Sources (Causes) of Variation

Heredity

The offspring’s of all the organisms (plants and animals) resemble their parents in several aspects. This is only due to the phenomenon of heredity. Hereditary literally means “tendency of like begets like” i.e. all living organisms tend to produce offspring’s like themselves.

Hereditary may be defined as the transmission of characters from one generation to successive generations or from parents to their offspring’s. Thus, heredity is the cause of similarities between the offspring’s, so that the individuals of the same parents resemble each other in many aspects. Heredity involves the transfer of genetic characters from parents to the offspring’s via the egg and sperm. These transferable characters are called “hereditary characters”.

Variation

Variation is the result of two main factors: genetic differences and environmental influences. Variability is also known as polymorphism (Greek – many forms)

_­Interspecific variation: When one species differs from another species.

Intraspecific variation: When members of the same species differ from each other.

Darwin divided variation into two categories, both of which played a significant role in the origin of new species.

Categories of Variations: Based on the degree of differences, variation is classified into two types:

1) Continuous Variation: Small and indistinct variations are called continuous variation.

a) These are fluctuating with environmental conditions.

b) These are non-heritable.

c) They have no role in evolution.

d) They are most common and occur in all organisms.

2) Discontinuous Variation: Large, distinct and sudden variations are called discontinuous variation.

a) These are relatively unaffected by environmental conditions.

b) These are heritable.

c) They provide raw materials for evolution on which selection is based.

d) They are not common and appear suddenly.

Types of Genetic Variation

These are due to the different genes that each individual organism possessed. These change from generation to generation.

Genetic variation occurs as a result of:

P = phenotypic variation G - gene mutation

P-Crossing over between chromatids of homologous chromosomes during Meiosis

P-Random fertilization of gametes

P-Random mating between organisms within a species

P-Independent Assortment: of chromosomes, and therefore alleles during meiosis.

G-Mutations: These sudden changes to genes and chromosomes may be passed to the next generation.

Though offspring’s receive all the characters from their parents, they are not exactly alike. Differences are found even between the offspring’s of the same parents. It is difficult to find out the identical individuals. The progeny differs not only in itself but also with the parents. These differences are called variations. Thus, variations may be defined as the visible differences between the parents and the offspring’s or between the offspring’s of the same parents.

Sources (Causes) of Variation

The variations may be classified into two types:

1) Hereditary variation: The variations which arise as a result of any change in the structure and function of the gene and are inherited from one generation to another are called hereditary variation.

2) Environmental Variations: Two individuals with the same genotype may become different in phenotype when they come in contact with different conditions of food, temperature, light, humidity and other external factors. Such differences among organisms of similar heredity are known as environmental variation. These are not heritable.

Based on the type of cells, variation is classified into two types.

1) Somatic Variation: The variation which occurs in somatic cells is called somatic variation. It is generally insignificant, because it is not inherited from parents. It is acquired by the organisms during their own lifetime and is lost with death. Hence, it is also called acquired variation.

2) Germinal Variation: The variation which affects the germinal or reproductive cells is called germinal variation. It is heritable and genetically significant. It provides raw materials for evolution.

What are the five main causes of Genetic Variation?

1. I __ __ __ __ __ __ __ __ __ __ A __ __ __ __ __ __ __ __ __ __ of chromosomes, and therefore alleles, during meiosis

2. C __ __ __ __ __ __ __ O __ __ __ between chromatids of H __ __ __ __ __ __ __ __ __ __ chromosomes during meiosis

3. R __ __ __ __ __ M __ __ __ __ __ between organisms within a species

4. Random fertilization of G __ __ __ __ __ __

5. M __ __ __ __ __ __ __ __

Genetic Variation produced P __ __ __ __ __ __ __ __ __ variation (what it looks like)

Numbers 1-4 remix existing alleles in the P __ __ __ __ __ __ __ __, where as M __ __ __ __ __ __ __ __ can produce completely new alleles.