Sources of acquired variations are apparent. Any external condition, influencing the organism, produces this type of variation.
The external factors may be light, heat, cold, food etc. These variations, because they are somatic, may not be inherited. The cause of evolution, really speaking, comes from the germinal variation which is inheritable.
When two animals or plants of the same species are grown under different conditions of light intensity, they show mark variations in their form. Cunningham proved experimentally that the flat fish, Solea developed pink colour on its ventral surface, which is normally white, when exposed to light. The upper surface, which is normally pink, turns white when the animals are kept in shed.
A number of variations can be observed by lowering or increasing the temperature. Tower and Agar by their experiment an mice have shown, that’ mice reared in a warm room at about 21°C were found to differ considerably from those reared in a cold room at 5°C.
It is also an important factor for normal growth. Plants growing in dry and arid soils undergo conserve water, while plants of moist areas show luxurious growth.
Nutrition, like water, is another very important factor for the growth of an organism.
5. Endocrine Glands
The hormonal secretions of endocrine glands, which influence the development and differentiation of various physical and mental characters, are also responsible for causing variations. For example Axolotl larva of Ambyostoma. Due to thyroxine deficiency it remains as larva.
6. The Use and Disuse of Organs
Use and disuse of organs also produce variations. The organ which is put to continuous use shows better development than less used. The constant disuse of an organ leads to atrophying of that organ.
7. Cytological Basis
The cytological or genetic sources of variation may be due to three reasons.
(i) Difference in the number of chromosomes
(ii)Change in the structure of individual chromosomes
The first are collectively called chromosomal aberrations.
(i) Differences in the number of chromosomes. It is also called heteroploidy. It is of two types:
(A) Changes involving entire sets.
(a) Haploidy. Having are genome only (n)
(b) Polyploidy. Each genome is represented more than twice like triploidy (3n), tetraploidy (4n) and so on.
(B) Changes involving the number of chromosomes in one genome.
(a) Monosomic. Loss of one chromosome from diploid set (2n – 1)
(b) Nullisomic. Loss of both the homologous chromosomes from diploid set (2n – 2)
(c) Trisomic. Increase of one chromsome in a diploid set (2n + 1)
(d) Tetrasomic. Increase in one pair of homologous chromosomes (2n + 2)
(ii)Change in the structure of chromosomes:
(a) Deletion. Loss of one or more genes.
(b) Duplication. Addition of one or more genes.
(c) Inversion. A chromosomal segment is inverted at 180°C.
(d) Translocation. Exchange of parts between non-homologous chromosomes.