Notes on Darwin’s Theory of Natural Selection of Evolution !
In 1831 Darwin got an opportunity to travel on H.M.S. Beagle (a ship in which Charles Darwin sailed around the world) for a voyage of world exploration.
The voyage lasted for five years (1831-1836). During that period Darwin explored the fauna and Flora of a number of continents and islands. Later Beagle was sailed to the Galapagos Islands. Galapagos Islands consist of 14 main islands and numerous smaller islands which lie on the equator about 960 Km off the West Coast of South America in the Pacific Ocean. These islands are volcanic in origin and are called “a living laboratory of evolution. Darwin visited these islands in 1835 and spent a month there. He observed great variations among the organisms that lived on these islands.
Darwin noticed giant tortoises, (Sp.: galapago- old spanish name for tortoise), metre-long marine and land iguanas, many unusual plants, insects, lizards, sea shells and birds on Galapagos islands. These giant tortoises may weigh as much as 275 kg, grow to 183 cm in length and attain an age of 200 to 250 years.
The Spanish word for tortoise, galapago, gives the islands their name. Birds of Galapagos Islands influenced Darwin to think about the evolutionary change. These birds were called finches. Finches were designated as Darwin’s finches by Dr David Lack (1947).
In 1798 T.R. Malthus, a British economist, put forward a theory of human population growth:
(i) He stated that population grows geometrically when unchecked, whereas the means of its subsistence like food grow only arithmetically,
(ii) Naturally, after some time an imbalance would occur in the population and the environment,
(iii) When the imbalance reaches a certain value, some factors like hunger, epidemics, floods, earthquakes, war. etc. will bring the population to a desired level. Such a population “crash” is called catastrophic control of population. These factors were called “Positive checks” by Malthus.
Darwin noticed the conflict between resources of population and its continued reproductive pressure. Darwin considered that like in humans, competition exists among all living things. Thus Darwin was much influenced by Malthus Theory of human population growth.
Darwin came to know that humans have been modifying wild plants and animals to suit their requirements. The breeders have successfully produced cows that give increased amount of milk. He also observed that humans have perfected the toy-like Shetland pony, the Great Dane dog, the sleek Arabian race horse and many cultivated crops and ornamental plants. Many crop plants like broccoli, cabbage, cauliflower (Fig. 7.47), etc. have also been produced through selective breeding.
While Darwin was busy in formulating his theory of natural selection, he received a brief essay from Alfred Wallace in June 1858. Alfred Wallace (1823-1913), a naturalist from Dutch East Indies was working on Malay Archipelago (present Indonesia). The essay was titled “On the Tendencies of varieties to Depart Indefinitely from the original type”. The thinking of both Darwin and Wallace in respect of organic evolution was similar.
Finally in November 1859 Darwin published his observations and conclusion in the form of book. The full title of his book was on the origin of species by means of Natural Selection.: The Preservation of Races in the Struggle for life. Actually Darwin gave brief description of origin of species; however he described in detail how populations become well adapted to their environments through natural selection.
Charles Robert Darwin returned to England in October 1836 from his 5-year expedition. In 1838 he came across with a book An Essay on the Principles of Population written by Thomas Robert Malthus (1766-1834) which was published in 1799. In 1798 T.R. Malthus, a British economist, put forward a theory of human population growth. Darwin was influenced by Malthus’s theory of human population growth.
The Principle of Natural Selection:
The principle of natural selection stems from five important observations and three inferences (Ernst Mayr 1982) which have been mentioned below.
Natural selection is differential success in reproduction and its product is adaptation of organisms to their environment. Thus natural selection occurs through an interaction between the environment and the variability inherent in the population.
Salient Features of Darwin’s Theory of Natural Selection:
The main features of the theory of Natural Selection are as follows:
1. over production (Rapid Multiplication):
All organisms possess enormous fertility. They multiply in geometric ratio. Some examples are cited below:
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 produces four litters in a year. Six- month-old rabbit is capable of reproduction. If all the rabbits survived and multiplied at this rate, their number would be very large after some time.
Each pair of mice produces dozens of young ones. It is assumed that elephant is the slowest breeder, which matures at the age of 30 years and lives for about 90 years. Each female gives rise to about six offspring.
Thus some organisms (living beings) produce more offspring and others produce fewer offspring. This is called differential reproduction.
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:
The struggle for existence can be of three types.
(i) Intraspecific Struggle:
It is the struggle between the individuals of the same species because their requirements like food, shelter, breeding places, etc. are similar. Many human wars are the examples of intraspecific struggle. Cannibalism (eating the individuals of its own species) is another example of this type of struggle.
(ii) Interspecific Struggle:
It is the struggle between the members of different species. This struggle is normally for food and shelter. For example, a fox hunts out a rabbit, while the fox is preyed upon by a tiger.
(iii) Environmental Struggle:
It is the struggle between the organisms and the environmental factors, such as drought, heavy rains, extreme heat or cold, earthquakes, diseases, etc. Thus, climate and other natural factors also help in restricting the number of individuals of particular species.
4. Appearance of Variations:
Except the identical twins, no two individuals are similar and their requirements are also not exactly the same. It means there are differences among the individuals. These differences are called variations. Due to the variations some individuals would be better adjusted towards the surroundings than the others.
Adaptive modifications are caused through the struggle for existence. According to Darwin, the variations are gradual (continuous) and those which are helpful in the adaptations of an organism towards its surroundings would be passed on to the next generation, while the others disappear.
5. Natural Selection or Survival of the Fittest:
The organisms which are provided with favourable variations would survive, because they are the fittest to face their surroundings, while the unfit are destroyed. Originally it was an idea of Herbert Spencer (1820-1903) who used the phrase ‘the survival of the fittest’ first time. While Darwin named it as natural selection.
It is to be noted that only survival of the fittest is not enough. But organisms should also adapt or change themselves according to the changed conditions of the environment as environment is always changing. To explain the phenomenon of survival of the fittest, the extinct reptiles can be cited as an example. During the evolution of reptiles, giant reptiles, the dinosaurs etc., appeared.
Majority of them were herbivorous, but due to certain climatic changes, the vegetation disappeared and, therefore, most of them became extinct. However, small animals who could change their feeding habits from herbivorous to carnivorous diet survived, because they could easily get adapted to the changed environment.
These, therefore, will survive more and hence are selected by nature. Darwin called it natural selection and implied it as a mechanism of evolution. Alfred Wallace a naturalist who worked in Malay Archepelago had also come to similar conclusions around the same time.
6. Inheritance of useful variations:
The organisms after getting fitted to the surroundings transmit their useful variations to the next generation, while the non-useful variations are eliminated. Darwin could not differentiate between continuous and discontinuous variations. In this respect, Darwin agreed with Lamarck’s views, because according to Darwin acquired characters which are useful to the possessor could be inherited.
7. Speciation (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.
Weakness of Darwinism:
Darwin was unable to explain the basis of variation and the mode of transmission of the variants to the next generation. In 1868, Darwin proposed the Theory of Pangenesis to explain the mechanism of inheritance. According to this theory, every organ of the body produce minute hereditary particles, pangenes or gemmules, for example, heart gemmules from heart, liver gemmules from liver, leg gemmules from the leg and so on.
He considered that the gemmules were carried through blood from every organ of the body and were collected into the gametes. However August Weismann’s Theory of Continuity of Germplasm refuted the theory of Pangenesis. The theory of continuity of germplasm has been described in the criticism of Lamarckism.
Criticism of the Natural Selection Theory:
(Objections against the Natural Selection Theory):
1. Inheritance of Small Variations:
According to natural selection theory only useful variations are transmitted to the next generation, but sometimes small variations which are not useful for the possessor, are also inherited. It is beyond understanding that if the appearance of small wings in birds could help them in flying.
2. Over-Specialization of Some Organs:
Some organs like tusks of elephants, antlers of deer have developed so much that instead of providing usefulness to the possessor, they often give hindrance to them. This theory cannot explain these facts.
3. Vestigial Organs:
Why vestigial organs are present in some animals when they have no function? According to the Natural Selection Theory, vestigial organs should not be present.
4. Arrival of the Fittest:
The theory only explains the survival of the fittest but, is unable to explain the arrival of the fittest.
5. Degeneration of Organs:
The theory does not account for the degeneration of certain organs in animals.
6. Discontinuous Variations:
The theory fails to explain the cause of sudden changes in the body. The main drawback of Darwin Is theory was lack of the knowledge of heredity and that is why he could not explain that how the variations are caused.
Darwin himself was conscious of the inadequacies of his theory, when he remarked that, “I am convinced that natural selection has been the most important but not the exclusive means of modifications.”
Evidences in Favour of Natural Selection:
1. Rate of Reproduction:
Rate of reproduction is many times higher than the rate of survival in all organisms.
2. Limitation of Resources:
Food, space and other resources are limited.
3. Struggle for Existence:
Competition or struggle for existence is seen in all organisms.
4. Abundance of Variations:
Variations are so abundant in nature that no two individuals of a species are similar, not even the monozygotic twins (they possess some dissimilarities due to their environment).
5. Production of New Varieties of Plants and Animals by Artificial Selection:
When man can produce various new varieties of plants and animals in a short period, nature with its vast resources and long time at its disposal can easily produce new species by selection.
6. Mimicry and Protective Colouration:
They are found in certain animals and are products of natural selection.
7. Correlation between Nectaries of Flowers and Proboscis of Insects (Entomophily):
The position of nectary in a flower and the length of proboscis in pollinating insects are wonderfully correlated.
8. Pedigrees of Some Animals:
Pedigrees of horses, camels and elephants also support the Natural Selection Theory.
Man has been taking the advantage of genetic variations for improving the qualities of domesticated plants and animals. He selects the individuals with desired characters and separates them from those which do not have such characters. The selected individuals are interbred. This process is termed as artificial selection.
Thus this process of selection is done through the agency of man or it is manmade. If it is repeated for many generations it produces a new breed with desired characters. If cows with high milk yield are desired, the animal breeders select those cows which produce a large quantity of milk.
The calves of high milk-yielding cows are interbred to get the new generation of calves. After repeating this process for a number of generations, a breed of high milk-yielding cows is produced.
By artificial selection animal breeders are able to produce improved varieties of different kinds of domestic animals (i.e., dogs, horses, pigeons, poultry, cows, goats, sheep and pigs) from their wild ancestors.
Similarly, the plant breeders have obtained improved varieties of useful plants such as wheat, rice, sugarcane, cotton, pulses, vegetables, fruits, etc. Artificial selection is similar to natural selection except that the role of nature is taken over by man and the characters selected are of human use.