Darwin’s theory:
Charles Darwin (1809-1882) was a famous English naturalist, who proposed that natural selection theory of evolution, he wrote a book “origin of species” in 1856ad. In that book, he explained about the cause of evolution. The principles of Darwin’s theory are:-
Overproduction
Struggle for existence
Heredity and variation
Natural selection/survival of fittest
Origin of species
Overproduction: according to this principle, each and every organism wants to produce maximum number of their off springs. Though they want to produce maximum number of offsprings to continue their generation, nature always maintains it. Nature tries to maintain equilibrium in all types of species. E.g. a salmon fish can produce 28 million eggs in a season. A pair of English sparrow can produce 275 billion offspring in 1 year. Thus, nature plays important role in maintaining balance in ecosystem.
Struggle for existence: according to this principle, overproduction causes competition among organisms for survival. This type of competition for survival is called struggle for existence. They can be categorized as:
Intra specific struggle: the struggle which takes place between individuals of same species is known as intra specific struggle.e.g. Struggle between tigers for food.
Inter specific struggle: the struggle which takes place between individuals of different species is known as inter specific struggle.e.g. Struggle between tiger and deer.
Environmental struggle: the struggle of organism against disaster, heat, cold etc is called Environmental struggle. E.g. struggle between fish and flood.
Heredity and variation: in nature every plant and animal shows variation and therefore, no two individuals are alike.darwin noticed two types of variations, useful and useless. He mentioned that individuals having useful variation have better chance of survival. Although he could not explain the cause of variation, he assumed it as an innate property of organism. He told that useful variation are inherited and also act as raw material for natural selection. e.g. Darwin noticed that the tortoises were clearly different from island to island even they had common ancestor. In isolation, each population had evolved its own distinctive features, yet all tortoises’ shows basic resemblances.
Natural selection: during struggle for existence only the successful variants survive and reproduce further while the rest are eliminated. This process is known as natural selection. In other words, nature selects the fittest or those individuals who are well adapted and eliminates those who are not well adapted. According to Darwin, two types of giraffe were present in ancient time; they were short necked and long necked. Nature selected long necked giraffe and rejected short necked giraffe. Dinosaur was extinted from earth because they could not adjust in changing environment and nature rejected them.
Origin of species: this is the final principle of Darwin’s theory. According to this principle, formation of new species takes place due to natural selection. Nature selects the organism with favorable characters and transmits character to next generation. After many generation, these characters are accumulated which leads to formation of new species. This process is called as speciation. Formation of various type of finch bird in Galapagos’s island is the example of it.
Drawbacks of Darwin’s theory:
Darwin believed that natural selection operates on small variation, which are heritable and lead to evolution after several generations. However these variations are rarely inherited. He did not mention the possibility of discontinuous variation, which are really responsible for origin of new species.
According to Darwin only useful organs are selected in natural selection. The vestigial organs are functionless, yet they are preserved generation after generation.
In certain animals, specialization occurs in some organs such as huge antlers of deer, tusk of elephant etc and have reached beyond the usefulness. These could not be explained on the basis of continuous variation and natural selection.
After Jurassic period dinosaurs were rejected and crocodiles were selected by nature although crocodiles didn’t develop any special features.
Darwin indirectly accepted lamarkism in inheritance of acquired characters.
Charles Darwin (1809-1882) was a famous English naturalist, who proposed that natural selection theory of evolution, he wrote a book “origin of species” in 1856ad. In that book, he explained about the cause of evolution. The principles of Darwin’s theory are:-
Overproduction
Struggle for existence
Heredity and variation
Natural selection/survival of fittest
Origin of species
Overproduction: according to this principle, each and every organism wants to produce maximum number of their off springs. Though they want to produce maximum number of offsprings to continue their generation, nature always maintains it. Nature tries to maintain equilibrium in all types of species. E.g. a salmon fish can produce 28 million eggs in a season. A pair of English sparrow can produce 275 billion offspring in 1 year. Thus, nature plays important role in maintaining balance in ecosystem.
Struggle for existence: according to this principle, overproduction causes competition among organisms for survival. This type of competition for survival is called struggle for existence. They can be categorized as:
Intra specific struggle: the struggle which takes place between individuals of same species is known as intra specific struggle.e.g. Struggle between tigers for food.
Inter specific struggle: the struggle which takes place between individuals of different species is known as inter specific struggle.e.g. Struggle between tiger and deer.
Environmental struggle: the struggle of organism against disaster, heat, cold etc is called Environmental struggle. E.g. struggle between fish and flood.
Heredity and variation: in nature every plant and animal shows variation and therefore, no two individuals are alike.darwin noticed two types of variations, useful and useless. He mentioned that individuals having useful variation have better chance of survival. Although he could not explain the cause of variation, he assumed it as an innate property of organism. He told that useful variation are inherited and also act as raw material for natural selection. e.g. Darwin noticed that the tortoises were clearly different from island to island even they had common ancestor. In isolation, each population had evolved its own distinctive features, yet all tortoises’ shows basic resemblances.
Natural selection: during struggle for existence only the successful variants survive and reproduce further while the rest are eliminated. This process is known as natural selection. In other words, nature selects the fittest or those individuals who are well adapted and eliminates those who are not well adapted. According to Darwin, two types of giraffe were present in ancient time; they were short necked and long necked. Nature selected long necked giraffe and rejected short necked giraffe. Dinosaur was extinted from earth because they could not adjust in changing environment and nature rejected them.
Origin of species: this is the final principle of Darwin’s theory. According to this principle, formation of new species takes place due to natural selection. Nature selects the organism with favorable characters and transmits character to next generation. After many generation, these characters are accumulated which leads to formation of new species. This process is called as speciation. Formation of various type of finch bird in Galapagos’s island is the example of it.
Drawbacks of Darwin’s theory:
Darwin believed that natural selection operates on small variation, which are heritable and lead to evolution after several generations. However these variations are rarely inherited. He did not mention the possibility of discontinuous variation, which are really responsible for origin of new species.
According to Darwin only useful organs are selected in natural selection. The vestigial organs are functionless, yet they are preserved generation after generation.
In certain animals, specialization occurs in some organs such as huge antlers of deer, tusk of elephant etc and have reached beyond the usefulness. These could not be explained on the basis of continuous variation and natural selection.
After Jurassic period dinosaurs were rejected and crocodiles were selected by nature although crocodiles didn’t develop any special features.
Darwin indirectly accepted lamarkism in inheritance of acquired characters.
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