Variation and its sources

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Individuals show great differences of from  size , colour,  habit and physiology among themselves. These differences are tremed as variations.  Variation can be defined as differences in traits of characteristics between individuals of the species.  No two individuals are like, not even identical twins. Variation are produced due to genetic properties and also sometime by exposure to certain types of environmental conditions.  Variation is necessary for naturl selection. Natural selection favors individuals with characteristics that are best adapted to their environment.  Variation is therefore necessary for species to become adapted to their environment and it enables them to change when the environment changes. Generally we can trace the variation to the following main sources.
     Variation in dogs


a. Genetic recombination
If you study the process of sexual reproduction carefully then you will see that three random components of sexual reproduction are responsible for bringing genetic diversity.  These are as follows:
1 . independent assortment of chromosomes
In sexual reproduction , the process of meiosis is required for the formation of gametes. The reduction in the number of chromosomes is achieved by separating the members of each pair of homologous chromosomes in to different nuclei. Independent assortment of homologous chromosomes takes place , which is responsible for the formation of new chromosome combinations in the daughter cells. 
Crossing over and independent assortment of chromosomes during different stages of meiosis

2. Crossing over
Sexual reproduction promotes variation because each gamete (sperm or egg) contains a mixture of genes from two different parents. Crossing-over promotes variation. During prophase of meiosis , crossing-over results in chromosomes that have some genes from one parent and some genes from the other parent.
The phenomenon of crossing over further increases the genetic variability produced by independent assortment of chromosomes.  If no crossing over occur, meiosis could produce only two genetic types of gametes. With crossing over more than two types of gametes  can result. 


C. Random fertilization
Each gamete has a unique genetic composition , because of the shuffling of genes and chromosomes in meiosis.  When gametes combine at random during fertilization,  the organisms produce are different from all others of the species. 


2.mutation
Mutations is a spontaneous change in a gene, which alter gene product.  It is defined as any change in the nucleotide sequence of DNA. Alleles arise by mutation.  The original allele is termed the wild type, and the new allele as the mutant allele. If there were no mutation , there would be no biological evolution. 
An extra limb due to mutation




3 . Environment and variation
Although environment has no role in the intitial production of variants , they are randomly produced.  Once the variants are produced they are then acted upon in the environment by the process of natural selection.  The variants , which are best suited in a particular environment,  have more chances to survive or being selected by nature.  Accumulation of such traits generation ofter generation results ultimately in the production of new species. 

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