KERNER von MARILAUN’s experiments illustrated the impact of environmental factors on the expression of certain phenotypes. A species may thus produce different phenotypes without changing its genotype. An approach to the speciation problem does nevertheless require different genotypes and methods for their identification. DARWIN did already address decisive aspects of this topic. His analysis of the origin of cultivars, for example, exemplifies, that a species may have several different stable varieties.
In 1922, the Swedish botanist G. TURESSON (1892 – 1970) furnished evidence for the existence of stable varieties occurring in nature by showing that species consist of genetically different populations (also called ecotypes or ecological races) subject to selection due to the environmental conditions in their respective habitat.
Hieracium umbellatum, for example, occurs at the Swedish west coast in two different ecotypes. One ecotype is a bushy plant with broad leaves and expanded inflorescences growing on rocky cliffs at the sea. The other is a prostrate plant with narrow leaves and small inflorescences, that grows on sand dunes. As the rocky cliffs and the sand dunes alternate along the coast, so does Hieracium umbellatum giving rise alternatively to its corresponding ecotypes. The plants keep their habitat-specific appearance under standardized experimental conditions. If, however, plants from one habitat were moved to the other type of habitat, they changed their appearance and adapted to the new environment. These experiments demonstrate the profound selective advantage of different genotypes in different habitats. Furthermore, they show, that a given genotype has enough flexibility to produce phenotypes, that are optimally adapted to the actual environmental conditions through modification.
During the 1940th and 50th, J. CLAUSEN, D. D. KECK, and W.M.HIESEY (Carnegie Institution, Department of Plant Biology, Stanford) collected further evidence of the distribution and stability of ecotypes with their studies of Achillea lanulosa. Achillea lanulosa is a perennial species belonging to the Compositae. It is common throughout the West of the United States. Along a transect through central California (at roughly 38° N latitude) conspicuous differences in the populations from different areas occur. The populations vary significantly in a number of parameters, like the height of the plants, the texture of their leaves, the number of flower heads or the number of flowers per head. The expression of a phenotype and the environmental factors correlate significantly. The tallest ecotypes, for example, came from the lowest altitudes, and plant height decreased more or less continuously with increasing altitude. The differences between neighbouring populations are gradual. This is a common feature in biology. J. HUXLEY defined it as a cline. The term ‘clinal variation’ is often seen in biological literature.
Achillea lanulosa can be propagated vegetatively. CLAUSEN and his collaborators collected plant parts from a number of different sites and cultivated them in parallel experiments under different, but standardized conditions. The experiments showed, that each ecotype performed best under condition resembling its natural habitat closest.
Clinal variations depend on the activity of a large number of genes. J. CLAUSEN postulated in 1951, that more than 100 genes determine the morphological and physiological differences allowing Potentilla glandulosa to adapt to its respective environmental conditions.
Despite habitat-specific factors, seasonal factors, too, play an essential role in the selection of optimally adapted races. The Compositae Madia elegans grows throughout California flowering both in spring (March to May) and in autumn (from August on). Following the winter rains, plants flowering in spring develop rapidly. Their shoots remain small. Plant flowering in autumn develop an extensive root system as an adaptation to the dry soil, a dense rosette of leaves, and tall shoots. These features of the plants are stable even under standard experimental conditions.
In Europe and the Near East, clinal variation manifests itself in the glycoside concentration of the leaves of Trifolium repens. Occurrence and distribution of the corresponding genes correlate strongly with the 0°C – January – isotherm. Other legumes display comparable differences. Glycosides (cyanide derivatives) are protective agents against browsing, by snails, for example.
As in the case of Lotus corniculatus, a temperature-dependent distribution of varieties containing glycoside correlates with the distribution of certain snail species sensitive to the cold. Whether, however, the distribution of its feeder is indeed the cause for the distribution patterns of the legumes’ glycoside genes has still to be proven.
Polymorphism
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