a) Directional Selection:

Directional selection is a type of natural selection where phenotypes at one of the extremes in phenotype range are selected for, meaning that individuals at one end of the curve have higher fitness than individuals at the other end. Allele frequency shifts in one direction. E.g. Industrial melanism, Bacteria + insects resisting against antibiotics and pesticides, etc.


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A graph displaying Directional Selection





b) Stabilizing Selection:

Stabilizing selection is a type of natural selection where the middle of intermediate traits in a population are favored (selected for) and the extremes are selected against, meaning that individuals near the center of the curve have higher fitness than individuals at either end. E.g. Sharks. It makes species very similar (variation decreases). Happens in punctuated equilibrium for a long period of time. It is environment's way of preserving traits in organisms for years (don't fix what is not broken).


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A graph displaying Stabilizing Selection






c) Disruptive Selection:

Disruptive selection is a type of natural selection where the two phenotypes (both) of the extremes are chosen. This selection selects against average traits and may result in two (or more) different species. This means that individuals at the outer ends of the curve have higher fitness than individuals near the middle of the curve. This type of selection usually happens when populations in an environment can use different types of resources. E.g. An island with several different types of plants.


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A graph displaying Disruptive Selection




Significance
As opposed to single-gene traits, the above selections occur in polygenic traits. Polygenic traits are controlled by more than one gene and the effects of natural selection are more complex. Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selections, as mentioned above. Natural selection on polygenic traits are important because it shows how different selections can happen in different situations, further providing evidence that natural selection is not fixed and is random.