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Dobzhansky's Experiments and Natural Selection

1. Briefly explain what is meant by the terms directional selection, stabilizing selection, and disruptive selection. Give an example of each.

2. Dobzhansky's laboratory experiments studying mating behavior evolution in the fruit fly Drosophila subobscura provided a striking example of how natural selection operates. Describe the methods and the results of Dobzhansky's experiments.

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1. Directional selection occurs when extreme phenotype is favored; the distribution curve shifts that direction.
a. A shift of dark-colored peppered moths from light-colored ones was correlated with increasing pollution.
b. Increases in insecticide-resistant mosquitoes and resistance of malaria agent to medications are examples of directional selection.
c. The gradual increase in the size of the modern horse, Equus, correlates with a change in the environment from forest-like conditions to grassland conditions.

2. Stabilizing selection occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored.
a. The average human birth weight is near optimum birth weight for ...

Solution Summary

Directional natural selection occurs when extreme phenotype is favored; the distribution curve shifts that direction. Stabilizing selection occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored. However, disruptive (diversifying) selection occurs when extreme phenotypes are favored and can lead to more than one distinct form.
Dobzhansky' proposed a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. Balance polymorphism is one of the key aspects of the operation of the natural selection

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