Genetic variabilityis either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing ingenotype,or the presence of genotypically different individuals, in contrast to environmentally induced differences which, as a rule, cause only temporary, nonheritable changes of thephenotype."[1]Genetic variability in apopulationpromotesbiodiversity,as it ensures that no two living things are exactly alike.[2]While many factors can cause genetic variability, some factors can also decrease genetic variability.
Causes
editThere are many sources of genetic variability in a population:
- Homologous recombinationis a significant source of variability. Duringmeiosisin sexual organisms (sexual reproduction), twohomologous chromosomescross over one another and exchange genetic material. This random process of exchanging genetic material is known as recombination, and it is governed by its own set of genes.[3]Being controlled by genes means that recombination is variable in frequency. Generally, it is more common in areas on the chromosome where there are a higher number of genes. It creates more combinations of genes.[3]After this occurs, the chromosomes are split apart and are ready to contribute to forming an offspring.
- Immigration,emigration,andtranslocation– each of these is the movement of an individual into or out of a population. When an individual enters a new population after coming from a genetically isolated one, they will increase the genetic variability of the next generation, provided that they reproduce.[4]
- Polyploidy– having more than two homologous chromosomes. This allows for more recombination during meiosis and more genetic variability in one's offspring. However, this phenomenon can also prove difficult forcell division.[5]
- Diffuse centromeres – inasexualorganisms where the offspring is an exact genetic copy of the parent, there are limited sources of genetic variability. One thing that increases variability, however, is having diffused centromeres instead of localizedcentromeres.Being diffused allows thechromatidsto split apart in many different ways, which produces chromosome fragmentation and polyploidy.[6]
- Geneticmutations– contribute to the genetic variability within a population and can have positive, negative, or neutral effects on a fitness.[7]This variability can be easily propagated throughout a population bynatural selectionif the mutation increases the affected individual'sfitnessand its effects will be minimized/hidden if the mutation is deleterious. If the individual can survive with the mutations they have, those mutations will likely be passed down to offspring. However, the smaller a population and its genetic variability are, the more likely the recessive/hidden deleterious mutations will show up causinggenetic drift.[7]
- DNA damages are very frequent, occurring more than 60,000 times a day per cell on average in humans. This is due tometabolicorhydrolyticprocesses as summarized inDNA damage (naturally occurring).Most DNA damages are accurately repaired by various naturalDNA repairmechanisms. However, some DNA damages remain and give rise to mutations.
- Additionally, not all types of mutations occur as much as others do. Some mutations might have a huge impact on the human body, and some might not. It depends on what combination of base pairs is changed.[8]
- Most spontaneously arising mutations result from error prone replication (translesion synthesis) past a DNA damage in the template strand. For example, in yeast more than 60% of spontaneous single-base pair substitutions and deletions are likely caused bytranslesion synthesis.[9]Another significant source of mutation is an inaccurate DNA repair process, non-homologous end joining, that is often employed in repair of DNA double-strand breaks.[10](Also seeMutation.) Thus, it seems that DNA damages are the underlying cause of most spontaneous mutations, either because of error-prone replication past damages or error-prone repair of damages.
Factors that decrease genetic variability
editThere are many sources that decrease genetic variability in a population:
- Habitat loss, including:
- Habitat fragmentationproduces discontinuity in an organism's habitat, so that interbreeding is limited. Fragmentation can be caused by many factors, including geological processes or a human-caused events. Fragmentation may further allowgenetic driftto lower localgenetic diversity.
- Climate changeis a drastic and enduring change in weather patterns. By driving species out of theirfundamental niche,climate change can lower population size and consequently lower genetic variation.
- Thefounder effect,which occurs when a population is founded by few individuals.
See also
editReferences
edit- ^Rieger, R., Michaelis, A., Green, M.M. (1968),A glossary of genetics and cytogenetics: Classical and molecular,New York: Springer-Verlag,ISBN978-0-387-07668-3
- ^Sousa, P., Froufe, E., Harris, D.J., Alves, P.C. & Meijden, A., van der. 2011. Genetic diversity of MaghrebianHottentotta(Scorpiones: Buthidae) scorpions based on CO1: new insights on the genus phylogeny and distribution.African Invertebrates52(1)."Archived copy".Archived fromthe originalon 2011-10-04.Retrieved2011-05-03.
{{cite web}}:CS1 maint: archived copy as title (link) - ^abStapley J, Feulner PG, Johnston SE, Santure AW, Smadja CM (2017-12-19)."Recombination: the good, the bad and the variable".Philosophical Transactions of the Royal Society B: Biological Sciences.372(1736): 20170279.doi:10.1098/rstb.2017.0279.ISSN0962-8436.PMC5698631.PMID29109232.
- ^Ehrich, Dorothy, Per Erik Jorde (2005)."High Genetic Variability Despite High-Amplitude Population Cycles in Lemmings".Journal of Mammalogy.86(2): 380–385.doi:10.1644/BER-126.1.
- ^Zhang S, Lin YH, Tarlow B, Zhu H (2019-06-18)."The origins and functions of hepatic polyploidy".Cell Cycle.18(12): 1302–1315.doi:10.1080/15384101.2019.1618123.ISSN1538-4101.PMC6592246.PMID31096847.
- ^Linhart, Yan, Janet Gehring (2003). "Genetic Variability and its Ecological Implications in the Clonal Plant Carex scopulurum Holm. In Colorado Tundra".Arctic, Antarctic, and Alpine Research.35(4): 429–433.doi:10.1657/1523-0430(2003)035[0429:GVAIEI]2.0.CO;2.ISSN1523-0430.S2CID86464133.
- ^abWills, Christopher (1980).Genetic Variability.New York: Oxford University Press.ISBN978-0-19-857570-2.
- ^Eichler EE (2019-07-04)."Genetic Variation, Comparative Genomics, and the Diagnosis of Disease".New England Journal of Medicine.381(1): 64–74.doi:10.1056/NEJMra1809315.ISSN0028-4793.PMC6681822.PMID31269367.
- ^Kunz BA, Ramachandran K, Vonarx EJ (April 1998)."DNA sequence analysis of spontaneous mutagenesis in Saccharomyces cerevisiae".Genetics.148(4): 1491–505.doi:10.1093/genetics/148.4.1491.PMC1460101.PMID9560369.
- ^Huertas P (January 2010)."DNA resection in eukaryotes: deciding how to fix the break".Nat. Struct. Mol. Biol.17(1): 11–6.doi:10.1038/nsmb.1710.PMC2850169.PMID20051983.