Briefly explain the difference between stabilizing selection, directional selection, and disruptive selection.

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These three terms are used in population genetics.  All three are related to natural selection, or the changes and shifts in gene frequencies in organism populations over time, resulting in particular phenotypes being favored.  A phenotype is a set of visible traits an organism has based on its genes.

Stabilizing direction is when overall genetic diversity decreases and a particular set of traits become dominant among a population.  This is the most common action of natural selection since it serves to stabilize over time by removing extremes and perpetuating a norm.

Disruptive selection is the opposite of stabilizing selection.  Standardized traits are disfavored and extreme traits become favored, thus resulting in a divergence of a population.  This is how different organisms can evolve from a common ancestor.

Directional selection is when a particular set of traits becomes consistently favored over time and gene frequency is constantly shifted in a particular direction.  This is how populations can change their characteristics over time, such as short necked giraffes becoming obsolete in favor of long necked giraffes.  Directional selection can occur in the face of a population changing their environment and having to adapt to a new set of ecological circumstances.

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