The sources of variation include two major sources; one is somatic and another one is blastogenic variation. Somatic variation caused by environment such as heat, cold, presence or absence of food, enemies intrusion etc. Its modification or variation is not heritable in nature but continuous somatogenic effects may produce blastogenic effects.Blastogenic variation occurred in genetic level and heritable in nature. Apart from this, other causes are also present which results in somatogenic or blastogenic effects. Let us discuss some of the causes of variation.
1. Environment :
Environment directly influences the organism life variedly though it may be morphologically or by genetically i.e. through long exposure to various factors of environment. The heat or cold, presence or absence of food, enemies intrusion or climatic variation so and so are directly influence the organism’s life. For example incase of human the twin earlier look alike in their morphology but they will subjected to different environmental condition their coping mechanism getting changed and results in variation of morphology we can observe. If it has been seen that in an experiment the shape of larvae of sea-urchins and frog is modified when lithium salts are added to water in which they live, in another observation it is seen that frogs are fed on thyroid of mammals passes through all stages of metamorphosis rapidly but remaining in small size as compared to as usual metamorphosis which take longer time to go through these stages.
2. Endocrine Glands :
Endocrine glands secrete hormones into the blood stream, which carries to different parts of the body and influence developmental as well as differentiation process. It directly involves in changes of physical and mental character of an organism and causing somatogenic or blastogenic variations.
3. Cytological Basis :
Cytological or genetic basis of variation can be discussed as follows;
i) Origin of new mutations.
ii) Recombination or New combination of old genes.
i) Origin of new mutations :
a) Gene mutation :
An expression of individual character though it may be genotype or phenotype is the interaction of genes. These genes control the development as well as differentiation of body of an organism. Some time a single gene controls many character or affect more than one character called ‘pleiotropism’ and when a group of particular genes control over a single character then this phenomena is called polygenes. The genes are nothing but segments of DNA sequence which carries the information for production of enzymes in the form of codon (triplet). This codons are form by combination of three nitrogenous base to form the amino acids; nevertheless enzymes always catalyzed the reaction whatever the physical or physiological process takes place in embryo or in adult, different enzymes catalyzed different reaction in developmental process and express different characteristics. Any changes occurs in DNA may be through addition, deletion, of specific nitrogenous base can termed as mutation. According to the alteration of genetic code through addition and deletion the mutations are of various types.
e.g Though codon are read by triple nitrogenous base therefore any change occurred at specific codon and beyond that the information get changed after that codon, because information get shifted from that point.
Normal –T A C C A T T A G ; (TAC) (CAT) (TAG)
Mutated-T A C C C A T T A G;(TAC) (CCA) (TTA) G
Where ‘C’ added.
Normal – T A C C A T T A G ; (TAC) (CAT) (TAG)
Mutated –T A C _ A T T A G; (TAC) (ATT) (AG)
Where ‘C’ deleted.
This type of mutation is called frameshift mutation. This results in the formation of new polypeptide chain. The mutation also caused by substitution of one or more nitrogenous base in polypeptide chain of DNA by other nitrogenous bases. The mutation caused by substitution is known as substitution mutation. In general mutation produce harmful effects but some mutation are beneficial in nature and help in coping mechanism to survive and reproduce in any circumstances. Mutation also leads to creation of new species and pace path for evolution.
b) Chromosomal mutation : The chromosomal mutation followed two types of changes i.e. one is changes in number and arrangements of genes in chromosome and another one is changes in one or set of entire chromosome number. The later one is called ploidy and former is called chromosomal aberration.
i) Chromosomal aberration : Though the chromosomal aberration indicates changes in number and rearrangement of genes in chromosome which further includes deletion or deficiency, duplication and inversion, translocation respectively, then let us first discuss the changes in number of genes.
e.g.1.Deletion/Deficiency –It causes lethal situation to organism.
e.g.2.Duplication/Repeat – A particular genes appeared more than once in a chromosome.
Duplication plays a significant role in evolution because it added genes which may cause the mutation in chromosome and any lethal mutation occurred in chromosome can reduces by these duplicated additional genes.
Now let’s have discuss the changes in arrangement of genes in chromosome.
e.g.1. Inversion – If there is a change in arrangement of sequence of genes altered through rotation of fragment by 180° in chromosome is called inversion.
Consequence of inversion is not so much lethal as compared to deletion and duplication though the genes are remained same.
e.g.2. Translocation –In translocation genes from one chromosome transferred to another chromosome i.e. the interchange of chromosome fragments of non- homologous chromosomes takes place. The following diagram ‘A’ and ‘B’ showing the resultant of translocation.
Consequence of translocation is normal as long as the position effect unless the gene play any lethal role.
ii) Changes in number of chromosomes or ploidy :
Changes in the chromosome number could be of two types i.e. one changing in entire sets of chromosome or second is changing in number of chromosome in one sets of chromosomes. e.g. 1. Changing entire sets.
a) Haploid – Having only one set of chromosomes i.e. n- chromosomes.
b) Polyploidy –Having multiple sets of chromosome i.e. more than twice sets.
Triploidy -3n
Tetraploidy- 4n
Polyploidy-5n
e.g.2. Changing number of chromosome in one set of chromosome.
i) Monosomic – 2n-1 ( loss of 1 chromosome).
ii) Polysomic –2n+1 or 2n+2 (addition of 1 or 2 number of chromosome).
iii) Nullisomy – 2n-2 ( loss from both the sets ).
C)New combination of old genes/Recombination :
New combination of old genes can be found by following aspects;
i) Mendelian recombination and random assortment genetic material.
ii) Crossing over or exchange of genes between chromosome.
iii) Introgressive hybridization.