Types of Natural selection

Stabilising selection

Disruptive (Diversifying) Selection

• increases the numbers of extreme types in a population at the expense of intermediate forms

• the median doesn’t change only the numbers do

• example a single snail population can have both plain and strip shells in short terms this is balanced polymorphism two or more phenotypes coexist over length of time disruptive selection may result and will lead formation of two different species

Directional selection

• Changing environmental conditions will give rise to directional selection where only one phenotype replaces the other in gene pool

• The median changes not numbers

• Example is the case of peppered moth in England that was discussed earlier

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