Introduction:
Charles Robert Darwin (1809-1882), an English naturalist publishes his famous book “The Origin of Species by Natural Selection” in 1859. Darwin’s theory of natural selection is based on a mass of accurate observations and prolonged experimental led the whole scientific world to believe on the doctrine of evolution. Darwinism is based on several facts on observation.
Facts of Darwinism:
Enormous fertility/Overproduction
Every organism has a remarkable capacity to reproduce. Such a great reproductive potential in various species may be easily observed in nature. Some examples are;
Housefly: In optimal conditions, a housefly can lay around six batches of eggs. Each batch has 120 to 140 eggs. The eggs are hatched in 0 days and became mature in 14 days. If all houseflies survived. Darwin’s calculated 191,000,000,000,000,000,000 progenies in one year.
Paramecium: Paramecium divides three times in 24 hours.
Oyster: An oyster lays approximately 80 million eggs each year.
Salmon: A Salmon produces 28 million eggs in one season. If all of them survived and continued to reproduce, a single salmon species will occupy all the sea after few generations.
Elephants: Elephants is a slowest breeding animal. They have life span of 100 years. During, their span a female may give rise to 6 young. If all these young survived and continue to reproduce. Darwin calculated in 750 years; descents form a single pair would be 19,000,000 elephants.
Struggle for existence
Due to excessive rate of multiplication and limited resources, the organism struggle with each other for food, space light, water, etc. The competition amongst organism for obtaining their essential requirements is called struggle for existence, Darwin’s observed three types of struggles for existence.
Intraspecific: It is competition among the individual of same species. Intraspecific struggle is the keenest because individual of the same species has similar requirements and similar types of characters.
Interspecific: It is competition among individual of two different species. Struggle may be for similar requirements as in case of plants of different species growing in same area or herbivorous of different types of feeding on similar types of plants. Prey-predator relationship is also example of interspecific struggle.
Environmental struggle: It is struggle with the natural calamities and environmental disasters such as excess of moisture or draught, excessive heat and cold, flood, earthquakes, volcanoes, etc.
Survival of fittest
As a result of this everlasting competition or struggle among the organism, the stronger win and survive, while weaker are rooted out. Only those individuals survive which are at selective advantages due to the presence of useful variations. The individual with favorable variations has a better chance of survival in the competition but others die. Nature is the super power that select best fitted individuals and rejects those are poorly developed and thus they usually die. This process is known as natural selection.
Variation and heredity
The everlasting competition among organisms has compelled them to changes according to the conditions so that they can utilize the natural resources and can survive successfully. This change is called variation.
Darwin noticed two types of variations: harmful or usefulness and useful. Harmful or useless variations make the individuals unit for struggle for existence whereas the useful variations are quite significant and make the organisms fit the nature. Such variations are transmitted (inherited) to the next generation so that the progenies are more suited or better chances of survival.
Origin of species
The surviving individuals selected by nature are the fittest and possess the maximum number of useful variations. They interbreed and passed on their useful variations to the progeny. The progeny will further show variations. Due to continuous variation, organisms became better adapted to their environment. The variation gets inherited to its progeny. Progeny also preserved or accumulated these changes or adaptations. After successive generation, the organisms became different from its parent. As a result, new species is established which was quite distinct from ancestors.
Objections to Darwin’s theory of natural selection:
- Darwin’s theory stressed upon small variations but it did not explain the usefulness of an organ in incipient stage (underdeveloped organ).
- It did not explain the effect of use and disuses of organs and the presences of vestigial organs.
- He did not differentiate between somatic and germinal variations.
- Darwin’s explained the survival of fittest but not the arrival of the fittest.
- Natural selection did not explain the over specialization of particular structure.
- The pangenesis hypothesis of Darwin was kept in dark.