Genetic variation

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Through Genetic variation new variants arise of a gene , such a variant is called allele . The totality of all alleles in a population is called the gene pool . The permanent change in the chemical structure of one or more genes is called a mutation .

A genetic variation can often be recognized by a changed appearance ( phenotype ) of a living being. Conversely, a change in phenotype does not suggest that genetic variation is the cause. Because the same genetic make-up can lead to different phenotypes under different environmental influences ( modification ; polyphenism ). In addition, different epigenetic characteristics , which in some cases are even passed on over several generations, can be a possible cause of a changed phenotype .

Genetic variation or variability is the basis of the origin and further development of species in the course of evolution . In the course of evolution, the frequency with which certain alleles appear in a population changes.

The genetic variation leads to polymorphism within the population .

Type of chameleon with a mimicry feature of the scales: simulated teeth to deter possible attackers. In other chameleon species, the simulated teeth are often not as distinct.

History of theory and research

Concept of variation before and with Darwin

The variability of the organisms of a species was discovered before Charles Darwin . The phenomenon was described in France by Georges Cuvier , Étienne Geoffroy Saint-Hilaire and in Great Britain by Darwin's grandfather Erasmus Darwin and Robert Chambers . C. Darwin spoke of a principle of divergence ( principle of divergence ). By this he meant that initially barely noticeable differences keep increasing and the races differ more and more from each other and from their common ancestors. Variations, as individual differences, are passages in the formation of geographic populations, and such populations are passages or precursors of species. Darwin thus took a position of the gradual, gradual transition of characteristics as new species emerge. At that time there was no knowledge of possible mechanisms of variation.

Mendelian variation

When studying the inheritance of traits, Gregor Mendel devoted himself to certain clearly distinguishable variations in peas, such as the shape of the seeds (round, wrinkled), the color of the seeds (yellow, green) and five other traits that vary in inheritance (see also Mendelsche Rules ). In doing so, he selected discrete distinguishing features which he suspected were based on the inheritance of specific, discrete units. According to Mendel, the variability of the offspring was due to the combination of already existing characteristics. His answer to the question of how traits are inherited initially seemed inconsistent with Darwin's considerations about when and how they change. Darwin came to the conclusion that speciation resulted from the slow, gradual accumulation of small, often imperceptible variations.

The incompatibility of Mendel's theory of inheritance with Darwin's theory of evolution with regard to the meaning of discontinuous variation was discussed by a number of well-known researchers, including William Bateson and Hugo de Vries at the turn of the 20th century. These researchers emphasized the importance of discontinuous variation in evolution. Such a view has been called saltationism or mutationism . Only the synthetic theory of evolution was able to resolve the supposed contradiction. In doing so, Ronald Aylmer Fisher created mathematical, population-genetic models with which he showed that quantitative characteristics, i.e. continuously varying properties that can be measured in numbers, such as body size, are possibly determined by many genetic loci . Individually, these would only make a small contribution to the development of such a feature or its variation.

The seven varying traits whose inheritance Mendel analyzed.

Geographic variation

Darwin pointed out the geographic variability of features. However, it was not until later that individuals from geographically separated populations were compared with the aim of determining whether their differences were hereditary or environmental. Such studies were first carried out by Richard Goldschmidt in 1918 with sponge spiders ( Lymantria dispar ), a butterfly that occurs in different species worldwide under different climatic conditions. A study of geographically separated populations of the jerboa in California from 1918 also indicated the heritability of traits. Individuals from local wild populations were moved to foreign regions. They retained their characteristics, which spoke in favor of their inheritance. In 1936, the Russian evolutionary researcher Theodosius Dobzhansky , together with Alfred Sturtevant , succeeded in demonstrating their phylogenetic relationship in fruit flies ( Drosophila melanogaster ) in geographically separate areas by mapping the geographical distribution of characteristics. The persuasiveness of these studies strengthened Darwin's theory and laid essential foundations for the later synthesis of the ideas of variation and mutation in the theory of evolution.

Variation in Classical and Molecular Genetics

Classical, pre-molecular genetics in the first half of the 20th century initially had to deal with two apparently opposing phenomena. On the one hand, one saw the genetic constitution of organisms, i.e. the inheritance of deviations. On the other hand, there were deviations that arose from the interaction of the genetic makeup with the respective environmental situation. Here, too, the above-mentioned synthesis was able to bring about extensive clarification, mainly through researchers such as Ernst Mayr .

With the advancement of molecular genetics, the variability of genetic makeup has become more and more apparent. In 1927 the American Hermann J. Muller was able to induce mutations in the fruit fly for the first time with the help of X-rays . Specific forms of mutation were later discovered: the mutation that triggers a variation can take place within a coding gene, in a transcription factor for a coding gene or in a non-coding cis element and can be inherited. Also linked to this include mutations (Example trisomy ). Variability always occurs in a specific context of DNA .

Natural selection is involved in the frequency with which variation occurs in a population . With so-called silent mutations, there is no difference in phenotype. In this case, natural selection does not have a point of attack.

Forms of variation

Single mutation

Virtually every gene is present in several different alleles in natural populations. If this only affects single base pairs, one speaks of single nucleotide polymorphism . These go back to point mutations , which mostly did not occur again (called " de novo "), but were inherited from the individual ancestors. Typically, such polymorphisms are selectively neutral; H. neither harmful nor useful. But even polymorphisms with a slight to moderate adverse effect can remain in the gene pool for a very long time because the effect of natural selection is not perfect. You can, for example, in "linkage disequilibrium" (can be translated as "coupling imbalance", but is usually untranslated as a technical term) with beneficial alleles.

Balanced polymorphism

Proportion of the population without the mutation that makes milk (sugar) tolerable even in adulthood (lactase persistence).
Source: Association for Lactose Intolerance / Die Zeit

In some cases, the emergence of an allele caused by a (point) mutation leads to a phenotype with a geographically limited selection advantage. Both variants (alleles), the old and the new, are then regionally promoted by the selection and then remain in the overall population at the same time. Typical examples are milk sugar tolerance ( lactase persistence ), a mutation that a few thousand years ago gave people in Northern Europe the opportunity to digest animal milk even in adulthood. In other cases, a specific allele is not read out at all, but the combination of several is selectively advantageous; one speaks of a balanced polymorphism . A classic example is the sickle cell anemia gene , a sickle-shaped deformation of the red blood cells with circulatory disorders, but which in heterozygous cases causes resistance to malaria .

See also

Individual evidence

  1. See also Günther Just : Variability. In: Concise dictionary of the natural sciences. Volume 10. 2nd edition. Jena 1935, pp. 135-154.
  2. Charles Darwin: The Origin of Species . German translation by Victor Carus after the 4th – 6th English edition. Nikol Verlag, Hamburg 2008.
  3. ^ A b Marcel Weber: Theories and debates in the history of biology. In: Phillipp Sarasin, Marianne Sommer (Ed.): Evolution. An interdisciplinary manual . JB Metzler, Stuttgart / Weimar 2010, p. 65ff.
  4. Ernst Mayr: The evolution of organisms or the question of why. In: Driving Force Evolution. Diversity, change, becoming human. Spectrum Academic Publishing House. Heidelberg 2008.
  5. Marcel Weber: Genetics and Modern Synthesis. In: Phillipp Sarasin, Marianne Sommer (Ed.): Evolution. An interdisciplinary manual . JB Metzler, Stuttgart / Weimar 2010, p. 102ff.
  6. J. Burger, M. Kirchner, B. Bramanti, W. Haak, MG Thomas: Absence of the lactase-persistence-associated allele in early Neolithic Europeans. In: Proceedings of the National Academy of Sciences USA. 104 (10) (2007), pp. 3736-3741, doi : 10.1073 / pnas.0607187104 .