Species living on earth today are products of 3 billion or more years of evolutionary process with natural selection operating over such a vast span of time.

Infinite variation occurs between sexually produced individuals even when they are closely related. Some of these variations are strongly different like hair and eye color while other variations such as height are imperceptible in gradation. Traits like height, skin color, hair color, eye color, shape of nose etc., which vary among people, may be due to heredity or environment or both.

Hereditarily controlled variations, both continuous and discontinuous arise from multiple factors. A trait may vary in direct relation to general fundamental biological factors.
a) Age factor: It may vary with age because many changes take place with the age. The sequence of events during growth is significant viz. sequence of fusion of epiphysis, age at menarche and spermatogenesis etc. Inter and intra group variations are also found in such traits.
b) Environmental factors: Size and color may depend upon varying amount of certain nutritional factors. Color may vary with sex; this is called sexual dimorphism. In general, females are smaller than males.

Variability is known by a special name polymorphism (Greek-many forms). Especially the hominidae, are extremely polymorphic group of animals. In fact, no two individuals are identical in their visible traits and invisible genetic makeup.

Categories of Variations:
Darwin has divided variations into two categories, both of which have played significant role in the origin of new species.
i) Continuous and
When the traits vary continuously along some scale, the variation is continuous between two extreme values on the scale. Height, face length, skull depth are such examples. Such variations are expressed by an average value, and certain range on each side of the average.
ii) Discontinuous (polymorphism)
These are controlled by a single gene locus e.g., ABO blood group.

Polymorphism or variability in the population occurs due to three components:
1) Environmental modifications-due to phenotypic plasticity
Phenotypic variations caused by environment are often phenotypic. It helps the organism to adjust better in a particular environment e.g., certain species of water butter cup plant which live in shallow pools, produce finely dissected leaves when remain submerged in water, and the complete leaves when float on the surface of the water. This is adaptive modification to increase the surface area for photosynthesis in the submerged leaves.

2) Genetic and Recombinational variability
In the process of sexual reproduction, genetic or inherited variability is caused by new gene combination, new genotype and hence new phenotypes or variants appear in the population. New gene combinations originated in three ways:
i) At the time of gamete formation (spore formation in plants), crossing over of homologous chromosomes during meiosis produced new combinations of genes.
ii) At metaphase of meiosis, random assortment of the homologous chromosomes on either side of the equatorial plate provides further reshuffling of the genes.
iii) Union of the gametes from genetically dissimilar individuals and out breeding also cause variability as maternal and paternal genes interact to produce a new phenotype.

3) Genetic and Mutational
Variation is fundamental biological phenomenon, which brings about evolution. The development of an organism is determined by its genes, or DNA, therefore, this must have the potential to change or mutate.
Genes in the population are not stable particles. They may occasionally change or mutate to a different state, or allele. Such mutations are so rare that their contribution to the variability of a population in a single generation is negligible. Nevertheless, mutations are the only source of new genetic variability.

The term mutation covers all types of changes in the genes, chromosomes, or other hereditary particles, which have a permanent effect on the genotype. The hidden genetic variability in cross-breeding population (higher animals and plants) exists because of mutational load (lag between the occurrence of mutation and their establishment or rejection by natural selection). Much genetic variability is held in the populations because certain gene combinations in the heterozygous conditions confer a particular adaptive advantage as compared to homozygotes.

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