In this article we will discuss about the Darwinism theory of evolution with its criticism.
In 1831, Charles Darwin on a voyage on HMS Beagle for five years noted the flora, fauna and geology of the islands of the South Pacific and collected numerous living and fossil specimens. He also sailed to the Galapagos Islands about 600 miles from the west coast of America (Fig. 7a and b).
He observed a number of variations or differences among the organisms that lived on these islands. The common birds of the Galapagos Islands were the finches that were remarkably different from the finches of the mainland. These closely related species of finches had beaks of different shapes and sizes and were adapted for feeding on completely different diets.
In 1838, Darwin read an essay on ‘The Principles of Population’ by Malthus who explained that the rate of reproduction in animal was very rapid and that animal population increases more rapidly than the available food supply. The food supply increases in arithmetic ratio while the population increases in geometric ratio.
Malthus noted that the human population was capable of doubling every 25 years. This increase in population would soon outstrip the food supply, leading to starvation, famine and war, which would ultimately reduce the population.
At the same time, Alfred Wallace, a young English naturalist made similar observations to Darwin. Wallace and Darwin adapted Malthus ideas about how scarce resources could affect populations. Darwin put forth all these ideas in the Journal of Proceedings of Linnean Society in 1859. Darwin also published his observations in a book titled the “The Origin of Species by Natural Selection”. Darwinism is the term coined for the explanation offered by Darwin for the origin of species.
Origin of Species by Natural Selection or Theory of Natural Selection:
The main points of the Theory of Natural Selection are as follows:
a. Over Production or Enormous Fertility:
Living organisms have an innate capacity to produce more individuals to ensure continuity of the race. For example, an oyster may produce over 60-80 million eggs per year. A rabbit produces six young ones in a litter and four litters in a year and the young rabbit becomes reproductively active in six months from birth. A single female salmon produces 28,000,000 eggs in a season.
b. Struggle for Existence:
Organisms multiply in a geometric ratio, while the food supply increases in an arithmetic ratio. This leads to intense competition between organisms to ensure living to obtain maximum amount of food and shelter.
Struggle exists at three levels:
i. Intraspecific struggle is the competition among individuals of the same species or closely related forms. This type of struggle is very severe as the need of the population is the same.
ii. Interspecific struggle is the struggle between organisms of different species living together. Individuals of one species compete with other species for similar requirements.
iii. Struggle with the environment means the various hazards of the nature like extreme heat or cold, excess moisture or drought, storms, earthquakes, volcanoes eruptions, etc. also affect the survival of various organisms.
c. Variations amongst Organisms:
Differences that exist among organisms are called variations. Variations may be harmful, neutral or useful. Variations that are passed on from generation to generation are called heritable variations and these form the raw material for evolution. These variations arise due to changes in the genes or the chromosomes.
d. Survival of the Fittest:
During the struggle for existence, the individuals that exhibit variations beneficial in facing the environment will survive, while those that cannot face the hardship will be eliminated. Those organisms best able to survive and reproduce will leave more offspring than those unsuccessful individuals. This is referred to as survival of the fittest.
According to Darwin, the giraffe exhibited variations in the length of the neck and legs. When the grass on the ground became scarce, giraffes with long necks and legs had an advantage over those with shorter neck and legs, as they could feed on the tall trees. So these forms survived and reproduced and became abundant. Over a period of time, giraffes with short necks starved and became extinct (Fig. 8).
e. Origin of Species:
As a result of struggle for existence, variability and inheritance, individuals that are better adapted, survived and became abundant. Slowly over a period of time, this group, which was remarkably different from the original population, becomes established as a new species. This group is also subject to the same forces of change as their ancestors were and this process continues to give rise to new species.
Members of this group may possess variations that may be beneficial to them in another environment. As a result, two or more species may arise from a single ancestral species. Over many generations, unequal reproduction among individuals with different genetic traits changes the overall genetic composition of the population. This is evolution by natural selection. This mechanism can cause a population to change so much, that it becomes a new species. This is known as speciation.
But neither Darwin nor Wallace could explain how the process of evolution occurred; how did the inheritable traits, i.e. variations pass on to the next generation? This was because of the fact that during this period, no one knew anything about genetics. During the twentieth century, genetics provided that answer, and was linked to evolution in Neo-Darwinism, also known as Modern Synthesis.
Criticism of Darwinism:
The following points have been raised against the theory of natural selection:
a. Darwin was unable to explain the mechanism of inheritance of characters. Darwin proposed the theory of pangenesis to explain this phenomenon. He said that every cell or organ produces minute hereditary particles called pangenes or gemmules. These were carried through the blood and deposited in the gametes. This theory was not accepted.
b. According to natural selection, only useful organs are favoured by natural selection. The existence of vestigial organs in organisms could not be explained.
c. In some species of deer, the antlers develop beyond the stage of usefulness. These structures are of no functional significance to the animal.
d. Darwin was unable to explain the source of variations in organisms.
Artificial selection is the isolation of natural population and the selective breeding of organisms with characteristic which are useful to humans. In this method, human exert a directional selection pressure that leads to changes in allele and genotype frequencies within the population. This is an evolutionary mechanism which gives rise to new breeds, strains, varieties, races and subspecies.
Darwin studied domestication in plants and animals in detail. He concluded that by artificial selection different varieties of plants and animals could be produced. Cabbage, cauliflower, broccoli, kale, kohlrabi, brussels sprouts could be produced from the common wild mustard (Fig. 9). Similarly, Darwin also raised several types of pigeons from the rock pigeon by artificial selection.
Similarly, the various breeds of fowl are all derived from the jungle fowl, Gallus gallus. Artificial selection has been used by breeders to produce high yielding cows, the Great Dane dog, the Shetland pony, the sleek Arabian horse, etc. The rate of species formation by artificial selection is fast. The analogous process that occurs in nature is natural selection, which proceeds very slowly.