~ INTRODUCTION
In the last chapter you have studied how organisms reproduce to produce new individuals. The offsprings produced as a result of reproduction are similar to their parents as well as other individuals of the species. For example, man resembles man in body shape size etc. Similarly dogs resemble dogs, cows resemble cows, and so on. Similarly the plant obtain from the seed of a mustard plant is similar to its parent. The offspring plant is similar to other mustard plants as well. But how is it that the offsprings are similar to other members of the species as well as to Parents. All this happens because of the presence of genes in parent/parents which are transferred to the next generation via gametes or other means. These genes transfer the biological information of parents to offsprings which gives rise to various characteristics in the offspring thereby making them similar to parents or other members of species.
These differences in characteristics shown by the individuals of a species, and also by the offsprings of the same parents is known as variations.
During the process of reproduction, not only resemblances but also variations are transmitted from one generation to the other. This is known as heredity.
The branch of science which deals with the study of heredity and variations is termed as genetic.
~ Accumulation Of Variation During Reproduction
All the variations which we observed in various organisms are not the result of variations of single generation. Occur due to accumulation of variations generation after generation.
Already discussed, no two individuals are in datical to each other. A sexual reproduction products produced identical of springs but these also have some variation due to the other. Asexual reproduction produces identical offsprings but these also have some variations due to the errors in DNA copying. As a consequence the off springs will get a common design but with subtitle changes. Now when this springs will reproduce they will transmit basic body design variations obtain from previous generation and some more variation resulted due to DNA copying to the next-generation. In this way the variations will continue to accumulate generation after generation.
This process can be easily understood by the following example :
Consider a bacterium. Suppose this bacterium is without any variation. Now when the bacterium divides and then the resultant bacteria divide again 4 individual bacteria formed would be very similar except for some variations which arise due to in Aquarius during DNA copying and some inherited from their parents.
Though variations are also produced during asexual reproduction, they are much greater in sexual reproduction. This easy because of crossing over taking place at the time of formation of gametes and fusion of gametes to form New combinations of chromosomes.
Variations can be both advantageous as well as disadvantageous. Some variations do not affect the organisms in any way. As discussed in previous chapter, variations can lead to survival of species. For example in the temperature of sea increases due to global warming then all the bacteria present in sea water will die except those variations which the resistance to heat. In this way environmental factors also choose the fittest organisms to survive and this process forms the basis of evolution.
Variations are of two types : (i) heredity and (ii) environmental. Heredity variations are inherited by the progeny from their parents.The characters which are present in parents can reassort and give rise to different combinations. These different combinations are responsible for heredity variations in the same progeny, e.g., young ones of same parents are never exactly similar to each other and to parents. Similarly, various members belonging to new species are never exactly similar. Environmental variations are those which are merely due to environment. While hereditary variations are transmitted from generation to another generation, the environmental variations temporary. They have nothing to do with the last or the next generation.
Hereditary variations with in a program result due to sexual reproduction. If there is no sexual reproduction there could be no hereditary variation except those due to mutations. In an asexually reproducing organism, the population appear fairly uniform. Thus, the sexual reproduction plays a very important role in evolution.
Sexually reproducing organisms have much more variations due to the process of crossing over, fusion of gametes, etc. These variations are accumulated generation after generation thereby leading to Greater diversity in population. Due to presence of large number of variations these organisms evolve at a very fast rate.
Before studying further you should know some terms associated with genetic. The knowledge of these terms will make your further understanding of the matter easier.
1. Gametes: A mature reproductive cell specialised for sexual fusion.
2. Character: A feature of the individual.
3. Trait: An inherited character and its detectable variant.
4. Cross: A mating between two individuals leading to fusion of gametes.
5. Zygote: The cell produced by fusion of male and female gametes.
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