i) Natural selection - Within populations, the inherited traits of some individuals make them more able to survive and reproduce than others under certain environmental conditions. As a result of the environment's selection against nonadaptive traits, only individuals with adaptive traits live long enough to transmit traits to the next generation. Individuals with adaptive traits are more likely to reproduce and increase the frequency of adaptive alleles in the population. Over time, natural selection can change the characteristics of populations.
ii) Genetic drift - Accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. This is the notion of genetic drift, which provides diversity without any adaptations.
iii) Mutation - Mutations introduce new alleles (new traits) into a population by altering old alleles.
iv) Gene flow - When migration of a section of population to another place and population occurs, gene frequencies change in the original as well as in the new population. New genes or alleles are added to the new population and these are lost from the old population. There would be a gene flow if this gene migration happens multiple times.
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