A modification is a change in the phenotype , the appearance of a living being, caused by environmental factors . The genes are not changed, which means that a modification - unlike a change through mutation - cannot be inherited , but an epigenetic transmission of this change cannot be ruled out.
The reaction norm determines how variably the phenotype can react to environmental changes.
A distinction is made between flowing modification (synonym: fluctuating variability), in which a flowing transition between the phenotypes can be determined, and reversing modification (also alternative or reversing modifiability ), in which there are no continuous transitions. The latter is also known as polyphenism .
The cause of the phenotypic variation can be recognized using the example of cultivated plants. The yield of these plants depends not only on the genetic make-up of the seed, but also on factors such as the condition of the soil, the nutrient content of the soil, precipitation and temperature as well as obstruction by other plants.
The fact that the same genetic basis can lead to completely different shapes can be seen particularly well in the surface and underwater leaves of the common arrowhead .
A plant that grows between crevices at an altitude of 1,500 m will grow less lush than a plant of the same species that grows 500 meters lower on fertile soil and is prevented from growing by other plants there.
The following example explains the modifiability very clearly: If a young dandelion plant (Taraxacum) is cut in half lengthways and one half is planted at sea level and the other in the mountains, the two hereditary plants can develop differently. Although the genotype is the same, the phenotype has changed due to external environmental influences. This was discovered by the French botanist Gaston Bonnier (1853–1922). This modification occurs through the activation of various genes , which are all present in both plants, but which are activated or deactivated according to the physiological state changed by environmental influences.
The general rule:
- Valley shape : longer stems + larger leaves + normally developed taproot
- Mountain shape : shorter stems + smaller leaves + deep, strong taproot
A paramecium population lives in an aquarium. If all paramecia go back to a single individual, all individuals of the progeny have the same genetic make-up , since paramecia multiply through mitotic divisions . If the length of the individual individuals of these progeny is determined, considerable differences are found despite the same genotype of all individuals. In this aquarium there are growth-inhibiting and growth-promoting environmental influences . The extreme values that occur are explained by the fact that the particularly large or small paramecia were primarily exposed to environmental influences that inhibit growth (size approx. 136 µm) or growth-promoting (size approx. 200 µm). However, since most individuals were subject to both favorable and unfavorable factors, the animals of medium length are the most common and the distribution pattern therefore often corresponds to a normal distribution (bell curve). Growth - promoting factors include dim light , a constant temperature and a pH value of 7–8, growth-inhibiting factors are among others. a. bright light and low oxygen levels. This experiment shows that the range of modification is genetically determined; i.e., a body length of 136 µm up to 200 µm. This hereditary area is also known as the reaction norm . If the smallest or the largest individual is selected as the starting animal for a new population, the same distribution of cell sizes results in the offspring, regardless of the size of the starting individual, but depending on the prevailing environmental variables.
There are also modifications of the phenotype in humans. The hereditary skin color is changed by UV rays; height, weight and obesity level depend on the type of food and diet.
- Elisabeth Günther: Grundriß der Genetik , 2nd edition, Gustav Fischer, Stuttgart 1971, p. 434 ff ( "23rd Influence of the Environment" )