Variations :
No two animals on the earth are exactly similar. Even the members of one species show differences and the generation of similar parent are also show differences. The differences in the characters of the animals of same species is termed as Variation. The variations are the main & progressive factors for evolution.
Causes of Variations :
There are many factors which cause variations in the animals. The main factors are as follows
1. Environment
It affects the organisms directly
2. Inherent tendency
The organisms have an inherent tendency to vary. Hence, under no circumstance, the two organisms can be exactly alike.
3. Endocrine glands
Abnormal secretions of hormones from the endocrine glands are able to cause somatic & germinal variations
4. Dual parentage
Bi parental parentage is the real cause of variation (Weismann). An organism receives ½ germplasm from its parents, ¼ from grand parents. 1/8 from grandgrand-parents & so on.
Type of variations :
(A) Somatic & germinal variations
The somatic variations are caused due to environmental effects. They are not heritable hence are lost with the death of the organism. They are also called as acquired variations.
Examples :
- More developed muscles in athletes.
- Mental development through education.
- Loss of body organ in an accident.
- Small feet of Chinese ladies.
- Sun-burnt skin in Europeans living in tropical regions.
The Germinal variations occur in the germplasm of the organisms. The germplasm is used in the formation of the gametes, hence, these variations transmit from generation to generation.
Sometimes somatic variations after repeated generations produce germinal variation effect.
Examples
1. Body length
2. Colour of hair
3. Colour of eye pupil
(B) Determinate and Indeterminate variations
The variations that occur controlled & in the definite direction of adaptation by unknown factor are called as determinate variations
The variations that are governed by some unknown force and imposed to occur in one line or certain lines, usually in an adaptive direction, Examples -overgrowth antlers of Iris deer & Tusks of Jefferson mammoth.
The variations which are not governed by any law but take place in some imaginary direction of change are called as indeterminate variations. The theory of natural selection of Darwin is based on this variation.
(C) Continuous & discontinuous variations.
They are also called as positive & negative variations. Darwin called discontinuous variations as fluctuating variation, because they fluctuate on either side of the average condition. Progressive accumulation of these fluctuations in one direction lead to the formation of new species.
Discontinuous variations are sudden & large and appear as totally new. They are also as mutations. They are mostly stable & heritable.
A few examples of discontinuous variations are absence of tail in kittens, absence of horns in calf, 6 fingers in man, absence of spines in the rose etc.
(D) Meristic & substantive variations
Repetition in the number of parts of a organism is called as meristic or quantitative variations.
Examples
- Presence of 6 arms in place of 5 in star fish.
- Presence of 13 ribs in place of 12 in man.
- Presence of 6 fingers in place of 5 in man.
- Absence of one kidney or one lung in man
Variation in form, size, shape & color of any organism or its parts is called as substantive or qualitative variations.
Examples
1. Hair color
2. Eye color
3. Shape of nose, ear & eye
4. Height of plant etc.
Significance of variations-
- Variations are the main factor of evolution
- Variations help organisms in adaptation.
Differences between continuous and discontinuous variations
Characters |
Continuous variation |
Discontinuous variation |
Presence |
Commonly found in all animals |
They occur suddenly |
Other name |
Fluctuation |
Salutation |
Cause |
Crossing over during gamete formation |
Change in the gene or genomation |