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Microevolution occurs due to the presence and development of variations in a population. The variations occur in what is known as the gene pool of the population. The gene pool is the collection of genetic information from all individuals of the same population. Any two individuals of the same population do not have the same combination of genes, unless they are twins. Some genes may be present in several different forms called alleles. Microevolution is possible because of the tendency to pass on different combinations of alleles to successive generations. Within a population, alleles are not distributed evenly. However, the frequency or distribution of alleles will tend to remain constant over time, especially in a well-adapted population. The study of allele frequencies is known as population genetics. Naturally, the study of population genetics would be concerned with the factors that change allele frequencies. These factors can be more or less, classified into five areas: Mutations are the only process through which new alleles are introduced into the gene pool of a population. Mutations are random changes in the composition or number of DNA molecules in a cell. External factors that create mutations include radioactivity, X-rays, and natural and artificial chemicals. Mutations can also occur due to inherent reasons, such as replication errors in DNA. Mutations are completely random and do not occur because they help or hinder an organism's ability to adapt to to its environment. Cases where mutations do help are rare, and such mutations are known as adaptations. Selection determines the chances of survival for an organism and its reproductive capability. This process works with mutations to determine which organisms survive and pass their traits to their offspring. Through this process, "weak" alleles are eliminated from the gene pool, whereas adaptive alleles are passed to a larger proportion of the gene pool. It is important to know that selection is independent of mutations. Environmental conditions do not create the need for the creation of certain mutations, but rather chooses which set of already present mutations survive and are passed on to future generations. Selection can occur both naturally and artificially. Natural factors in selection, include both abiotic and non-biotic factors. Artificial selection is performed by humans to obtain animals with certain desirable traits. Migration is one of the less subtle factors that affects allele frequencies in a population. Migration is the movement of organisms into and out of a population. This movement represents the introduction and removal of alleles from the gene pool. The effect of migration on the gene pool of a population depends on the alleles that incoming and outgoing organisms carry. If these organisms carry a genotype different from the population, the effect is greater on allele frequencies than if these organisms carry a largely similar genotype. Whereas migration is known as the movement of organisms, the movement of genes into and out of the gene pool is known as gene flow. Genetic Drift is the most random process by which allele frequencies can be drastically affected. Very often, the effect can be so drastic that a whole new population can evolve. Genetic drift occurs when a group of organisms within a population, happen to have a higher frequency of an allele than the surrounding organisms. Slowly this group starts to drift away from the gene pool of the main population by selectively mating within this group. In many ways, genetic drift is similar to nonrandom mating. Nonrandom Mating is the fifth and last factor that affects the gene pool. Nonrandom mating occurs when certain individuals in a population choose to mate with others with similar or different phenotypes. This does not change allele frequencies, but the occurrence of homozygous individuals.
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