Updated by: Arabinda Naik
Darwin theory of evolution(Darwinism): The theory of Darwin is known as Origin of species by natural selection or Theory of Natural selection or Descent with modification and is also known as Darwinism. This theory is based upon the following observations from which certain logical deductions have been made.
Observations Deductions
Rapid Multiplication Number of survivors remain constant
Limited food and space Struggle for Existance
Struggle for existence variations Survival of the fittest or Natural selection
Survival of the fittest Inheritance of useful variations over many generations Formation of new species
1. Over production: (Rapid multiplication): All organisms possess enormous fertility. They multiply in geometric ratio. For continuity of race. Ex. Insects lay hundreds of eggs. A cod fish lays several hundred eggs at a time. A female rabbit gives birth to six young ones in one litter and produce four litters in a year. If all the rabbits survived and multiplied at this rate their number would be very large after some time. The elephant will produce 19000000 elephant in a period of 750 years. But we observed that overcrowding never takes place. The population of each species remain constant. This is because a large number of offspring perish before attending adulthood.
2. Limited food and space: Despite of rapid multiplication of all types of species food and space and other resources remain limited. They are not liable to increase.
3. Struggle for existence: Because of excessive multiplication by the parents and limited space and food supply there starts a severe competition among the offspring for their requirements. Every individual makes efforts for fulfilment of its basic need as space, food , mate and protection from enemies. This competition for the primary necessities of life is called struggle for existence. The struggle continues from zygote stage to death. Large number of individuals are destroyed in the struggle. The active competition or struggle for maximum amount of food and better shelter takes place in the following three areas.
I) Interspecific: Struggle between organisms of different species living together for food and space. A tiger struggles to kill a deer for food and deer struggle to escape from tiger.
II) Intraspecific: Struggle between individuals of the same species for food, habitat and mating partner.
III)Environmental: Struggle between organisms and the environmental conditions such as excess of rain, drought, excessive heat or cold, natural calamities like earthquake, cyclone, lighting, volcanic eruption, flood, pollution results in death of large number of higher plants and animals.
4. Variations: No two individuals are similar. These difference are called variations. Due to the variations some individuals would be better adjusted towards the surrounding than the others. The variations are gradual and which are helpful in the adaptations of an organisms towards its surrounding would be passed on to next generation while the others disappear.
5. Natural selection (survival of the fittest): In the struggle for existence the individuals which have more favourable variations will enjoy a competitive advantage over others and will survive and reproduce. Ex. During a severe drought the plants which can maximum absorption of water and can minimise its water loss have a greater chance of survival than the others. The fast running rabbits are more likely survive a chase by a fox than the slow ones. The competition is so severe that only few individuals survive out of millions. This individuals with useful variations has been called natural selection by Darwin and survival of the fittest by the Herbert Spencer. The individuals which survive in the struggle for existence grow into adults and produce young ones.
6. Inheritance of useful variations: The organisms after getting fitted to the surrounding transmit their useful variations to the next generation, while the non-useful variations are eliminated. Darwin could not differentiate between continuous and discontinuous variations. He agreed with Lamarck’s views. Because according to Darwin acquired characters which are useful to the possessor could be inherited.
7. Formation of new species: Darwin considered that useful variations are transmitted to the offspring and appear more prominently in succeeding generations. After some generations these continuous and gradual variations in the possessor would be so distinct that they form a new species.
Objections against natural selection
Darwin’s theory received widespread approval from 1859-1900. This period is known as romantic period of Darwinism. In 1900 Mendel’s principle of heredity came into light. Since then the validity of Darwinism was subjected to serious doubts and criticism.
1. Arrival of the Fittest: Natural selection explain the survival of fittest. But it fails to explain the arrival of the fittest.
2. Variation : This theory is based upon small variations but does not explain the usefulness of an organ in its initial stage.
3. Vestigial organs: A member of vestigial organs are found in organism. There are 180 vestigial organs in humans. Natural selection operates only for useful traits. It cannot allow the functionless vestigial organs to exist generation after generation.
4. The theory did not differentiate between somatic and germinal variations.
5. Over specialisation: Overspecialisation organs are those which have grown maximum usefulness. They have become a hindrance .Ex. branched antlers of Deer, tusks of elephant. Such organs should not be selected beyond the level of optimum usefulness.
6. Pangenesis: Darwin 1868 put formed a theory of inheritance called theory of pangenesis. According to this theory every organ of the body produces minute heredity particles called pangenes. There will be liver pangenes, leg pangenes, hand pangenes , eye pangenes. Pangenes of different body organs are passed into blood which carries them to gametes. Theory of pangenesis was disapproved by theory of germplasm.
7. New characters: Darwinism could not explain the origin of new characters. It only provides for a mechanism of modification.
8. Unchanged form: Natural selection believes that variations occurs by passing characters into a new species after several generations. Some organism to have remained unchanged for past several million years. Ex.Selaginella. The occurrence of such unchanged form cannot be explained by Darwinism.