Evolution

Defination 

"Development of more advance and complex organisms from pre-existing relatively simple ancestors over course of time is called evolution

Evolution

 

 

 

Homologous Organs: 

 The organs that are same in structure but different in functions are called homologous organs.'

 Divergent Evolution : 

evolutionary process producing homologous organs is called divergent evolution. Analogous Organs The organs that are similar in functions but different in structure are called analogous organs. 

Convergent Evolution: 

The evolutionary process producing analogous organs is called convergent evolution. Hereditary Characteristics:

 Definition: 

 "Characteristics which are passed from parents to offspring are called hereditary characteristics." Examples: Eye colour, hair colour, tongue rolling, etc. in human beings are all hereditary characteristics. 

Non-hereditary Characteristics:

 Definition:

 "These are characteristics which are not transferred from parents to offspring." Examples: Parent's habits and loss of any body part of a person due to diseases or accident are all examples of non-hereditary characteristics. Explanation: Human beings have many similarities and differences. For example in height, colour and other body feature, one can differentiate between individuals due to these characteristics. 

Variation: 

Definition: 

 "The differences shown by the individuals of one kind of organisms are collectively called variation." or "Variation means differences in characteristics of organisms, belonging to the same species in a natural population."

 Types of Variations: 

 Variations are of two types:

 Continuous Variations Discontinuous Variations Continuous Variations In this type of variations, there are small differences among the individuals of the same species and many intermediate forms are seen. So there is a complete gradation of characteristics in a population. 

Source of Continuous Variations: 

Characteristics showing continuous variation. are produced due to combined effect of many genes. and environmental factors. Examples: Height in human beings is an example of continuous variation. Intelligence is another example of continuous variation. 

 Discontinuous Variations: 

 In these variations, there are sharp differences among the individuals of same species. Examples: Tongue rolling, blood group in man, etc. which do not show any intermediate forms are examples of discontinuous variations. 
 Sources of Variations: 
There are two major sources of variations. 
Sexual reproduction 
Mutation 

Sexual Reproduction: 

Great variety of variations occur in sexually reproducing organisms. Crossing over and independent assortment during meiosis and random fusion of gametes in fertilization are the important sources of genetic variation in population. The variation in organisms produced by these sources is not under influence of environment. The environment affects at a later stage. 

Mutation:

 Sudden heritable change in genetic material is called mutation. Mutation is an important primary source of variations among organisms. It is of two types. 
 (i) Gene mutation
 (ii) Chromosomal mutation 

Gene Mutation:

 The gene mutation takes place in the nucleotide and is at gene level so called gene mutation. 

Chromosomal Mutation: 

 In this type of mutation a complete chromosome or a part of chromosome is deleted or added into gene complement. The variations which occur due to changes in genetic material have occurred gradually and continuously since the life started on earth. These variations are heritable because these are passed from parents to their offspring in sexual reproduction. These heritable variations are the raw material for evolution.