Let us make in-depth study of the meaning, types and importance of variations in different organisms.
Variations mean the differences (morphological, physiological, cytological and behaviouristic) amongst the individuals of the same species and the offspring of the same parents.
They are found in all the characters and in every conceivable direction. Therefore, no two individuals are similar.
Types of Variations:
Variations are classified variously according to:
(i) Affected Trait:
Morphological, physiological, cytological and behaviouristic.
Useful, harmful and neutral or indifferent
Meristic (number of parts and their geometrical relations) and substantive (appearance),
Continuous and discontinuous,
(v) Cells Affected:
Somatic and germinal,
(vi) Phenotypic (observable) and genotypic (constitutional).
I. Somatic or Somatogenic Variations:
The variations affect the somatic or body cells of the organisms. They are also called modifications or acquired-characters because they are acquired by an individual during its life time. Lamarck (1801, 1809) based his theory of evolution on the inheritance of acquired characters. However, as proved by Weismann (1892), somatic variations die with the death of the individual and are hence non-inheritable. They are caused by three factors—environment, use and disuse of organs and conscious efforts.
(a) Environmental Factors:
The environmental factors are medium, light, temperature, nutrition, wind, water supply, etc. The environmental factors bring about only slight modifications in animals but in plants the modifications are much more conspicuous. This is due to the environmental effect on the meristems of various parts. A slight change in the meristematic activity can have permanent effect on the plant. Environment can also change the amount of flowering and bring about non- inheritable changes in the floral parts.
Some of the more important environmental factors are:
It is common knowledge that some amphibious plants show heterophilly with dissected leaves inside water and entire leaves outside, e.g., Ranunculus aquatilis. Stockard placed eggs of fish Fundulus in sea water containing magnesium chloride. The young ones reared in such medium possessed one median eye instead of the two usual lateral eyes. Hydrangea bears blue flowers in acidic soil arid pinkish flowers in alkaline soil.
In the absence of light the plants remain etiolated. Shade produces elongated internodes and thinner and broader leaves. It increases the succulence of many vegetables. Strong light, on the contrary, helps in the production of more mechanical tissue and smaller and thicker leaves. Palisade parenchyma becomes multilayered under strong light but remains single layered under moderate intensities of light (e.g.. Peach).
The effect of light has also been observed by Cunningham in flat fish Solea. The fish habitually rests on left side. It develops pigmentation and eyes on right side, the side exposed to sun. If left side is exposed to sunlight in the young fish, both eyes and pigmentation develop on that side.
Temperature directly affects the metabolic activity of the organisms and rate of transpiration in plants. Plants growing in hot area show stunted growth of the aerial parts and greater growth of the root system. Strong sunlight and high temperature bring about sun-tanning of human skin by production of more melanin for protection against excessive insulation and ultraviolet radiations.
The individual provided with optimum nutrition grows best while the under nourished shows stunted growth. The abundance or deficiency of a mineral salt produces various types of deformities in plants. A larva of honey bee fed on royal jelly grows into queen while the one fed on the bee bread develops into worker.
Plants growing in soils deficient in water or in areas with little rainfall show modifications in order to reduce transpiration and retain water, e.g., succulence, spines, reduced leaves, thick coating, sunken stomata, etc. Those growing in humid and moist area show luxuriant growth.
(b) Use and Disuse of Organs:
This is mostly observed in higher animals. The organ which is put to continuous use develops more while the organ less used develops little. A wrestler or a player who performs daily exercise develops a stronger and more muscular body than another man who does not do any exercise. A lion, tiger or bear kept in a zoo is weaker than the one living in jungle.
(c) Conscious Efforts:
Modifications due to conscious efforts are observed only in those animals which have intelligence. Receiving education, training of some pets, slim bodies, boring of pinna, long neck, small feet, mutilations in pets, bonsai, etc. are some of the examples of conscious efforts.
II. Germinal or Blastogenic Variations:
They are produced in the germ cells of an organism and are inheritable. They may be already present in ancestors or may be formed suddenly. Accordingly, the germinal variations are of two types, continuous and discontinuous.
1. Continuous Variations:
They are also called fluctuating variations because they fluctuate on either side (both plus and minus) of a mean or average for the species. Continuous variations are typical of quantitative characteristics. They show differences from the average which are connected with it through small intermediate forms.
If plotted as a graph, the mean or normal characteristic will be found to be possessed by maximum number of individuals. The number of individuals will decrease with the increase in degree of fluctuation. The graph will appear to be bell shaped (Fig. 5.9). The variations are already present in different organisms or races of a species.
They are produced by:
(i) Chance separation or segregation of chromosomes at the time of gamete or spore formation.
(ii) Crossing over or exchange of segments between homologous chromosomes during meiosis.
(iii) Chance combination of chromosomes during fertilization.
Therefore, these variations are also known by the name of re-combinations. They make an organism better fitted to struggle for existence in a particular environment. They also enable human beings to improve the races of important plants and animals. However, they are unable to form a new species though Darwin (1859) based his evolution theory of natural selection on continuous variations.
Continuous variations are of two types:
They influence appearance including shape, size, weight and colour of a part or whole of the organism, e.g., height, shape of nose, skin colour, colour of eyes, hair, length of fingers or toes, yield of milk, eggs, etc.
They influence the number of parts, e.g., number of grains in an ear of wheat, number of epicalyx segments in Althaea, tentacles in Hydra or segments in earthworm, etc.
2. Discontinuous Variations:
They are also called sports, saltation’s or mutations. They are sudden unpredictable inheritable departures from the normal without any intermediate stage. The organism in which a mutation occurs is called a mutant. Discontinuous variations form the basis of mutation theory of evolution proposed by de Vries (1902).
Discontinuous variations or mutations are caused by:
(a) Chromosomal aberrations like deletion, duplication, inversion and translocation,
(b) Change in chromosome number through aneuploidy and polyploidy,
(c) Change in gene structure and expression due to addition, deletion or change in nucleotides.
The discontinuous variations are of two types:
They affect the shape, size and colour, e.g., short legged Ancon Sheep, Hornless cattle, Hairless cats. Piebald patching in man, etc.
They affect the number of parts, e.g., Polydactyly (six or more digits) in humans.
Importance of Variations:
1. Variations make some individuals better fitted in the struggle for existence.
2. They help the individuals to adapt themselves according to the changing environment.
3. Discontinuous variations produce new traits in the organisms.
4. Variations allow breeders to improve races of useful plants and animals for increased resistance, better yield, quicker growth and lesser input.
5. They constitute the raw material for evolution.
6. Variations give each organism a distinct individuality.
7. Because of variations, species do not remain static. Instead, they are slowly getting modified forming new species with time.
8. Pre-adaptations caused by the presence of neutral variations are extremely useful for survival against sudden changes in environment, e.g., resistance against a new pesticide or antibiotic.