The term evolution has been derived from the Latin word 'evolvere' means unroll.
Evolution can be defined as a sequence of gradual development of complex form of life from simple form of life over the course of geological time " Descent with modification."
Evolution is of two types –
(i) Chemical evolution
(ii) Organic evolution.
|S.No.||Features||Chemical Evolution||Organic Evolution|
|1||Definition||It is the formation of the complex organic compounds from simple compound or element.||It is the formation of complex form of life from simple form of life.|
|2||Time of occurrence||It occurred at the time of origin of life.||It is still occurring.|
|4||Speed of evolution||It is fast process.||It is slow process.|
|5||Index was given by||A.I Oparin||Charles Robert Darwin|
After origin of a living cell the next questions that arose was how did so many different species of complex life forms come into existence? Here are various view points.
Carolus Linnaeous: Said that no species is new i.e. each species originates from pre-existing species.
Jeans baptist Lamarck: Explained in his book namely "philosophie zoologique", the theory of inheritance of acquired characters or Lamarkism. This theory states that use and disuse of an organ leads to change in the organ, which is inherited by the offsprings. The favourable variations which remain for longer period of time results in evolution of new species.
This theory was discarded by August Weismann as he experimentally proved that even after cutting tails of mice for 21 generation tailless mice was never born.
Darwinism: Charles Robert Darwin was born on 12 Feb, 1809 in England. He travelled by HMS Beagle ship along with Dr. Henslow. He visited many islands of south America, South Africa, Australia and Galapagos Islands. Darwin was influenced by two books. "Principal of population" of Malthus. "Principal of Geology" of Charls Leyell.
Charles Robert DarwinIn his book origin of species he answered this questions. The theory presented by him is called theory of natural selection or Darwinism.
Alfred Russel Wallace: He travelled south eastern Asia and South America. The idea of natural selection striked in his mind. Wallace wrote an essay and sent it to Darwin. On the tendency of varieties to depart indefinitely from original type, there is striking similarity between the views of Darwin and Wallace.
(i) Enormous rate of reproduction among animals.
(ii) Constant number of animals of a species.
|Struggle for existence.|
(i) Struggle for existence.
(ii) Heritable variations.
|Survival of the fittest or natural selection|
(i) Survival of the fittest.
(ii) Continues environment changes.
|Continuous natural selection leading to evolution of new species.|
Charles Darwin explained the mechanism of origin of new species by natural selection. But he failed to explain the mechanism of source of heritable variations. This was explained by Hugo De Vries, a Dutch botanist. According to him, heritable variations arise when there is a change in genes of the germplasm (protoplasm of germcell). He called it mutation.
Variations provide raw materials for evolution. These may be inheritable or non-inheritable, only inheritable variations participate in evolution.
Genetic Variations: The differences in the DNA sequences among every organism leading to the diverse gene pool are called genetic variations. These differences lead to different/varied physical characters or biochemical pathways.
Natural Selection: It is the phenomenon by which a favourable trait in a population of a species is selected. Changing natural conditions exert equal pressure on all the existing species. The species/organisms which are better adapted to the changing conditions survive and reproduce i.e. selected by nature and species/organisms which cannot adapt perish i.e. rejected by nature.
Type of Variations:
On the basis of nature of cells where variations occur, variations are of two types:
(i) Somatogenic variations or Acquired traits
(ii) Germinal (Blastogenic) variations.
|S.No.||Somatogenic variation||Germinal variations (Blastogenicvariations)|
They occur only in somatic (nongerminal) cells of an individual.
These occur in germinal cells of
reproductive organs of an individual.
|2||These are acquired during the life span of an individual.|
These occur at the time of formation of
gametes in reproductive organs.
These are result of enviromental factors
like changes in light, temperature, food availability etc.
These are developed either due to mutation or recombination of genes.
|4||DNA of cells is not changed so it is non-inheritable.|
These are in hertiable variations.
So they are transmitted from one generation to another
e.g. – Polydactylyinman.
Consider the following example.
1. A group of twelve red beetles living in bushes with green leaves.
2. Beetles in the population can generate variations because these are reproducing sexually.
3. Crow can eat the beetles. The more beetles the crow eat, the fewer beetles are left for reproduction.
Variation in a population inherited and otherwise.
Now consider the following situations :–
|1st Situation||2nd Situation||3rd Situation|
|A green colour variation arises during reproduction.|
A blue colour variation arises during reproduction.
Appearance of plant disease in the bushes.
|One beetle.||This blue beetle can pass the blue to its progeny||Amount of plant (leaf) material for the beetles decreases.|
|This green beetle can pass the green colour to its progeny.||Crows can see blue as well red beetles on the green leaves of bushes so can eat them.||Beetles are poorly nourished.|
|Crows cannot see green beetles on the green leaves of the bushes so cannot eat them.||Most of the beetles are killed by elephant foot.|
Average weight of beetles decreases.
|Observation : More population as compared to red beetles.||Beetles which survived are mostly blue. This is by chance.|
|Conclusion : Variations have survival advantage||Observation : Population of beetles grows slowly and blue beetles are more in number.||Amount of plant (leaf) material for the beetles increases.|
|Rare variation came as a common characteristic in the population.||Conclusion : Variations do not have survival advantage.|
Beetles are properly nourished.
|In other words, frequency of certain gene traits (genes control the traits) changed over generations. This is the idea of evolution.||Frequency of certain traits/genes can be changed by accidents in small populations.|
Size of the saturated beetles increases to normal.
|The number of red beetles decreases as the number of crows increases (Natural selection is directing the evolution).||Both above changes provide diversity without any adaptations.||Conclusion:|
The change in the weight of beetles is not inherited over generation.
Acquired characters: The traits that are acquired by an organism over the period of its lifetime are termed as acquired characters. These characters may or may not get transferred to the next generation.
Inherited characters: The traits that are inherited from the parents are called inherited characters. These traits always get transferred to the next generation, but depending on the dominance or recessiveness it may or may not be expressed. Examples are height, skin colour and eye colour.
The reason why organisms resemble their parents lie in the precise copying of their genes, which carry hereditary characters from one generation to the next. On the other hand no two offspring have exactly the same genes.
This is because offspring of sexually reproducing organisms receive varying combination of genetic material from both parents such variations result from mutations (errors in DNA copying). Variations also result from genetic recombination during sexual reproduction.
The random changes in the gene frequency occurring by chance alone. The effect of genetic drift is very small in large population and large in small populations.