Directional Selection

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In directional selection the population average value for a phenotype say the weight of birds in this example shift direction because the more extreme phenotypes at one end of the distribution of phenotypes is at a disadvantage in their environment. So in a harsh winter small birds might be slected against. Thus the average weight of the remaining birds becomes larger, simply because the smaller ones do not survive as well.

Examples.

Antibiotic resistance

Industrial melanism

Heavy metal tolerenace