Darwinism or Origin of Species by Natural Selection or Theory of Natural Selection
The theory of evolution proposed by Charles Darwin (1809-1882) is commonly known as Darwinism. With adequate evidence Darwin proved that species evolved from previously existing species; that means evolution had taken place. Secondly, he explained the mechanism of evolution by presenting a theory, known as the theory of natural selection.
Darwin traveled around the world on the ship H.M.S Beagle with a surveying expedition for 5 years (1831-1836). He not only observed events but also collected materials of varied natures based on which he developed his theory of evolution.
In Brazil, Darwin marked that certain animal species of the southern part were slightly different from those of the northern part. The differences were so small that they did not form different species but could be called local varieties. In Argentina, Darwin found fossils of huge creatures, the features of which resembled those of the living armadillos of that region. From this and other similar evidence he concluded that new species evolved from older species and therefore, both had some common characteristics.
At a distance of about 900 kilometers west of Ecuador lies the Galapagos archipelago, from where he collected specimens of plants, animals, insects, and reptiles. But the most important thing to which his attention was drawn was that the finches inhabiting the islands differed greatly in the structure of their beaks. Some had stout beaks, others had small beaks; in some, the beaks were straight, in others those were parrot-like. Slender beaks were suited for eating small insects, large ones were for eating large seeds, while parrot-like was for eating fruits, buds, etc. As regards slight differences among certain animal species in Brazil as well as in other places. Darwin thought that those were perhaps because of variations in climate and natural surroundings. But in the islands of the Galapagos archipelago, there was no climatic variation and the other natural conditions were more or less the same. How do account for this variation? At that time he had no answer.
Darwin had to work for about twenty years after his return from the voyage to develop his idea of evolution and also the mechanism involved in the process of evolution. During that period he observed more facts, collected more information, and studied those from various angles. His studies on domesticated plants and animals gave him some clues. He observed that to get the desired type of pigeons, the breeders allowed mating among the selected pigeons only, and thereby they developed a new strain, which had demand in the market. Through this process, the breeders gradually eliminated the undesired type and increased the number of the new variety, which after several generations showed variation from the ancestral form. Darwin gave serious thought to it. If the breeders of pigeons could develop a new variety, then why not nature could bring more remarkable changes? The long-continued process through different geological periods will be more effective. This led him to conclude that in domesticated as well as in natural populations individual variation occurred, and that variation ultimately caused evolutionary changes. In the case of pigeons, the selection was done artificially by the breeders, whereas in the case of natural populations some natural selective forces encouraged certain members of a species to reproduce more and discouraged others to do the same. This is what is called natural selection.
Darwin was highly impressed by the writings of an economist, Thomas Malthus (1766-1834), more particularly by his “Essay on Population” where he foresaw a “struggle for existence” among future humans, because of their ever-growing numbers with a limited food supply. Darwin applied this principle to nonhuman populations. He observed that in natural conditions most organisms tend to increase their numbers. Because every breeding pair did not reproduce only two to replace them, but many more. However, all the offspring did not survive. Some were killed by predators, some died because of inborn fatal disorders, some could not survive till the time of reproduction because of their inability to procure food in the world of competition, where resources were limited, and the like. He noted that in each generation the number of those who were better-adopted slightly increased, because of selective pressure. Individuals with unfavorable characteristics were gradually eliminated and they were replaced by favored individuals. The favorable traits were passed on gradually from generation to generation. After several generations, those favorable traits became the common characteristics of the serving individuals. This process produces major evolutionary change.
Alfred Wallace (1823-1913) was also a widely-traveled person. He was a naturalist from Dutch East Indies, working in the Malay Archipelago, now Indonesia. He studied the flora and fauna of South America and South-East Asia. He also got an evolutionary idea and expressed it in his essay “On the Tendency of Varieties to Depart Indefinitely from the Original Type“.
Most biologists call the theory of natural selection, the Darwin-Wallace theory. But since Darwin’s thesis was much better documented, the theory is often known as Darwinism. It was later explained by Darwin in his monumental book “On the Origin of Species by Means of Natural Selection” in November 1859. Darwin died on April 19, 1882.
The publication of Darwin’s Book raised controversy. The concept of evolution was opposed primarily on ethical and religious grounds. However, Darwin’s logic finally convinced the people.
Factors of Darwinism Theory:
Darwinism theory was based on several facts, observations, and influences. They are:
Overproduction or Prodigality of production:
All living beings have an inherent tendency to produce offspring of their own kind in large numbers for the perpetuation of their race. This is called geometric increase. The number of their offspring is much more than can be supported by a particular environment and can possibly survive. For example-
- Paramecium- The animal divides every 16 hours. If all its offspring survive and multiply, in five years the mass of paramecia will be equal to ten thousand times the mass of earth.
- Cod Fish- A codfish produces one million eggs in one year. If all the egg hatch, within five years the descendants of a single fish will have a volume that would fill the entire Atlantic Ocean.
- Oyster- Oyster lays about 1,14,000,000 eggs in a single spawn. If all the eggs develop and young ones survived, after five generations, there would be more oysters than the estimated number of electrons in the visible universe.
- Rabbit- A rabbit produces about 6 young ones in a litter (at one time) and four litters per year and the young ones start reproducing at the age of 6 months.
- Elephant- The elephants are the slowest breeders. An elephant begins to breed at 30 years of age. It goes on breeding till it is 90 years old. A female elephant can give birth to 6 young ones during its lifetime. Under these conditions, a pair of male and female elephants can cause the production of 19 million elephants in 750 years.
- Evening Primrose- A single plant produces 118,000 seeds. If all of them are to produce plants, within five generations the population of Evening Primrose will be 14 x 1080.
Limited Environmental Resources (Space and Food):
Earth has limited space. So is the availability of food. Each ecosystem has a limited carrying capacity for its different members depending upon the number of producers it support. Carrying capacity is maintained by food chains and Biogeochemical cycling.
Struggle for existence:
Because of excessive multiplication by the parents and limited space and food supply, there starts severe competition among the offspring for their requirements. Every individual makes efforts to fulfillment of its basic needs, namely suitable space to live, food to eat, mate to reproduce, and protection from enemies. This competition for the primary necessities of life is called the struggle for existence. The struggle for existence is three-fold for every individual:
- Intraspecific Struggle- It is competition amongst members of the same species for obtaining optimum amounts of their requirements of food, shelter, mate, water, light, etc. This is the keenest form of struggle as the needs of the individuals of the same species are identical. An extreme example of an intraspecific struggle is cannibalism. War is also a similar struggle.
- Interspecific Struggle- It is competition between members of different species. The struggle may be for similar requirements as in the case of plants of different species growing in an area or herbivores of different types feeding on similar types of plants. Prey-predator relationship is also an example of an interspecific struggle. It is important for maintaining the population of different species at their optimum level.
- Environmental Struggle- Living organisms also struggle with adverse environmental conditions, i.e., excess moisture, drought, heat, cold and also lightning, volcanic eruptions, earthquakes, etc. An interesting example of this is the reptiles of the Mesozoic era. These reptiles were gigantic and lead a very successful life in that period but subsequently, they could not face the changing environmental conditions and perished.
The offspring of the same parents also differ from one another and show variations. In addition to visible phenotypic variations, hidden genetic variations also exist. In a population, some variability is always present for almost every characteristic. The variations are very important for evolution as some of these may help the individuals to survive in a certain type of environment while others may not.
Survival of the Fittest or Natural Selection:
Darwin believed that as in artificial selection man selects animals or plants with the desired character, the same way nature selects only those individuals which are with more favorable variations and are best adapted to the environment. The less fit and unfit organisms are left out by selection. This sorting out of individuals with useful variations was called natural selection by Darwin and survival of the fittest by Wallace. Individuals with favorable variations survive, reach adulthood and reproduce offspring, others fail to do so. This is called differential reproduction. Darwin has called this differential reproduction as natural selection.
Darwin strongly believed that using the natural selection concept, all evolutionary processes in the living world can be explained. As an example, he differed from the explanation provided by Lamarck while explaining the lengthening of the neck in giraffes. According to Darwin, the population of giraffes had individuals having varying neck lengths. Those that had longer necks had more survival value since they had more food and remained healthy. Gradually natural selection encouraged them to survive. Thus in course of time, the average length of the neck increased.
Inheritance of Useful Variation:
The individuals, after their selection by nature in the struggle for existence, pass on their useful variations to the next generation. Thus, the offspring of the fit individuals will also be fit. Darwin did not differentiate between continuous and discontinuous variations. He held that any variation, which is favorable to its possessor, could be inherited. His inheritable variations included even the acquired characters. In this respect, Darwin agreed with Lamarck.
Formation of New Species:
Due to the above facts, Darwin concluded that the struggle for existence leading to the survival of the fittest tends the successive generations to become better adapted to their environment. So, all the modifications produced by variations and selected by natural selection are collected from generation to generation till finally a generation is produced which is much more adapted than the previous ones, and has, therefore, much more chances of survival. However, this gradual improvement of organism generation after generation leads to the emergence of such organisms which are quite different and better organized than their remote ancestors. Thus, these forms are identified as new species.
However, Darwinism can be summed up with the help of the following chart as devised by Alfred Wallace.
Criticism of Darwinism:
Although Darwinism can explain the mechanism of evolution to some extent, most scientists do not agree with the view of Darwin. There are some views in its favor and some views against it.
The facts supporting Darwinism are as follows:
- By artificial selection, it is possible to produce various domesticated varieties.
- The number of ancient animals decreased due to changes in the environment. The animals of the present day have some favorable characteristics which helped them to survive in changing environments and unfavorable surroundings.
- The evolution of horses provides strong support for Darwinism. Modern horses evolved as a result of modifications of their legs and teeth due to changes in the environment. The evolution of cows, elephants, deer, etc. also suggests strong proof of Darwinism.
- When black, white, and checkered-colored chickens were allowed to move freely in a field, it was found that hawks spread the checkered-colored chickens as their color matched with the background. This experiment was performed to support Darwinism by Davenport.
Facts disapproving Darwinism are as follows:
- Darwin gave importance to slow and small variations but could not explain the usefulness of the organs in the incipient stage (organs not yet perfectly developed).
- He was silent over the use and disuse of organs and the presence of vestigial organs.
- Darwin explained the survival of the fittest but not the arrival of the fittest.
- He did not differentiate between the stomatal and germinal variations and said both were heritable, which is not true.
- How some species when crossed produce sterile off-springs whereas when varieties are crossed fertile off-springs are produced.
- His theory could not explain the origin of coordinated structures to produce the huge antlers of the Irish Elk (deer). These antlers are hindrance rather than a help.
- Many differences seen in the species are not at the adaptative value, but are the result of the incidental effect of genes. How and why should these be inherited, Darwin could not explain.
- In order to explain the transmission of variations from one generation to the next, Darwin (1868) put forward a theory of inheritance, called the theory of pangenesis. According to this theory, every organ of the body produces minute hereditary particles called gemmules or pangenes, and so forth. Pangenes or gemmules of different body organs are passed into the blood which carries them to gametes. The theory of pangenesis was disapproved by the theory of germplasm (Weismann, 1892).
- Many Darwin’s examples for variations are found today to be not heritable. Only those variations are heritable which occur due to changes in genes.
- Whether instincts are acquired and modified through natural selection, he did not say anything.
- He could not speak about the evolution of Homosapiens.
- He could not solve the problem of morality associated with evolution.
Most modern biologists do not accept the views of Darwin. Darwinism does not explain the presence of vestigial organs as well as mutations that arise during the lifetime of an individual. Further, the fittest characteristics are not always inherited by individuals of the next generations. For example, a man with a very good body does not produce a child with a good body. Natural selection cannot explain the universal occurrence of variations but accept this as a factor of evolution. Natural selection is not the only factor for the origin of a new species because mutation can also produce a new species.