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Motivation within populations towards the fixation of new adaptations.
Directional selection contrasts explicitly with stabilizing selection. That is, directional selection is natural selection towards something new rather than towards the retention of that which has already become fixed within a population.
Directional selection is the taking of alleles that are found in low frequency and increasing their representation within that population. This, furthermore, is distinct from stabilizing frequency dependent selection because rather than more than one allele being selected particularly because that allele is found in low frequency, instead directional selection is positive selection for alleles that are found at lower frequencies solely because those alleles provide a fitness advantage, and one that is independent of their frequency within a population.
Stabilizing selection, similarly, is positive selection for alleles that are found at higher frequencies solely because those alleles provide a fitness advantage, and again one that is independent of their frequency within a population.
When we think of natural selection, generally it is directional selection that one imagines. Directional selection furthermore is typically seen when the environment within which a population resides changes in some manner, or when rare but beneficial alleles enter into a population via mutation or migration.
Note that directional selection can be resisted within clonal populations by what is known as clonal interference.
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