Processes-07-web - Lecture3: Thusfar,(p.504 1 2 3 4 Thisrais

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Lecture 3: Processes of Evolution Thus far, we have seen that natural selection consists of four components (p. 504): 1. Species can overproduce offspring 2. There are limits to population growth 3. There is variation among individuals 4. Differential reproductive success This raises the notions of fitness and adapation: Fitness: is the ability of an individual to produce surviving fertile offspring, relative to other individuals in the population Adaptation: a heritable trait that increases the fitness of an individual relative to another individual that lacks that trait. To sum: Natural selection is differential success in reproduction due to competition for resources. Natural selection acts on individuals, but it results in the adaptation of a population of organisms to their environment. Biston betularia: a famous example of Natural selection at work 1. Industrial melanism: Many moths in Britain come in two different colors, a lighter "natural" form, and a darker, melanic form. In Biston betularia , a single gene controls the difference between the “peppered” form and the melanic form. In short, there is variation among individuals and it is heritable. 2. Historical records: In the last 150 years, the relative frequency of the two morphs has changed dramatically. Old collections from the early 1800's show that the pepper morph was far and away the most abundant. Probably only 1 out of 100 moths was the melanic, or black, morph. Beginning in the early to mid 1800's, however, melanic form increased in abundance dramatically, until the 1950s, in some areas virtually all moths were melanic. There has thus clearly been a dramatic change in the frequency of the melanic form - i.e. natural selection has occurred. 3. What was the mechanism of Natural Selection? To answer this, one first had to consider some observations. The change in morph frequencies corresponded with the onset of the industrial revolution in Britain. One consequence of the industrial revolution was that the smoke and soot put out by all of the factories that sprung up across Britain caused a darkening of the tree trunks in many areas, particularly near urban industrial centers. Originally, tree trunks had been covered with light-colored lichens, but after the industrial revolution, these lichens and the tree trunks they covered became dark in these areas; many lichens were killed. 4. Kettlewell's hypothesis: Because moths spend a great deal of time during the day resting on tree trunks, Kettlewell hypothesized that they are exposed to predation. It would therefore benefit a moth to blend in with the backgound… in this case, the tree trunks on which they sit.
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5. Kettlewell tested his idea in two ways. First, he compared the survival of melanic and pepper moths. He released marked moths and then recaptured them. He found the following: Percent Recaptured Woodland Melanics Pepper Urban (black trees) 27.5 13.0 Rural (lichen covered trees) 6.3 12.5
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