CHAPTER 5:- ORIGIN AND EVOUTION OF LIFE 

Q.: Give the three key factors of Modern synthetic theory of evolution:

Ans.: The three main concepts of Modern synthetic theory are –

a) Genetic Variations, b) Natural selection c) Isolation.

Q.:  Define genetic population.

Ans.: A group of similar individuals interbreeding among themselves occupying a geographical area is called as genetic population or Mendelian population.

Q.:  Define gene pool?

Ans. The sum total of genes of all individual of interbreeding population is called as gene pool.

Q.: Define gene frequency.

Ans.: The proportion of alleles in the gene pool as compared with other alleles at the same locus is known as gene frequency.

Q.: Explain modern synthetic theory of evolution?

Ans.: 1) The basic concept of Modern synthetic theory is based on Dobzhansky’s book Genetics and the origin of species.

2) The main architects of this theory are Dobzhansky, Mayer, Julian Huxley, Simpson Stebbins, fisher Haldane and Sewall Right etc.

3) The theory supports organic evolution by natural selection and inheritance of characters.

It explains Darwinism and Hugo De Vries theory in the focus of genetics.

4) The theory is based on three main concepts which can be explained as follows:-

A) Genetic variations:

1) The group of interbreeding organism in a specific geographical area is known as population and considered as unit of evolution.

2) Genetic variations actually act as raw material in the process of evolution.

3) The different reasons for genetic variations can explained as follows:-

I) Gene mutation or point mutation:

1) Changes in the chemical makeup of a gene is known as gene mutation or point mutation.

2) Point mutation bring about variation in gene pod and change the gene frequency.

II) Gene Flow:

1) Transfer of genes between two genetically different populations from one another is called as gene flow.

2) Migration of animals to different regions transfer genes to the gene pool of new area altering the gene frequency.

III) Genetic recombination:

A. It is the result of exchange of genetic material between two homologous chromosomes by the process of crossing over.

In sexual reproduction, gametogenesis brings about new genetic recombinations.

B. These genetic recombination’s are responsible for variations.

IV) Genetic drift / Sewall Right effect:

1) Any alteration in frequency of an allele in the natural population by pure chance is genetic drift.

2) Sewall Right proposed this concept of genetic drift.

3) Due to genetic drift addition of elimination of specific gene can occur in a population.

4) Smaller populations have greater chances of genetic drift. These small population under favourable conditions can give barge number of variations.

V) Chromosomal aberrations:

1) It refers to structural alteration in chromosomes changing the arrangement of genes.

2) The different process like deletion, duplication, translocation and inversion can bring about changes in Mendelian population also.

B) Natural Selection:

1) It is the fundamental unique operating mechanism of evolution.

2) The process refers to survival of those organisms which appear physically, physiologically and behaviorally better adapted to environmental conditions.

3) Such useful adaptations are passed into next generation through natural selection.

4) Selection depends upon the existence of phenol type variations within population and is part of mechanism by which species adapts itself to the environment.

5) Hence, through transfer of such favourable adaptation better suited off springs will be produced.

6) The best example is of Month Biston betularia and Biston Carbonaria during industrial revolution in Great Britain.

C) Isolation:

1) Separation of a single population into several subunits is called isolation.

2) The process of interbreeding is not possible between isolation groups.

3) Each group may develop a set of new characters which lead to evolutionary changes and formation of new species.

4) The isolating mechanisms are either geographical or reproductive.

5) In geographical isolation population get divided into subunits due to rivers, oceans, island formation forests, mountains, etc. and fails to reproduce.

6) In reproductive isolation the mechanism operates through change in genetic Material, gene pool and structure of genital organs.