In evolutionary terms, differences between closely related organisms are termed as variations. Thus differences between individuals of the same species, subspecies or race can be called variations and not differences between two genera, families or classes.
Only heritable changes have evolutionary significance, since they only make the population change from generation to generation as the evolution goes on. Variations can be of several types and can be classified in different ways based on the type of character taken into account.
Kinds of variations
1. Group variations: When one population of a species differs from the other, it can be termed as group variation. For example, African population of man is black while European population is white. Insect populations show group variations even within a small range of distribution.
2. Individual variations: These are differences among the individuals of the same population. They are very important in taxonomic studies, when extent of variation within a deme is taken into account for comparing and assigning the population to a taxon.
Variations can also be classified in the following 3 categories based on the type of character considered:
1. Meristic variations: These are variations that can be counted in numbers. For example, man has 12 pairs of ribs but if some individual has 13 pairs of ribs, it will be called a meristic variation. Similarly some people possess 6 fingers instead of 5 due to trisomy, and a starfish may have 6 arms instead of the usual five.
2. Quantitative variations: Variations that can be measured in size or weight, such as tall versus dwarf animal, heavy versus light body, long tail versus short tail etc.
3. Qualitative variations: These include characters, which depict identification quality of an individual, e.g. presence or absence of spots, hairs, colour, stripes, specialized feathers etc.
Based on the continuity of a character the variations can be classified into the following two categories:
1. Continuous variations (=Clinal variations) (=Minus-plus variations): Variations which fluctuate above or below the average, with intermediate stages also found. For example, in a population some individuals are larger, some smaller and some intermediate. In Indian population, some people have lighter skin, some darker and all kinds of intermediate shades are also found.
2. Discontinuous variations: These variations deviate greatly from the average individuals. Major mutations and disruptive selection produce some individuals, which are distinct from the others. For example, white tiger, hornless calf and albino peacock or cow.
Based on the inheritance, variations can be classified into the following two types:
1. Somatic variations: These are also called somatogenic variations, which are produced in the body due to the effect of environment. They are not heritable as they are not due to genetic changes, as for example, muscles of a wrestler, or weak individuals under starvation conditions.
2. Genetic variations: They are germinal variations that occur in genes. They are blastogenic, heritable and produce phenotypic changes in the populations. If somatic variations persist for a longer time, they tend to become genetic and become important in evolution.
Some variations not classified in the above-mentioned ways are as follows:
1. Age variations: When young ones differ from the adults distinctly, as caterpillars differ from the adult butterfly or tadpole differ from frog, it is called age variation. These variations have adaptive value but not evolutionary significance, although they help in taxonomic identification.
2. Seasonal variations: When animals change their appearances in different seasons, mainly as climatic adaptation, they are called seasonal variations, e.g. winter fur in temperate animals like snowshoe hare, whose fur becomes white in winter and brown in summer. Seasonal variations are commonly found in insects, like butterflies, grasshoppers, bugs etc.
3. Habitat variations: In sedentary animals like sponges, corals, oysters as well as in plants, variations are produced due to local environmental influences of the particular habitat. Some of the mobile animals, namely grasshoppers, locusts and plant bugs also show habitat variations. Chameleon can change its colour according to the habitat.
4. Castes in social insects: Division of labour in social insects, e.g. termites, honey bees, ants and wasps creates castes which have specialized organs to carry out a particular job in the colony. Therefore we can see different types of individuals moving about in the same population.
5. Polymorphism: When different types of individuals occur in a single interbreeding population of a non-social animal, it is called polymorphism. It is very common among insects, like butterflies and beetles, which show dry and wet season forms.