Darwin’s theory  of evolution tells us how evolution occurs from simple to more complex forms. Mendel’s experiments on pea plant explained the hereditory mechanism of transfer of gene from parents to progeny. But neither of them tell us anything about how life began on Earth in the first place.

In 1929, JBS Halden, a British biologist, biochemist and geneticist, proposed  life must have originated from the simple inorganic molecules. These molecules might have originated on the Earth through a long series of physiochemical changes to more complex organic molecles that were brought about first by chemical evolution. This gradual changes or deveopment of complex forms from simpler forms we term as organic evolution.

In 1953, Stanley L. Miller and  Harold C. Urey  gave an experimental support  to Halden’s theory of origin of life.  They created an atmosphere similar to that which existed on early Earth which contained hydrogen, ambetween the two populationmonia, methane, hydrogen sulphide and water vapourbut no oxygen over water in an air- tight apparatus. This was maintained at a temperature just below 100°C. Continuous  sparks were passed through the mixture of gases to stimulate lightening. At the end of a week, 15% of the carbon from CH4 had been converted to simple carbon compounds, including amino acids which make up protein molecules.


 When one  or more species of living organism develops from an existing spesiec then we can term it as speciation in origin of life. It occurs when two populatins are isolated  in both geographically and reproductively lteading to almost no gene  flow between the two population,

Even in an isolated habitat inbreeding process continues to increase more generations. In a sub-population there is an accumulation of genetic drift  generation after generation. Also,  in various geographical locations variations come through natural selection. As for example, eagle eliminates crow in some territory of sub-population.

 But in all other sub-population number of crows will be very high. As an outcome of this, the green variation will not be selected at the fist site, while it will be strongly selected at the second.

Together ,the process of genetic drift and natural selection may make two isolated sub-populations more and more different from each other, this will lead to generation of new species.


  • If the DNA changes are severe enough, such as a changes in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other.
  • If a new variation emerges in which females are only able to mare with few males due to their specific trait. This factor helps to raise special as well as strong natural selection.


Evolution is the process by which newer tpes of organisms are developed from the pre-existing ones through modifications in their genetic make-up. It is a naturally occurring show, continuous and irreversible process of change. Characteristics are the details of appearance or behaviour. We can say in other words, a particular form or a particular function. For example, the forelimbs of human beings is a characteristic and that the plants can perform photosynthesis is also a characteristics.

 We may define classification as  the scientific arrangement of living organisms into a partcular groups  which is done ccording to their  physiology, anatomy and biochemical relationship. All systems of classification are hierarchial.


  • Cell is the basic unit of life in all living organisms.
  •  The Cells of some organisms do not have organised nucleus e.g. bacterial cell, whereas the cells of some other organisms have a well-organised nucleus.
  • Amongst organisms with well-developed nucleus, some are unicellular and others are multicellular.
  • Among multicellular organisms also, some organisms are can manufacture their own food by photosynthesis process while others depend upon other organisms.
  • Other difference between unicellular and multicellular organisms is that some multicellular organisms have skeleton inside the body, while others have skeleton around the body.


Qs1: Define speciation. State four factors whch lead speciation.

Ans1: Speciation is defined as the process by which new species develop from the existing species.

Four factors leadind speciation are

  • Genetic drift
  • Mutation
  • Natural selection
  • Migration or geographical isolation

Qs2: Mention the factor which cannot be a major factor in the speciation of self-pollinating plant species? Justify your answer.

Ans2: Geographiical isolation cannot be a major factor in the speciation of a self-pollinating plant species. Because, new genes do not enter in the population and no new species is formed.

Hence, there is only a little variation possible. Speciation takes place only when aration is combined with geographical isolation.

Qs3: If two groups of organisms of a single species never exchange genes then what would happen?

Ans3: There may be a situation when two organisms of same species do not cross reproduce by exchanging  genes i.e. do not undego reproduction , there will be no evolutionary  process. Reproduction forms the basis of evolution, by fusing genes of  two parents to form a new geneation with new set of genes.

Qs4:  It is told that all the human races of all continents may have common ancestors. Justify your answer.

Ans4: Human races do not have biological basis to the notion. All humans are a single species irrespective of  their race. All  human body consist of similar  structure in body , both in physiological and metabolical. Their genetic make up is also similar as 23 pairs chromosomes , almost 99.9% DNA  is same in all humans. All of them can freely interbreed to produce offsprings. Not only this, study of evolution of human beings indicates that all of us come from Africa.  Scientists have traced the earliest human species Homo sapiens  in Africa. From all these evidences we can say that habitats of Asia, Europe, Africa are having a common ancestor.


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