Evolution Lecture 2

Evolution Lecture 2 - Evolution Lecture 2: Descent with...

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Evolution Lecture 2: Descent with modification is the fact of evolution. The pattern of evolution is gradual, change in any species was so slow that you could almost not detect it happening. Darwin rejected that change has occurred suddenly, or discontinuously. He also introduced natural selection, the mechanism. Chapter 5: Laws of Variation. Darwin tried to discover laws of variation and inheritance as was understood from studies of domesticated plants and animals. In the end, though, he didn’t discover laws of variation. Darwin didn’t know about Mendel and was not influenced by him. Chapter 6: Problems on Theory. Darwin recognized that the theory he proposed would be controversial. He tried to anticipate the principal objections: 1. The lack of intermediate forms. Darwin meant that if you look at living species today, there isn’t a whole range of types available. There are elephants with great long trunks, and then things without long noses. But there are not species with large trunks, and then smaller and smaller trunks so there is a continuous variation. If you look in the fossil record, you don’t find the intermediate forms that you would expect to see if Darwin’s pattern of continuous change were true. Intermediate forms were not seen. Darwin’s answers to this was that the fossil record was incomplete. If the fossil record is incomplete, (mostly incomplete, not slightly) then the lack of intermediate forms becomes what you expect to see. Suppose you have an elephant with a long nose and short nose. If you looked closely at what people knew about the history of life, and look at a section of rock with layers below you see a series of strata: They looked like layers in a layer cake. They were also different, and were named because of fossils. In one layer you might find trilobites, and it would be full of trilobites but the layer above and below it would have none. The recognition of strata had been practiced for about 100 years before Darwin (mining industry). People knew about fossil strata. As you go from one stratum to another, you see an abrupt change. You will find something very different from the trilobite in the layer above it. These strata had been named the major Geological periods well before Darwin’s time. Paleozoic, Mesozoic, etc… People did this to recognize strata for their mineral properties. Darwin said these strata correspond to different time periods. What happened in between was lost. Naturally, it would appear to be a discontinuous change which doesn’t mean there wasn’t continuous gradual change in between. We just don’t have the record of these events. Natural selection predicts continuous change. But we don’t see it because of the imperfections in the fossil records.
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The lack of intermediate forms in living species: if there is something in the history of life that makes long noses a great advantage, then you wouldn’t expect to find living species in the intermediate range anyway. They evolved into elephants. There was a single
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Evolution Lecture 2 - Evolution Lecture 2: Descent with...

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