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Evolution at the DNA Level Natural Selection and Alleles • We saw how natural selection impacts the frequency of alleles for a SINGLE gene trait • What about polygenic traits, where individuals have more than two genes for a trait? Types of Selection Directional Selection: individuals at one phenotypic extreme have lower fitness than those in the middle. Types of Selection • Stabilizing Selection: individuals at both phenotypic extremes have lower fitness than those in the middle. Types of Selection • Disruptive Selection: individuals at phenotypic extremes have higher fitness than those in the middle. Hardy-Weinberg Principle • Alleles in a population will stay in genetic equilibrium (no net change) unless: – Mating is not random – Population is small – Immigration or emigration occur – Mutations occur – Natural selection occurs Any of these five things will cause a population to evolve!