2. Seasonal variations: 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: 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: 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: 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.

## SOURCES OF VARIATION

Somatic as well genetic variations can be produced in the following ways: 1. Environment: Environment: Environmental factors, namely, heat cold, rain, drought, food etc. influence populations drastically and normally cause somatic variations within a short period. These variations are generally not heritable and disappear if environmental conditions revert back. If tadpoles are fed on thyroid extract, they quickly metamorphose into small frogs, without growing in size. 2. Endocrine glands: Endocrine glands: Optimum activity and balance in the functioning of endocrine glands is very important for the normal growth of the body. Over or under functioning of pituitary, thyroid, adrenal etc. can produce variations in the body, which are again only somatic and not heritable. Such variations are also due to the environmental effects. 3. Blending inheritance: Darwin laid emphasis on blending inheritance as a major source of variation in animals, without understanding its genetic mechanism. Characters of both the parents merge in the first generation of offspring, due to crossing over between homologous chromosomes in diplotene stage of meiosis. But the characters segregate in the subsequent generations according to Mendel’s laws, but still the variations are produced. 4. Mutation: Mutation: Mutation is a change in the sequence of nitrogenous base pairs in DNA. For example, if AGC is a codon on DNA, its complementary codon on m-RNA will be UCG, which will code for the synthesis of amino acid serine. If a small mutation replaced Guanine with Adenine in the codon of DNA, the complementary codon in m-RNA will also change to UUG, which will now synthesize a different amino acid, Leucine. This is a point mutation that will alter the relevant character on the body. Similarly, point mutation produces sickle-cell anaemia in man, when codon CTT coding for glutamic acid is changed to CAT, which codes for valine. Mutations can be produced by excessive environmental conditions or by radiation but most of the times they are caused naturally during the duplication of DNA, when new bases are synthesized. As these mutations are recessive, they remain hidden and do not express for a long time, and since they are genetic they play very important role in evolution.