Genetic variability

Genetic variability is either the presence of, or the generation of genetic differences.

It was defined as

the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally induced differences which, as a rule, cause only temporary, nonheritable changes of the phenotype.^[1]

Genetic variability in a population is important for biodiversity,^[2] because without variability, it becomes difficult for a population to adapt to environmental changes and therefore makes it more prone to extinction.

Causes

There are many sources of genetic variability in a population:

• Homologous recombination is a significant source of variability. During meiosis in sexual organisms, two homologous chromosomes cross over one another and exchange genetic material. The chromosomes then split apart and are ready to contribute to forming an offspring. Recombination is random and is governed by its own set of genes. Being controlled by genes means that recombination is variable in frequency.

• Immigration, emigration, and translocation – each of these is the movement of an individual into or out of a population. When an individual comes from a previously genetically isolated population into a new one it will increase the genetic variability of the next generation if it reproduces.^[3]
• Polyploidy – having more than two homologous chromosomes allows for even more recombination during meiosis allowing for even more genetic variability in one's offspring.
• Diffuse centromeres – in asexual organisms where the offspring is an exact genetic copy of the parent, there are limited sources of genetic variability. One thing that increased variability, however, is having diffused instead of localized centromeres. Being diffused allows the chromatids to split apart in many different ways allowing for chromosome fragmentation and polyploidy creating more variability.^[4]
• Genetic mutations – contribute to the genetic variability within a population and can have positive, negative, or neutral effects on a fitness.^[5] This variability can be easily propagated throughout a population by natural selection if the mutation increases the affected individual's fitness and its effects will be minimized/hidden if the mutation is deleterious. However, the smaller a population and its genetic variability are, the more likely the recessive/hidden deleterious mutations will show up causing genetic drift.^[5]

DNA damages are very frequent, occurring on average more than 60,000 times a day per cell in humans due to metabolic or hydrolytic processes as summarized in DNA damage (naturally occurring). Most DNA damages are accurately repaired by various DNA repair mechanisms. However, some DNA damages remain and give rise to mutations.

It appears that most spontaneously arising mutations result from error prone replication (trans-lesion 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 by translesion synthesis.^[6] 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.^[7] (Also see Mutation.) 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

There are many sources that decrease genetic variability in a population:

• Habitat loss, including:
  • Habitat fragmentation produces 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 allow genetic drift to lower local genetic diversity.
  • Climate change is the drastic change in weather patterns. By driving species out of their fundamental niche, climate changes can lower population size and consequently lower genetic variation.
• The founder effect, which occurs when a population is founded by few individuals.

See also

• Evolvability
• Human genetic variation
  • Personalized medicine

References

1. Rieger, R.; Michaelis, A.; Green, M.M. (1968), A glossary of genetics and cytogenetics: Classical and molecular, New York: Springer-Verlag, ISBN 9780387076683
2. Sousa, P., Froufe, E., Harris, D.J., Alves, P.C. & Meijden, A., van der. 2011. Genetic diversity of Maghrebian Hottentotta (Scorpiones: Buthidae) scorpions based on CO1: new insights on the genus phylogeny and distribution. African Invertebrates 52 (1).
3. Ehrich, Dorothy; Per Erik Jorde (2005). "High Genetic Variability Despite High-Amplitude Population Cycles in Lemmings". Journal of Mammalogy. 86 (86.2): 380–385. doi:10.1644/BER-126.1.
4. 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 (35.4): 429–433. doi:10.1657/1523-0430(2003)035[0429:GVAIEI]2.0.CO;2. ISSN 1523-0430.
5. Wills, Christopher (1980). Genetic Variability. NewYork: Oxford University Press.
6. Kunz BA, Ramachandran K, Vonarx EJ (April 1998). "DNA sequence analysis of spontaneous mutagenesis in Saccharomyces cerevisiae". Genetics. 148 (4): 1491–505. PMC 1460101. PMID 9560369.
7. 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. PMC 2850169. PMID 20051983.