This is given by a British biologist Charles Robert Darwin in 1859 AD. In this theory, Darwin explained about the origin of species by natural selections, this theory is also called, “Theory of Natural Selection”.
Facts of Darwinism:-
Darwinism is based on the following important/fundamental facts:
- Over population of off-springs:
Nature has provided an enormous power of reproductions or fertility, which can lead to over population of off-springs. This may cause rapid increase in organism number.
Salmon: A salmon produces 28 million eggs in one second. If all of them survive, a single species will occupy all the sea after few generations.
- Struggle for existence:
Due to increase in the number of organisms, there arises the scarcity of food and space. Similarly, organisms have to struggle for climate and breeding place.
Darwinism tell about the following struggles:-
- Intro-species: This may occur between the members of same species. For E.g. two dogs fighting for a meat-piece.
- Inter-species: This occurs between the members of two different species. E.g. cat chasing mouse and mouse being escaped.
- Environmental Struggle:
The organisms have to struggle against the environmental factors such as extreme heat or cold, lightning, earthquake, volcanic eruption etc.
- Natural Selection (Survival of the fittest):
Due to overpopulation, the scarce of food, shelter and breeding place takes place. In this course, organisms struggle against different factors in environment. Those who are best acclimatized in the environment will be selected by the nature and other become extinct soon.
Herbert Spencer coined this term for the first time and Darwin termed as natural selection.
- Variation and Heredity:
According to this, variation are of two types; useful and harmful variations. Harmful variation cause the organisms unfit in the struggle for existence and cause their elimination while useful variations make them adapted in the environment.
Variation are of 2 types:-
- Continuous: It is minute variation between generations to generations. For E.g. variation between two brothers etc.
- Discontinuous: It is the sudden variation which is very different, called as mutation. For E.g. Occurrence of five legs in a new calf.
- Origin of new species (Speciation):
In this course of struggle of existence and adaptation in the environment, the successful and advantageous variations are transmitted to the off-springs by the organisms. The changing environment causes variation in generations to generations over a long period of time, and finally results in the origin of new species.
Darwin’s theory of evolution is based on five key observations and inferences drawn from them. These observations and inferences have been summarized by the great biologist Ernst Mayr as follows:
|1) Species have great fertility. Individual multiply in geometric proportion.
2) Populations remain roughly the same size, with modest fluctuations.
3) Food resources are limited, but are relatively constant most of the time.
|From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals.
|4) In sexually reproducing species, generally no two individuals are identical. Variation is rampant. Much of this variation is heritable.
|From this it may be inferred: In a world of stable populations where each individual must struggle to survive, those with the “best” characteristics will be more likely to survive, and those desirable traits will be passed to their offspring ie survival of fittest.|
|5) Continuous change||These advantageous characteristics are inherited by following generations, becoming dominant among the population through time. This is natural selection. It may be further inferred that natural selection, if carried far enough, makes changes in a population, eventually leading to origin of new species.|
These observations have been amply demonstrated in biology, and even fossils demonstrate the veracity of these observations.
To summarise Darwin’s Theory of Evolution;
1. Variation: There is Variation in Every Population.
2. Competition: Organisms Compete for limited resources.
3. Offspring: Organisms produce more Offspring than can survive.
4. Genetics: Organisms pass Genetic traits on to their offspring.
5. Natural Selection: Those organisms with the Most Beneficial Traits are more likely to Survive and Reproduce.
Drawbacks Of Darwinism:-
- Darwin described about slow and small variations during organic evolution but he didn`t describe about usefulness of underdeveloped organs at initial stage.
- He didn`t mention abut use and disuse of organs and also about vestigial organs.
- Darwin didn`t mention about mutation which is major factor of organic evolution.
- Darwin described about survival of the fittest but not about the arrival of the fittest.
- Darwin didn`t explain (differentiate) somatic and germinal variation.
- Darwin did not describe about over specialization of certain organs which may lead to the extinction of organisms.
- The Pangenesis hypothesis of Darwin was kept in dark.
- No explanation of over specialisation.