1. Hereditary Variations:
These arise in the germplasm and consequently can be passed on to the next generation. These are also called heritable or permanent variations as they go on from one generation to the other. It is primarily the hereditary variations that are responsible for the variations between one species and the other.
2. Environmental Variations:
These arise in the somatoplasm due to the influence of environmental factors and consequently cannot be passed on to the next generation. They are called transient variations because they will last only for one generation.
Between the two types of variations, from the evolutionary point of view, heritable variations are of more significance as they tend to carry the variation for several generations.
Some interesting experiments have been made by clausen al in the Californian (USA) plant Potentilla glandutosa, a member of Rosaceae, concerning the difference between heritable and environmental variations.
It is generally known that the individuals of a plant species growing at different altitudes in a mountainous region tend to show morphological differences.
The individuals that grow at sea level have long stem and leaves and take nearly six months to produce flowers from the seedling stage.
A much taller form is encountered at about 4000-4500 ft above sea level and it takes only 50-60 days to flower in its natural habitat.
At about 10,000 ft above sea level (alpine region) there is a dwarf from of Potentilla glandulosa. Which takes about 50-55 days to produce flowers to what extent these differences encountered are due to environment and to what extent due to heredity?
In order to find out an answer, the plant from each locality was cut into three pieces and replanted in three experimental gardens at coastal level, at 4500 feet and at 10,000 feet. The alpine race (form) grown at sea level resembles the original race, but grows to a taller height at 4500 ft.
The mid altitude race (4500 ft) becomes dwarf at the coastal level and extremely dwarf at the alpine region.
The coastal race grown at 4500 ft altitude and at 10000 ft the plant dies. The above experiment clearly illustrates that the members of the same stock (clone) react differently to different environments as shown in the case of height. Hence variations of this type are called environmental variations.