Altering Existing Genetic Variation
Genetic Drift
Genetic drift is a change in in genetic variation due to chance changes in allele frequencies. For example, genetic drift may occur as a matter of luck; one phenotype may simply never meet a member of the opposite sex in order to mate and produce offspring. Genetic drift is not affected by the relative fitness of individuals to their respective environments. In most cases, genetic drift occurs in small populations to the smaller gene pool compared to large populations. After many generations, genetic drift may cause an allele to be lost or fixed- the allele will reach either 0% frequency or 100% frequency. Genetic drift may also be observed in large populations, but its effect (loss or fixation of an allele) happens much more slowly than it would in a small population.
Two examples of genetic drift are the founder effect and the bottleneck effect. The founder effect occurs when a subset of a population breaks off from the larger population and establishes its own colony. Because the subset is not likely a representative sample of the larger population, genetic drift will cause the smaller colony to have a much different gene pool than that observed in the larger colony. The bottleneck effect occurs when a random event decreases the size of the population dramatically. This can be caused by such events as natural disasters, such as hurricanes or earthquakes, or through the destruction of habitat by humans or other forces. Because the decrease in the gene pool is random, the allele frequencies in the surviving population may be different from that of the original population. Sometimes, alleles can be completely eliminated by these chance events.
Gene Flow / Migration
These bears lived in different populations with different allele frequencies. By meeting each other, mating, and producing offspring, they will have caused gene flow between their populations.
Gene flow refers to the movement of genes between two populations that may have different gene frequencies. For example, a deer population that lives on the east side of a river may have different allele frequencies than the deer population on the west side of the river. If one year there is a drought and some deer are able to move to the other side of the river, mate, and produce offspring, the allele frequencies of each population may change as a result of the new gene pool. Gene flow is also sometimes called migration because it is often caused by migration of one population into the territory of another population. Thus, the genes in each population also “migrate” to the other population.
Gene flow has two important consequences. First, allele frequencies in each population experiencing migration is reduced. Scientists can evaluate the relative similarities and differences in allele frequencies in order to determine how isolated the populations are. The more similar the allele frequencies between two populations are, the less isolated the populations are in turn. Second, gene flow can introduce new alleles into a population. Since mutations are so rare, and beneficial mutations even rarer, populations that do have a rare beneficial mutation introduced into their population are most likely to share the allele via gene flow.
Non-Random Mating
Would you rather mate and produce offspring with the man above
or with this man? If you expressed a preference for either, you are practicing nonrandom mating based on phenotypic characteristics.
Non-random mating occurs when individuals choose mates based on phenotypic characteristics or genetic lineage. In non-random mating situations, the allele frequencies do not change, but the proportions of heterozygotes to homozygotes may change. Inbreeding is one example of non-random mating that increases the frequency of homozygotes in a population. Conservation biologists keep track of non-random mating in a population in order to make sure increased homozygosity does not lead to inbreeding depression. If evidence of inbreeding depression is seen, biologists may introduce animals from another population to increase the gene pool.
Natural Selection
Natural selection is a phenomenon in which the environment selects for individuals with beneficial traits and against individuals with unfavorable traits. Because natural selection is discussed in greater depth in other articles, it will only be mentioned as a factor here.
Sources of New Genetic Variation
Random Mutations
The different tail types for manx cats were brought about by random mutations.
Random mutations are changes in existing genes that can introduce new alleles into a population. However, mutations are very rare, and beneficial mutations are even rarer. In most cases, mutations are detrimental to an individual and those that do not result in death, disfigurement, or other forms of severe disadvantage are, at best, neutral. When a beneficial mutation does arise, other mechanisms such as genetic drift or natural selection must act upon them in order for the allele to rise in frequency in a population.
Gene Duplication
Abnormal events during crossover (meiosis) may increase the number of copies of a gene. Through several generations, the new gene family may have new presence in a gene pool and may have different gene products.
Exon Shuffling
Sometimes, instead of full gene duplications during crossover, exons may be inserted into another gene. The new gene may have new functions and will be acted upon by mechanisms such as genetic drift or natural selection before increasing in frequency in a population.
Horizontal Gene Transfer
Typically, horizontal gene transfer is seen in bacteria. Through events such as endocytosis, genes from one species may be introduced into the genes of another species.
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