Gene Flow
Gene flow is the movement of genes within and among populations. Gene flow is a major force in evolution that can act to increase or decrease the types and numbers of genes available to a local breeding group.
One type of gene flow occurs whenever individuals change their local genetic group by moving from one place to another. Such movement or migrations may allow the introduction of new variations of the same gene (alleles) as new group members mate and produce offspring with members of the existing group. In such cases, gene flow acts to increase the gene pool (number of available genes) within the group.
Gene flow can also act to decrease genetic variation. If certain alleles are carried away from a group then those alleles are no longer a part of the local mating group gene pool. However, because genes are usually carried by many members of a large population that has undergone random mating for several generations, random migrations of individuals away from the population or group usually do not significantly decrease the number and types of alleles within the gene pool of the group left behind.
Movement of genes is the product of mating and gene exchange within populations. A genetically isolated population is known as a deme, gene flow can occur within demes or between demes when populations meet. On a limited scale, gene flow can occur across species barriers. This potential occurrence is one of the arguments against releasing genetically modified organisms into the environment, particularly those modified to include antibiotic resistance for screening purposes. Because of gene flow, these genes can spread through the wild population of that species and antibiotic resistance can be spread between species.
Gene flow is also a method by which advantageous genes or alleles can be spread throughout a population, making them more common and more likely to remain within the population. If genes or alleles do not spread within a population, they are very quickly lost and, in evolutionary terms, they are a dead end. Gene flow creates new combinations of genes in individuals that can then be tested against the environment. If the new combination is more able to survive in the given environment, that gene and combination of genes is more likely to spread within the population.
Gene flow is usually dependent on sufficient migration of alleles. Although there rare cases where individual introductions or migrations of genes significantly alter the gene pool of small isolated populations, it is rare to for one individual to have a significant genetic impact on a population. Generally, alleles must be introduced into groups in sufficient numbers to avoid chance extinction of the alleles.
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