Introduction

The word evolution comes from the Latin word evolvere. That means gradual development. Evolution refers to the process by which a simple organism evolves into a complex organism. The evolutionary process occurs slowly. At first, unicellular and simple organisms emerged on Earth. Evolution describes the emergence of more advanced and complex species from these unicellular and simple organisms. There are various theories about evolution. But of all the theories, Darwin’s evolution theory is the most significant in the world. That is why Darwin is called the father of evolution (1).

Darwin’s theory

In 1859, the English naturalist Charles Darwin presented the theory of the evolution of living things in his book, “Origin of Species by Means of Natural Selection”. In 1831-1835 Darwin traveled in the islands of the Atlantic Ocean as a naturalist on the ship HMS Beagle.  He collected some samples and presented a theory about the expression of living things by observing his travel experiences and samples.

According to Darwin, the characteristics of the offspring that each organism produces are slightly different from those of the mother organism. Only organisms that are best adapted to the natural environment can sustain their existence through natural selection. New species are created through these processes. These new species differ from each other. Darwin’s theory is based on observation and experimentation. Some notable aspects of this theory are discussed below (2).

1. Prodigality of production

Organisms reproduce at an exorbitant rate to survive. These are their innate properties. Plants and animals produce more organisms than they need. The number of organisms increases at a geometric progression.

Example

1. Such a tobacco tree produces 3, 60,000 seeds per year. Although not all growths result or produce offspring plants, the relationship that plants form from these seeds is sufficient to sustain the species.
2. A female salmon fish lays about 30 million eggs during the breeding season.

• If all the elephants produced from a pair of elephants survived, the number of elephants in 750 years would be about 19 million (1).

2. Constancy of food and shelter

Food and the size of the earth do not increase in line with population growth. Therefore, if the number of organisms increases, there will be a food crisis for their habitat and livelihood (3).

3. Struggle for existence

Excessive production leads to a struggle among the organisms to obtain food, shelter, light, water, and other environmental elements. If plants were to grow from all the seeds of a particular plant, they would occupy a large area of the earth’s surface, but it is not. This is because there is intense competition among the created plants for the need of food, shelter, light, etc according to Darwin, this competition is the struggle for existence. There are basically three types of struggle.

• Intraspecific struggle

Intraspecific struggle is the struggle between different organisms belonging to the same species.

• Interspecific struggle

The struggle between different species of organisms is an interspecific struggle.

• Environmental struggle

The struggle of organisms against natural disasters such as floods, earthquakes, volcanoes, etc. is an environmental struggle.

Through all these struggles, the creatures that adapt themselves survive. On the other hand, those who are incapable of adapting to this form become extinct from nature forever. Adapted organisms survive by struggling for survival (2) & (3).

4. Variation

Not all the organisms belonging to the same species have the same characteristics. There are some differences between them. The reason is the variation. According to Darwin, the constant struggle created differences or variations between organisms. These differences are transmitted to the offspring’s body during reproduction. Some variations help the organism to struggle for life. The variety that helps in the struggle of life is called the favorable variety. Adverse variation does not help in the struggle for existence. According to Darwin, a tiny series of changes creates species. Sudden change is a game of nature. It does not affect the species (3).

5. Survival of the fittest

The fittest means the best of all organisms. And the emergence is to survive in the struggle of life. Only favorable variation organisms win the battle of life and they are considered to be the most deserving. Thus, the emergence of the most deserving is considered to be the survival of the fittest. On the other hand, the adverse variation species are defective in the struggle for life and gradually become extinct from Earth. For example, polar bears in temperate countries are unsuitable for living in tropical environments. This is because tropical environments are not ideal for polar bears. So Darwin thought that if polar bears lived in the trophic region, they would become extinct forever. On the other hand, organisms in tropical environments cannot survive in polar regions. This is because the polar region is not suitable for them.

6. Natural selection

Natural selection is an important aspect of Darwin’s theory. The main theme of Darwin’s theory is natural selection. The role of nature is most important. Nature is the main. The creature chosen by nature is the best. According to Darwin, natural selection is the selection of suitable organisms by nature. This is the main process of evolutionary change. The organisms that have favorable variations in their bodies win the struggle for existence. And the creatures that cannot survive in the struggle for life are not chosen by nature. As a result, they become extinct from the earth forever. That is, according to this theory, the best creatures chosen by nature find a place to survive on earth and reproduce (1) & (5).

7. Origin of new species

The characteristics of the organism that has won the struggle for existence are passed to the next generation of organisms. All the traits that helped the creature to win the struggle for existence became one of the characteristics of the offspring. According to Darwin, these variations accumulated over several generations and brought about significant changes in the structure of the organism. In this way, newer and more advanced creatures are created from the previous species (1)

Examples in support of Darwinism

Darwin gave some examples in favor of the theory.

• Leaf insects, stick insects, etc. are shaped like leaves and sticks. They are shaped for self-defense. As a result, they merge with the plants and are protected from the enemy. It is for this shape of the body that they survive and are naturally selected.
• The antenna of the insect is of such length that it can reach the honeycomb of the flower. They are naturally selected for this length of the antenna (3).

Criticism of Darwin’s theory

Although Darwin’s theory is observational and experimental, there are some aspects of this theory that have been criticized. Such as-

• According to Darwin, the origin of new species is through variation. But he could not give any idea about the origin of the variation.
• Darwin also said in his theory about the survival of the fittest. But could not explain the appearance of the most qualified creature.
• Natural digesters often lead to the complete extinction of some organisms. As a result, many weaker organisms emerged in nature. Darwin does not discuss any information about this in the theory of evolution.
• Darwin did not realize the importance of mutations. He simply ignores mutations as a game of nature.
• Variation is more important than natural selection. The causes of variation are mainly responsible for evolution. But Darwin could not explain it.
• According to Darwin’s theory, the overgrowth of an organism is conducive to evolution. But organ overgrowth causes the extinction of the creatures. Darwin could not give any accurate information about it.
• Darwin’s evolution theory said the struggle for survival among new offspring. But there has been no discussion of the effect of the struggle for existence on previous organisms (2) & (4).

Neo-Darwinism

Based on the above criticism, it can be said that Darwin’s “natural selection” did not accurately explain the process of the creation of new species. Although his theory established the fact of evolution, his theory became baseless due to a lack of sufficient evidence. According to Darwin, the most appropriate variations evolved through natural selection. But he could not give an explanation as to how these variations occurred. Then new information is discovered through mutation theory, genetics, method analysis, ecology, etc. The combination of Darwin’s theory and these theories (the mutation theory, genetics, method analysis, ecology) led to the emergence of a new theory. This theory is now accepted. This new and modified theory is known as Neo-Darwinism. This is the modified version of Darwin’s theory (1)

Discoverer of Neo-Darwinism

Neo-Darwinism is the combined result of the theories of modern biologists such as Darwin, Hugo de Vries, Thomas Henry Huxley, Hermann Joseph Muller, and Emil Fischer (3).

The main theme of Neo-Darwinism

The main point of this theory is that genes are rearranged through bi-parental reproduction under the influence of natural selection over a long period. Evolution is caused by these genetic changes.

According to neo-Darwinism, not all variants of a species are evenly distributed throughout all populations. That is limited to a certain part of the population. This part of the exceptionally variant population is called the subspecies. All these subspecies gradually produce a new species through natural selection (1).