Describe the process of natural selection

Natural selection is differential reproductive success. It occurs when a population consisting of multiple variants exist in a limited environment. Some variants will be more successful - in terms of producing offspring - than other variants. The term used to indicate the measure of reproductive success of a variant is 'fitness'. The alleles (variations of a gene) of a 'fitter' variant will have a larger representation in the next generation than the alleles of a less 'fit' variant, and so the 'fit' alleles spread throughout the population at a greater rate than less 'fit' alleles. Eventually, less 'fit' alleles may even vanish from the population entirely. When an allele is so successful that it becomes the only variant for a given gene, we say that the allele has become 'fixed'. Note that the term 'fitness' has no direct relation to the health, strength, intelligence or speed of the variant: the only measure of 'fitness' is the reproductive success of the variant.