Natural selection is exerted on mixed populations composed of organisms with the parental genetic makeup and of derivatives thereof carrying different genetic variations.
Each of these variants will have its own characteristic ability to deal with the encountered living conditions, so that in a growing population, the fittest organisms will finally take over, while others may eventually become eliminated. Natural selection is exerted on expressed functions. Natural selection, together with the variant phenotypes that are available in mixed populations, determine the directions taken by the evolution of the organisms in question.
So the direction and process of evolution is determined depending on which factors are selected at a particular point of time by nature.
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