Directional Selection
-- also called positive selection
-- an extreme phenotype is favored above all other phenotypes
-- caused when an extreme allele is the most advantageous
-- because this allele increases survival or reproductive capability, carriers of it will produce more offspring
-- because they produce more offspring, the allele will increase in frequency
-- first described in Charles Darwin’s “On the Origin of Species”
-- examples -- African cichlids -- sockeye salmon
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