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the presence in a single species of two forms, which differ in morphophysiological features while inhabiting the same territory.
Dimorphism is the common and most usual instance of polymorphism. In animals, sexual dimorphism is most often encountered—that is, there are differences in overall appearance (size, color, and so forth) between the female and the male (the rooster and hen, the male and female stag beetle). Dimorphism is also observed in alternation of generations, metagenesis (for example, in hydroids and medusae), and cyclomorphosis (for example, in Daphnia). A particular form of dimorphism is the alternation of phases, where the species is encountered with diminished population during the so-called solitary phase and with increased population during the swarming phase (for example, in grasshoppers and springtails). Seasonal dimorphism is related to a change in the temperatures at which the development of the organism occurs. For example, in the moth Araschnia levana the typical spring form is smaller and has a reddish-yellow color, while the autumn (prorsa) form is larger and black-brown in color. Another case of dimorphism is that which appeared as a result of mutation in the peppered moth (Biston betularia) in Great Britain, where the trunks of the birch trees in industrial regions of the country have become covered with soot so that the dark mutants that appeared more than 100 years ago are now replacing the original light-winged form. This kind of dimorphism can be viewed as the beginning of the divergence of a species. The coexistence of dexiotropic and leiotropic gastropods of the same species may also be considered an example of mutational dimorphism.
In plants, a distinction is made between the dimorphism manifested in the overall appearance of a plant and that manifested only in the structure of individual organs. The first case is found more rarely—for example, in such diclinous plants as hemp. Seasonal dimorphism in plants is expressed in the presence of spring and autumn forms (for example, in cow wheat). An example of group ecological dimorphism might include arrowhead, where specimens growing in the water at a depth of over 1.5 m have only taeniate aquatic leaves, while those growing along the water’s edge have only sagitate epigeal leaves. Dimorphism is also manifested in flower structure—for example, in the varying length of the stamen and pistil in the flowers of buckwheat (heterostyly), or the ligulate and tubular florets in the inflorescence of the sunflower. Examples of dimorphism are also known in bacteria which, in the same medium, produce S and R colonies (which differ in terms of configuration—“smooth” and “rough”). There are dexiotropic and leiotropic forms among spirilla of the same species.
Cases of the transfer of features are known for all types of dimorphism (false hermaphroditism, gynandromorphs, and intersexes in dioecious animals). Ordinarily the term “dimorphism” is not used for organisms that change appearance during every ontogeny (the caterpillar and butterfly, the gametophyte and sporophyte of the fern).
M. S. GILIAROV