Darwinism: The theory of Natural Selection


Charles Darwin proposed the theory of natural selection to explain the origin of species. This article highlights the main postulates of Darwinism. You will also find the criticisms of Darwinism and about main features of Neo-Darwinism.

Introduction

The theory of natural selection was proposed by Charles Robert Darwin to explain the origin of species through the process of organic evolution. He made an extensive study of nature over 20 years, especially in 1831-1836 when he went on a voyage on the famous ship H.M.S. Beagle. The H.M.S. Beagle steamed off from Davenport on 27 December 1831and during five years of it cruise, visited many islands of the Atlantic ocean, some coasts of South America and some islands of the South Pacific, of which the Galapagos, Archipelagos is the most important. He collected the information on animal distribution and the relationship between the living and extinct animals.

In 1959, Darwin published his famous book "The origin of species" in which Darwin stated that the origin of species takes place by means of natural selection. So, his theory is known as the theory of natural selection.

Postulates of Darwinism

Darwin's theory of natural selection was based on six facts. The main postulates of Darwinism are as below:

  • Overproduction or Enormous fertility: All the living beings in the absence of environmental checks, tends to increase in a geometrical manner. We can easily observe such a great reproductive potential of different species in nature. If a population of a given species doubles in one year and if there are n checks on its increase, it will quadruple the next year and so on. If this tendency of multiplication is allowed to go unchecked, in a very short time, the earth will be overcrowded with organisms.

  • Struggle for existence : Since individuals multiply in geometric ratio and space and food remain almost constant, there is an intense competition and threefold struggle for existence. The competition between living beings for basic needs like food, space, mate etc. is known as a struggle for existence. There are three types of struggle:

    1. Intraspecific: This kind of competition takes place between the members of same species. E.g., struggling of two lions for a piece of meat.

    2. Interspecific: The competition takes between the members of different species is called interspecific competition. E.g., competition between predator and prey.

    3. Environmental: It is the struggle of organisms with natural forces and environmental conditions, such as excess of moisture food or drought, excessive heat or cold, earthquakes, parasites, predators and disease-causing germs, etc. All these factors keep a check on the number of organisms.

    Out of these three types of competitions or struggles, the intraspecific struggle is the strongest type of struggle as the needs of the individuals of the same species are most common. In this struggle, the majority of individuals die before reaching sexual maturity and only a few individuals survive and reach the reproductive stage. Thus, the struggle for existence checks the over-increasing population of each species. Therefore, the number of offspring of each species remains nearly constant over a long period of time. Thus, the struggle for existence is a continuous process in nature.

  • Heredity and variations: Two individuals are not alike. There are always some differences between the parents and offspring or between the members of the same species. These differences are called variations. These variations either take place due to changes in genetic material or due to changes in environmental conditions. Due to these variations, some individuals would be better adjusted towards the surrounding environmental conditions than the others. The variations which help the organism to survive successfully in its environment reach in the offspring through inheritance, while the others disappear.

  • Survival of the fittest or Natural selection: According to Darwin, in the struggle for existence only those individuals survive which are with useful continuous variations in structure and functions and are best adapted to the environment while the others or unfit individuals are rejected by it and perish. Thus, nature exercises its selection and only those individuals who are fit survive and reproduce successfully. Darwin called this sorting out of the individual with useful variations from a heterogeneous population by nature as natural selection and Wallace called it as survival of the fittest.

  • Inheritance of useful variations: According to Darwin, the selected individuals transmit their useful continuous variations to the next generation. So that they can survive in the changing environment. The individuals of this generation again produce the same variations. Thus useful new variations are selected generation after generation under one environmental set and it's called continuous variation.

  • Origin of new species or speciation: As a result of the struggle for existence, variability and inheritance, the successful variations are transmitted to the succeeding generations. The successive generations in this way tend to become better adapted to their environment. As the environment changes, further adaptations occur. Useful variations appear in every generation and are inherited from one generation to the next generation.


  • Criticism of Darwinism

    Darwinism or theory of natural selection was so reasonable and well documented that it is accepted by most of the biologist s of the world. However, some criticisms given by different biologists are as below:

  • Darwin's theory of natural selection explains the inheritance of small variations which are to a large extent non-heritable and has no role in evolution. This theory does not explain the usefulness of an organ in the incipient stage which was thoroughly explained in Lamarckism.

  • Darwin's theory states that only useful organs are selected in the struggle for existence. But it fails to explain the presence of vestigial organs generation after generation, though they do not serve any function.

  • Darwinism remains concerned with the survival of the fittest but not for the arrival of the fittest.

  • This theory does not explain the overspecialization of organs. E.g., antlers in deer and tusks in an elephant.

  • Darwin's theory fails to explain the actual cause of sudden changes in the body. This is because of the lack of knowledge of heredity.

  • Darwinism also fails to explain the emergence of terrestrial animals from the aquatic forms.


  • Neo-Darwinism

    Darwin's theory of evolution has been modified in the light of modern pieces of evidence from genetics, molecular biology, palaeontology, ecology, and ethology. This modified theory is called Neo-Darwinism. The new theory is defined as the theory of organic evolution by the natural selection of inherited characteristics. This theory is based on the following three facts:

    1. That the evolution has taken place in the past - past evolution.

    2. That this demonstrates a mechanism which results in evolution – natural selection of genes.

    3. That this includes evolution taking place today – evolution in action.

    The theory of New-Darwinism is also not fool-proof. Thus there is still a debate, not so much about whether evolution takes place, but about how it takes place by natural selection.


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