Natural Selection Natural selection is the process by which traits become more or less common in a population due to consistent effects upon the survival or reproduction of their carriers. It is the key factor of evolution. Natural selection acts on an organism's phenotype, or physical characteristics. Phenotype is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. Requires: Members of a population to have heritable variation More offspring produced than can be supported Individuals with favorable traits survive and reproduce more than those lacking the traits Across generations, a larger proportion of the population possesses the favorable traits and become adapted to the environment Fitness Individuals which are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest" Fitness depends crucially upon the environment. Conditions like sickle-cell anemia may have
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