Presumably, each species is fairly well adapted or fit, if it has been in its particular environment for many generations. The range of tolerance of a species may be narrow (i e., species has a low ecological amplitude) or broad (i.e., species has a high ecological amplitude).
Thus, organisms are “slaves” to the physical environment; they adapt themselves and modify the physical environment so as to reduce the limiting effect of temperature, light, water and other physical conditions of existence. Such factor compensation is particularly effective at the community level of organization but also occurs within the species.
Species with wide geographical distribution almost always develop locally adapted populations called ecotypes that have optima and limits of tolerances adjusted to local conditions. Compensation along gradients of temperature, light or other factors may involve genetic races (with or without morphological manifestation) or merely physiological acclimatization.
Acclimatization describes all the non-genetic modifications of the organisms in response to an environmental stress during their own life-time. For example, nude man exposed to moderately cold temperatures shiver violently however, after several weeks of exposure.
This response decreases and they become better able to withstand the cold. Likewise, within a few days after a move from low to high altitudes, people experience an increase in the number of red cells in their blood. Red blood cells carry oxygen, so these people can then tolerate the lower oxygen level in at high altitudes without signs of stress. Acclimatization is an important weapon in the arsenal of the organism trying to maintain homeostasis.
The concept of ecotype was developed by the Swedish botanist, Gote Turesson as a result of a series of observations and experiments conducted in the 1920s and 1930s on variation on Swedish plant species such as Piantago maritima. Turesson collected live plants from different parts of the country and grew them together in the same garden.
Turesson described the study of ecotypes as a new research field, called genecology (Krebs, 1972). In fact, he started a procedure for transplant experiments that has since been followed by many genecologists or biosytematists, as they are now more commonly called.
Turesson’s basic conclusions include the following aspects: (1) Widely distributed species exhibit a variation in morphological and physiological characters; these vary from place to place within the habitat. (2) This variation is largely correlated with observable habitat differences. (3) This correlated variation, where it is not simply phenotypic variation, is the result of the natural selection of particular genotypes from the pool of genetic variability available within the species.
The population sampling and subsequent procedures that Turesson followed led him to conclude that species were composed of a mosaic of populations—the ecotypes — each adapted by natural selection to a particular habitat and each being discontinuously distinct from the others.
Thus populations of ecotypes were seen to differ from clinal populations or ecoclines, in which variation is both continuous and directional in response to a habitat gradient. Ecotypes can also be distinguished from the ecads or ecophenes, which are the morphological variants of the same population in relation to some habitat characteristic but not fixed by genetic composition.
One of the earliest studies in India showed that the two closely related species of Lindenbergia namely; L. polyantha and L. urticaefolia were not two distinct species but only two ecotypes of the same species (Mira and Sivarao, 1948). In another study, Ramakrishnam showed that the two populations, red and green in colour, of Euphorbia thymifolia were two ecotypes which had their distribution related to the exchangeable calcium content of the soil.
The red ecotype grows in calcium rich soil but may also occur in calcium deficient soil. On the other hand, the green ecotype is an obligate calcifuge, i e., it cannot grow in calcium rich soil. These ecotypes were found to be caused by paired dominant alleles (red) and paired recessive alleles (green) in simple Mendelian way.
Ecotypes have been reported in several plant species such as Euphorbia hirta, Cassia tora, Xanthium strumarium, Ageratum conizoides, Cenchrus ciliaris, Portulaca oleracea, etc.