The difference between closely related animals called variation. Each individual differs from the others by height, body shape, body colour, voice, behaviuor etc. Variation provides raw material for evolution.
Types of Variation:
1. Meristimic Variation:
It is the differences in the number of body parts e.g. presence of six fingers instead of five.
2. Substantive Variation:
It is a variation where difference occurs in the shape size and colour etc. e.g colours of skin, hair and shape of ear ,nose body etc.
3. Continuous Variations:
These are minute differences in a population where one individual shows continuous differences from the other with intermediate forms.
4. Discontinuous Variation:
Those are large differences and appear suddenly with out intermediate stages. These sudden changes are called mutations e.g. appearance of six fingers in a man.
5. Somatic Variation:
The variation of somatic cells by the influence of environmental factors called somatic variation e.g. Lamarck’s acquired characters in Giraffe.
6. Germinal variation:
The differences appearing in germ cells is called germinal variation. These variations are heritable.
Sources of Variation:
The following factors are the sources for variation:
- Gene mutation
- Chromosomal aberration
- Natural selection
- Genetic drift
- Founder principle
- Migration and gene flow
1. Gene Mutation:
Sudden change in the genetic make upon of an individual is called gene mutation. This mutation produces a different phenotype characters. Thus mutation produces variation.
2. Chromosomal Aberration:
Each cell of a species contains specific number of chromosome and genes are arranged in a specific sequence in the chromosome. The changes occurring in the chromosomes are called chromosomal aberration or chromosomal mutation. Any change in chromosome affects the phenotypic characters. Thus chromosomal aberration causes variation. The chromosomal aberrations are as follows:
- Deletion– the loss of chromosomal parts resulting in loss genes is called deletion.
- Duplication – The presence of same type of genes more than once in a chromosome is called duplicative.
- Inversion– It is a change of the sequence of genes by the rotation of gene blocks with a chromosome by 180°.
- Translocation– It is an exchange of chromosomal parts between homologous or non-homologous chromosomes.
- Haploidy– When only one set of chromosomes is present; this condition is called haploidy (n).
- Polyploidy– Each chromosome is represented by more than twice. The normal having three sets (3n) or four sets (4n) or five sets (5n) etc. is called polyploidy.
- Monosomic (2n-1) – It represents to the loss of one chromosome from a pair.
- Polysomic (2n+1, 2n+2)-It represents addition of one or more chromosomes to a pair.
- Nullisomic– It presents loss of both chromosomes of a pair (2n-2).
Recombination is a process of mixing and reshuffling of the available genes to produce a new gene combination. The new genetic make up causes variation. The recombination of genes occurs by following factors.
a. Sexual reproduction, b. Mendelian recombination, c. Crossing over.
a. Sexual reproduction:
There is randommixing of genes of male and female gametes during fertilization. This makes a new gene combination in the zygote(fertilized egg). Thus sexual reproduction provides greater potential for variation.
b. Mendelian recombination:
During gamete formation, the genes segregate at random. During fertilization different gene groups recombine at random. Thus during gametogenesis and fertilization, a variety of recombination of a pair of genes occurs.
c. Crossing Over:
The exchange of chromosomal segments between non-sister chromatids of the homologous chromosom is to produce new genetic make up in an individual, is called crossing over. This occurs in the meiosis during gameto genesis.
The hybridization is a cross between two species to form a hybrid. This brings about new gene recombination and establishes variation in the population. Hybrids are generally sterile but a few are fertile.
The interbreeding between individuals of species can be prevented by barriers such as water, land, desert, maintain etc. This is called isolation. Isolation separates two populations. The gene pool of the separated population remains isolated from the other due to geographic barrier. This leads to genetic differences that cause variation.
6. Natural Selection:
The variations in organisms may be favourable or under-favourable in an environment. If the environment favours the variable characters, the organism will be the fittest to survive. The organisms with unfavourable variation may die. This means the nature select the fittest one.
7. Genetic Drift:
It is also called Sewal Wrights effect. The genetic drift is a random change in gene frequency from one generation to another in a population. This leads to complete elimination of character.
8. Founders Principle:
It was proposed by Sheppard (1960) and Mayr (1963). This principle states that the gene pool of an isolated new population is not similar to that of parent population because of sampling error. This leads to genetic variation. For example, in a large population of birds, the gene pools are G and g. According to Mendels principle the population consists of 25% GG individuals, 50 Gg individuals and 25% gg individuals.(1:2:1 ratio) Out of this large population 10 birds migrated to an island. These 10 birds are called founder because they will give rise to a new population.
These 10 birds may not show of the parent population. There may be 7 GG, 2Gg and 1 gg individuals or in any other combination or all the 10 individuals may be GG or gg. This is called sampling error. If all the individuals are GG, the island population lacks g gene. In this way the gene pool of founders will be different from the parent population.
9. Migration and Gene Flow:
The migration brings two species together and then they interbreed. Therefore there is gene flow from one species to another. This brings about variations.