticsModernSynthesis

a201-11f-09-PopulationGeneticsModernSynthesis -...

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Introduction to Biological Anthropology: Notes 9 Population genetics and the modern synthesis of evolutionary theory Copyright Bruce Owen 2011 - Recall from an earlier lecture that variation can come in two forms: - discontinuous variation refers to traits that come in separate, distinct variants - like red, pink, or white snapdragon flowers - one kind of discontinuous variation is dichotomous variation : either-or characteristics; they have only two possibilities, like green vs. yellow peas - continuous variation (or continuously variable traits ) refers to traits that have an unbroken range of variation, rather than distinct types - like height, beak depth in finches, etc. - there are many, many continuously variable traits that are interesting and important, probably a lot more than discrete or dichotomous traits - We have seen that: - Mendel’s model explains dichotomous variation, - like green vs. yellow peas - it also explains discontinuous variation with three or more options, although we did not work through the details - But what about all those continuously variable traits, like height or hair color? - Mendel’s model can explain continuously variable traits, too - The key idea: - dichotomous traits are controlled by just one pair of loci - like the pair of loci for pea seed color - but continuously variable traits are controlled by many pairs of loci - each of the many pairs of loci influences the trait somewhat - the principles are the same - the details are more complicated, because there are many more combinations of alleles to consider - traits that are controlled by multiple pairs of loci are called polygenic - this is very common, and what we will consider today - Example: Let’s use the beak depth measured on the finches studied by Peter and Rosemary Grant on Daphne Major in the Galapagos - Imagine that beak depth is controlled by one pair of loci with codominant alleles - we could do this example imagining simple dominant and recessive alleles, too, but it would get slightly more complicated - say the alleles control the amount of a beak-stimulating hormone - H+ (more hormone), contributes to deeper beaks - H- (less hormone), contributes to shallower beaks - so the possible genotypes are - H+H+ (deep beak) - H+H- (intermediate beak)
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Intro to Biological Anthro F 2011 / Owen: Population genetics, modern synthesis p. 2 - there are two ways to get this (H+H-, or H-H+), so there are twice as many of these - H-H- (shallow beak) - This only gives three kinds of beaks, not a continuous distribution - it works just like any other codominant Mendelian trait - but now, imagine that alleles at another pair of loci also affect beak depth - say, by controlling the amount of calcium available for beak formation - C+ leads to more available calcium, hence deeper beaks - C- leads to less available calcium, hence smaller beaks - so now the genotypes are - H+H+C+C+: biggest beak - H+H+C+C-: slightly smaller - H+H+C-C-: slightly smaller yet - H+H-C+C+: smaller than the biggest, probably not exactly the same as either of the other two, either -
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