One of the most fascinating Biology Topics is the study of the brain and behavior.
The Importance of Variation – The Evolution and Source of Variation
Variation makes the individuals of a species different from one another. This variation is measured by the phenotypic expression of a feature and this may be associated with the morphology of the organism, physiology, or behaviour of the organism. Darwin gave much importance to the variations as raw material causing the origin of species in the process of evolution. According to Darwin accumulation of variation makes an organism successful in fighting with others for survival in nature. Variation is said to be a natural occurrence, but it may appear in certain individuals due to some genuine cause and that appears usually due to a change in the genetic makeup.
If the variation is a phenotypic expression, the genotype becomes the root of the expression of the phenotype. Adaptability or fitness is taken to be the essence behind success in the process of struggle for existence and variation ensures this adaptability of the members of the species. Hence, without knowledge of variations and their appearance in the population study on evolution cannot be completed. No two individuals of a species are exactly alike and they vary appreciably. The differences between closely related organisms occurring in the same environment are called variations. Darwin observed slight variations occurring in all parts of the organism and he gave much importance to this for evolution.
Primary Category of Variations
1. Useful and Unuseful Variations:
The variations which have better survival value in the struggle for existence, are useful whereas the variations with less survival value, are unuseful.
2. Hereditary and Non-hereditary Variations:
The variations which are inherited in successive generations are hereditary, whereas non-hereditary variations are somatic variations due to environmental effects or the effect of conscious effort. They are restricted to the first generation and are not inherited by filial generations.
3. Continuous and Discontinuous Variations:
The hereditary variations, if transmitted through successive generations, are known as continuous, whereas discontinuous variations are inherited only for a few generations, after which they are discontinued.
The role of useful, hereditary, and continuous variations in the process of evolution, is immense. Darwin also considered useful, hereditary, and continuous variations of the species as the most important factors in the., mechanism of evolution. But Darwin did not explain the origin of such variations.
The proposers of modern synthetic theory feel that the variations in the members of a species are genetically determined. They also proposed that the variation in the individual level is not very important, rather variation in the whole species population is helpful for the evolution of species. For this reason, the gene pool of a population is more important rather than the genome of a member of the species. The collective gene reserve of a species population is known as a gene pool. Through changes in gene frequency of the gene pool evolution may occur. Any change in the phenotype (i.e., index of variation) may occur only through changes in gene frequency. Therefore, a change in gene frequency is antagonistic to the stability of a species.
Nature of Variation and its Classification
Variation affects the phenotype of an organism and therefore, the phenotype that is affected by a variation may be a basis for classification of variation.
- Morphological Variation: In this variation, the size, shape, or structural modification of an organism or its body parts may be observed.
- Physiological Variation: When the activity of a body part is influenced by a variation, it is called physiological variation.
- Psychological Variation: Mental status of the organism is altered in this variation, as a result, the behaviour of the organism is changed.
All the variations as mentioned above may again be of two types heritable variation and non-heritable variation. The non-heritable variations are confined to a single generation and cannot be inherited by the succeeding generations. On the other hand, heritable variations may be transmitted from one generation to the other. It is noteworthy that heritable variations are important for evolution and non-heritable variation has no value in the process of evolution.
Types of Variation
All variations may be divided into 4 types of variations as-
1. Meristic and Substantive Variation:
The character which is considered based on the number is known as a meristic character. When a variation is related to some meristic character, it is known as meristic variation. On the other hand, a character judged based on quality is known as substantive character. A variation concerned with substantive character is known as substantive variation. The number of digits in the hand and feet in man, the number of ribs in the chest, the arm number in starfish, etc. come under the meristic character. On the other hand colour of the eyes, hair, the shape of the nose, ears, and eye, height of the plants, etc. come under substantive character.
2. Continuous and Discontinuous Variation:
The characters whose expressions may be continuously measured by qualitative values are known as continuous variations. On the other hand, a suddenly appeared distinct character is considered a discontinuous character. The variables in continuous variation contain a mid-value or average when most of the members in the population possess this average value. The variable values of the continuous variation of the population are so distributed that if the values of the character concerning individuals in the population are placed on graph paper, then a bell-shaped curve may be obtained. Darwin termed these variations as fluctuations and according to Darwin continuous variations are useful for evolution. The discontinuous variations are not only sudden, they are large and unique. This type of variation is also called saltation or mutation. Mutations are heritable and stable, de Vries opined that discontinuous variations are favourable for evolution.
3. Determinate and Indeterminate Variation:
The variations which are targeted to some final achievement, are known as determinate variation. Such variation helps an organism adapt to the environment. For example, the number of digits in the modern horse is one. But their forefathers contained more than one digit on their limbs. Over time for millions of years, the number of digits has been reduced to one in the modern horse and this has made them swift runners on the ground. Similarly industrial melanism in Biston betularia was a directional change to achieve a state as in their dark variety and this made them more adaptive in comparison to their normal variety in the darker environment. The indeterminate variations are spurious and haphazard and this type of variation is not at all helpful for the adaptation of its possessor.
4. Somatogenic and Biastogenic Variation:
The body of the living organisms is usually (in many cases) formed of two entities namely somatoplasm and germplasm. Based on this body cells may be categorized into somatic cells and germ cells. Germ cells in the body produce gametes for reproduction and somatic cells to constitute the body structures. In this consideration, the variations which are confined to somatic cells of the body and are used in the formation of body components may be designated as somatogenic variations. On the other hand, the variations which influence the germ cells of the body and are transmitted into the gametes, are known as blastogenic variations.
Differences between Somatogenic and Blastogenic Variation:
|Somatogenic Variation||Blastogenic Variation|
|1. Confined to somatic cells.||1. Confined to germ cells.|
|2. Expressed during the formation of body structure.||2. Does not express but may be transmitted into gametes.|
|3. Usually develops under the influence of the environment.||3. Usually appears due to mutation or recombination.|
|4. Not heritable.||4. Usually heritable.|
Differences between Continuous and Discontinuous Variation:
|Continuous Variation||Discontinuous Variation|
|1. Small showing continuity.||1. Large, unique, and discrete.|
|2. Measurable by qualitative values.||2. Not measurable by any means and haphazard by occurrence.|
|3. Called fluctuations.||3. Known as saltation or mutation.|
|4. Appears under the influence of the environment.||4. Appears due to gene mutation.|
Differences between Meristic and Substantive Variation:
|Meristic Variation||Substantive Variation|
|1. Changes concerned with characters expressed by numerical values.||1. Changes concerned with the form and structural makeup of the body.|
|2. Such change does not come to the consideration of the whole body of the organism.||2. Comes to the consideration of the whole body of the organism.|