You might have observed that human beings have many similarities and differences, for example, in height, weight, color and other body features. One can differentiate between individuals due to these characteristics. The differences shown by the individuals of one kind of organisms are called variations.
Variations are due to hereditary and non-hereditary characteristics. Characteristics, which are passed from parents to offspring, are called hereditary characters. E.g. eye color, hair color, tongue rolling etc. in human beings. Non-hereditary characters are those which are not transferred to offspring for example, parent’s habits and loss of any body part of a person due to disease, accident etc.
In general the term variation means the differences in characteristics of organisms belonging to the same species in a natural population.
Types of Variations
Two types of variations, i.e., continuous and discontinuous, occur in any large population. In continuous variation, there are small differences among individuals of the same species and many intermediate forms are seen. So there is a complete gradation of many characteristics of organisms from one extreme to the other in a population. E.g., height in human being. Characteristics showing continuous variations are produced due to combined effects of many genes and environmental factors. In discontinuous variation, there are sharp differences among the individuals of a species e.g., tongue rolling, blood groups in man etc, which do not show any intermediate forms. Characteristics showing this type of variations are usually controlled by one or two major genes which may have two or more allelic forms and their phenotypic expression is unaffected by environmental conditions.
“a person having extra fingers (six fingers instead of normal five fingers) is an example of discontinuous variation. Intelligence is an example of continuous variation. Can you name any other discontinuous and continuous variations in human beings?”
As a result of these variations, no two organisms are photogenically identical. In asexually reproducing organisms, there is a little possibility of variation. Any apparent variation between these organisms is certainly the result of environmental influences. Great variety of variations occurs in sexually reproducing organisms. Crossing over and independent assortment during meiosis and random fusion of gametes in fertilization are the important sources of genetic variation in population. The variation in organisms produced by these sources is not under influence of environment.
The environment affects at a later stage. In addition, another important primary source of variation among organisms is mutation. These variations which occur due to change in the genetic material that have occurred gradual and continuously since the life started on earth. These variations are hereditable as these are passed from parents to their offspring in sexual reproduction. These hereditable variations are the raw material for evolution.