Directional Selection

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Enviromental conditions change so the phenotypes needed for survival also change

Some individuals will be to the left or the right of the mean, they are known as having extreme phenotypes

These individuals will posses phenotypes more suited to the new conditions

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More likely to survive and reproduce

Contribute more offspring to the population

Mean will shift in the direction of the offsprings phenotype

The phenotypes at one extreme will be favoured for whilst the opposite extreme is favoured against

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In a popualation where ther is a range of fur lengths...

The original temperature of the enviroment is 10 degrees. The mean fur length is 15mm.

If the temperature falls, individuals with longer fur (to the right of the mean) will be favoured for, whilst those with shorter fur (to the left of the mean) are favoured against.

Over many generations, the mean fur length increases as individuals who posses long fur phenotypes are surviving and reproducing. Those with shorter fur do not reproduce as successfully as they either die or are not healthy enough.

The mean fur length continues to shift till the optimum is reached for the lower temperature.

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