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a201-11f-07-DarwinsProblemMendel - Introduction to...

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Introduction to Biological Anthropology: Notes 7 Darwin’s big problem and Mendelian genetics Copyright Bruce Owen 2011 - Darwin’s big problem - We have seen that natural selection works by favoring the most successful variants among the individuals in a population - it only works if individuals vary in ways that affect their survival and reproduction - offspring must be similar to their parents, but not exactly the same - if offspring were identical to their parents, they would be identical to each other, and there would be no “more successful” and “less successful” individuals for natural selection to pick from - so there could be no change in the next generation: no evolution - Darwin had no good explanation for why offspring resemble parents, but also vary - he knew that this was a big gap in his theory - The prevalent idea of inheritance in Darwin’s time was blending inheritance - blending inheritance holds that the characteristics of offspring are mixtures of the characteristics of their parents - the idea was that the material from the two parents that controlled inherited characteristics blended like two colors of paint - this is a reasonable approximation, based on everyday experience - so, every mating should produce offspring that are intermediate between the parents - for example, if a six-foot man mates with a five-foot woman… - then the offspring should all be between five and six feet tall - no offspring are expected to be more extreme than either parent - there are two huge problems with the blending model of inheritance - First, it obviously isn't true - lots of parents have children who are taller, or shorter, than both of the parents - many kids have traits like hair color, eye color, etc. that neither of their parents have - so blending simply can't be what is going on - Second, in a few generations, blending inheritance would cause the whole population to be mixed to the same average of all traits - the same medium height - the same medium hair color, eye color, skin tone, etc. - after a while, all the individuals would be the same - selection would have no variation to work on - and evolutionary change would stop - this obviously does not happen, so inheritance must work in some other way - So Darwin needed to explain: - why do offspring vary? - why doesn’t that variation get averaged away after a few generations? - natural selection weeds out poorer variants, but where do new variants come from? - That is, if natural selection weeds out some types, why aren't they all eliminated, leaving just the one, best type?
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Intro to Biological Anthro F 2011 / Owen: Darwin's problem and Mendelian genetics p. 2 - remember the corn example - researchers started with corn kernels that varied from 4% to 6% oil content - they picked the oiliest seeds every generation - so wouldn’t they eliminate all the low-oil variants and end up with a population of only plants that produce 6% oil?
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a201-11f-07-DarwinsProblemMendel - Introduction to...

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