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Theory of Evolution

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Generally speaking, Biology is a diverse and has many sub-categories yet this introductory book is a preliminary textbook and thereby it will present an essential account of biology chapters in year 2 through to year 6.


That is, the theory of evolution, cell and gene theory, growth and reproduction theory and immune system. The brief historical events at the first place and secondly, the general meaning.


A number of classical Greek philosophers believed in the gradual evolution of life. But the philosophers who influenced Western culture most, Plato and his student Aristotle, held opinions that opposed any concept of evolution in which species are permanent, are perfect, and do not evolve. In Judeo-Christian culture, the Old Testament account of creation fortified the idea that species were individually designed and permanent.

In the 1700s, biology in Europe and America was dominated by natural theology, a philosophy dedicated to discovering the Creator’s plan by studying nature. Natural theologians saw the adaptations of organisms as evidence that the Creator had designed each and every species for a particular purpose. A major objective of natural theology was to classify species in order to reveal the steps of the scale of life that God had created.

In 18th century, Carolus Linnaeus, a Swedish physician and botanist, sought to discover order in the diversity of life “for the greater glory of God”. Linnaeus was the founder of taxonomy, the branch of biology concerned with naming and classifying the diverse forms of life. He developed the two-part, or binomial, system of naming organisms according to genus and species that is still used today.

In addition, Linnaeus adopted a system for grouping similar species into a hierarchy of increasingly general categories. For example, similar species are grouped in the same genus, similar genera (plural of genus) are grouped in the same family, and so on. To Linnaeus, clustering similar species together implied no evolutionary kinship, but a century later his taxonomic system would become a focal point in Darwin’s arguments for evolution.

In the early part of 19th century and late 18th century, natural scientists have started studying fossils. Fossils are remains or impressions of a prehistoric organism preserved in petrified form or as a mold or cast in rock. There were many opinions and arguments on the geological formation and how life evolved on it by relying on fossils.

Paleontology, the study of fossils, was largely developed by French anatomist Georges Cuvier. Realizing that the history of life is recorded in strata containing fossils, he documented the succession f fossil species in the Paris Basin. He noted that each stratum is characterized by unique group of fossil species, and that the deeper (old) the stratum, the more dissimilar the flora (plant life) and fauna (animal life) are from modern life.

Cuvier even recognized that extinction had been a common occurrence in the history of life. From stratum to stratum, new species appear and others disappear. Yet Cuvier was a staunch opponent to the evolutionists of his day. Instead, he advocated catastrophism, speculating that each boundary between strata corresponded in time to a catastrophe, such as flood or drought that had destroyed many of the species living there at that time.

He proposed that these periodic catastrophes were usually confined to local geographical regions, and that the ravaged region, such as mountain sliding to form ground and bulging of planes forming mountain, was repopulated by species immigrating from other areas.

The leading geologist of Darwin’s era, a Scot, named Charles Lyell had criticized the theory of gradualism. He incorporated Hutton’s gradualism into a theory known as uniformitarianism. The term refers to Lyell’s idea that geological processes have not changed throughout Earth’s history. Thus, for example, the forces that build mountains and erode mountains and the rates at which these forces operate are the same today as in the past. Darwin was strongly influenced by two conclusions that followed directly from the observations of Hutton and Lyell.

First, if geological change results from slow, continuous actions rather than sudden events, then Earth must be very old, certainly much older than the 6000 years assigned by many theologians on the basis of biblical inference. Second, very slow and subtle processes persisting over a long period of time can cause substantial change. Darwin was not the first to apply the principle of gradualism to biological evolution, however.

Toward the end of the 18th century, several naturalists suggested that life had evolved along with the evolution of Earth. But only one of Darwin’s predecessors developed a comprehensive model that attempted to explain how life evolves: Jean Baptiste Lamarck. Lamarck published his theory of evolution in 1809, the year Darwin was born.

He agreed that living organisms on Earth evolved from much simpler nature toward greater complexity and he proposed to explain specific adaptations evolve. It incorporated two ideas that were popular during Lamarck’s era. The first was use and disuse, the idea that those parts of the body used extensively to cope with the environment become larger and stronger while those that are not used deteriorate.

Among the examples Lamarck cited were a blacksmith developing a bigger bicep in the arm that wields the hammer and a giraffe stretching its neck to reach leaves on high branches. The second idea Lamarck adopted was called the inheritance of acquired characteristics. In this concept of heredity, the modifications an organism acquires during its lifetime can be passed along to its offspring.

The long neck of the giraffe, Lamarck reasoned, evolved gradually as the cumulative product of a great many generations of ancestors stretching ever higher. There is, however, no evidence that acquired characteristics can be inherited.

Not only had Charles Darwin (1809-1882) established firmly the evolutionary theory but also showed clearly how the evolution occurs and therefore transformed it into an essential claim of biology. Even as a boy he had a consuming interest in nature. Darwin was 22 years old when he sailed for five years from Great Britain aboard HMS Beagle in December 1831 across the seas on his voyage to different continents and islands of the world. During this long voyage, Darwin had the chance to observe and study in detail a wide variety of birds, plants, flowers, fossils, etc., on those regions of the world. During this long voyage, he found the evidence to change the fundamental claims of evolution.

Soon after returning, Darwin started reassessing all the information and evidence that he had observed during the voyage of the Beagle and finally came up with the idea that the existential diversity of life on earth happened from evolution and by means of natural selection. That is, evolution of living organisms occurs by means of natural selection and natural selection itself occurs through organisms’ adaptation to its environment.

It occurred to him that a new species could arise from an ancestral form by the gradual accumulation of adaptations to a different environment. Darwin developed his famous ideas of evolution and compiled with extensive evidence to publish the book titled The Origin of Species in 1859. After the publication of the book, it spread wide and open and became a manifesto of biology.

Descent with Modification

Secondly, in order to talk about general meaning of evolution, there are five things to talk about. These are, descent with modifications, variation, natural selection, adaptation and origin of species.

Darwin perceived unity in life, with all organisms related through descent from some unknown prototype that lived in the remote past. As the descendants of that inaugural organism spilled into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life.

Such descent of external phenotypic characteristics and internal genetic variation unto descendants is called descent with modification. This is the core idea of Darwin’s view on the nature of life and comprised the views of the descent of genetic based ancestry characteristics onto descendants.

If asked, can it be possible to pass on all the characteristics of ancestors to descendants, which is not as genetic based characteristics cannot be passed down. For example, in human being, colour of eyes and skin, blood type, baldness and so forth are transferable characteristics. Most of the characteristics that have been acquired through learning and other factors in this life are not transferrable such as big arms of blacksmith and degeneration of organs due to concussion.

Transferrable characteristics are passed down from parent to offspring through DNA, a gene-holding molecule. More details will come up in chapter 3. So, because of the changes in the transferrable characteristics due to causes and conditions, new characteristics are passed onto descendants and gradually spread across the population of that organism, and thereby evolution takes place in the population of that organism.


Secondly, variation refers to differences that are visible in the population. When we pay attention to our surrounding environment, we can see so many different variations among population such as differences in skin colour, facial structure, body height and so on in human population, different characteristics in same species of domestic animals and different types of colour and size in same species of flowers.

Visible characteristics are given the term dominant gene and that characteristic based genes are referred to as genetic composition. We can predict the differences in genetic composition the population from the differences in dominant gene of population and differences in dominant gene emerge from the differences occurring in the genetic composition.

There happens change in genetic composition of the population with time and spread across the population and when it brings change in the nature of population, it is considered that evolution has happened in the population.

And then if we asked by depending on what has the variation occurred? We can say that it occurred from the changes in genetic composition, gene alteration from mating and also from the gene flow, gene flow from one population to another due to the migration between populations. More details will come up in the second year chapters.

Variation is one important cause of the happening of evolution and that evolution will not occur if there are no diverse variations in the population. And if we are asked about the way evolution occurred in population with such variation, the way of happening of this evolution is called natural selection that is explained below.

Natural Selection

Thirdly, there are three ways by which we can talk about natural selection and these are:

general meaning, examples of its occurring and types.

At first, the meaning or definition. The organisms that have characteristics that let them adapt better to the environment than rest of the same species have better chance of survival and reproduction thereby having more descendants and due to that have chance of prolonging the advantageous genetic composition onto future generations. In this way, the genes of the organisms with advantageous traits are preserved and passed onto descendants. This is called Natural Selection.

One popular empirical example of natural selection is that during the industrial revolution, lot of factories were running in so many places producing smokes from the burning of coal. Nearby forests were turned black by the smokes. There were two different kinds of moth, white and black. Because of the darkening of trees and branches from the black smokes, it gave favourable conditions for the survival of black moth relative to white moth and thereby can survive longer and advantageous in terms of reproduction. Due to the similar colour of moths and tree branches, birds cannot see and

so it’s easy to escape from the danger. And because it’s easier to see white moths on black branches, they became prey to birds. Gradually, the empirical evidence of the transformation of earlier population of mixed moths to black population is one of the simplest and most popular examples of natural selection.

Secondly, if we were to show how natural selection would occur through example, such as there are few factors for natural selection to occur in the population of beetles.

First condition is that of variation in traits. For example, there are many different kinds of physical colour for beetles such as green and brown, and white and black traits in population of moths.

Second condition is that of the differences in the hereditary reproduction. Not only can the population not expand limitlessly but also can it extend the full use of the ability for hereditary reproduction and therefore there emerges differences in the hereditary reproduction in the population of organisms. In this example, because of the visibility of green beetles, they became prey to the birds and so the ability for the hereditary reproduction is comparatively less than the brown beetles.

Third condition. Traits have genetically inherited nature. For example, brown beetles reproduce brown offspring and green beetles reproduce green offspring because these characteristics have genetic basis.

Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage thereby spreading within the population. The whole organisms in the population turn into the population with such traits. And if there are variations in traits within the population and that there are variations in the reproduction of each organism of the population, traits having genetically inherited nature, then consequently it follows that evolution occurs through the means of natural selection.

Thirdly when we talk about different types of natural selection, there is directional selection, disruptive selection, stabilizing selection, sexual selection, artificial selection and so forth. When we ask about the consequences of evolution by means of the mechanism of natural selection, generally speaking, although evolution do influence the features and behavior of organisms, the most visible consequences are the organism’s behavioral adaptation and physical adaptation. Therefore, we will discuss little bit about adaption.


Fourthly, there are three things in order to talk about adaptation, the nature of adaptation, and mechanism of adaptation and consequences of adaptation.

Firstly, adaptation is the process that makes the organisms better suited to their habitat.

Secondly, Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats. Organisms have to compete for suitable conditions to survive and reproduce and in undergoing such processes, organisms having traits that adapt to the environment are being naturally selected and pass onto next generation.

Thirdly the regarding consequences of adaptation, adaptation enables or enhances the probability of that organism surviving and reproducing. And if the organism lacks adaptive traits then their survival and reproduction weakens. Therefore, adaptation helps increasing the population quantity.

The Origin of Life

Sixth thing regarding the origin of life, in general, scientists popularly claim that the universe formed about 13.7 billion years ago and Earth formed about 4.5 billion years ago. Scientists cannot recognise the exact dateline of the origin of life and neither have they found the standard evidence. According to the claim, when the earth started forming, all the inner and outer parts are made up of fire and gradually the temperature started dropping.

Earth is situated at a suitable distance from Sun and many factors such as huge water bodies have conditioned for life to form on it and developed continuously. Planets Mercury and Venus are too close to Sun and Planets Mars and Jupiter are too far from Sun and so pose challenges to form life on these planets.

Carbon is the necessary atom for all organisms and it is estimated that it has conditioned the origin of diverse forms of life in the early part on earth. One of the primary origins of carbon is from the vapours from volcanic eruption. Carbon and hydrogen, oxygen, nitrogen are joined together to form an important form of chemical called amino acid, the building block of protein.

Since amino acid is the building block of protein, it is also the basis of early forms of life. Enzymes and catalyst made up of proteins become inevitable for organisms to carry on their different bodily functions. Generally, it is a phenomenon of organisms to develop from simplest to more complex state. For example, it is like even though organisms originated from very small size and number yet gradually expanded and become diverse in nature.

What is clear for us is that an organism that depends on other cell cannot be considered as the origin of life. This is because one has to depend on other organism in order to survive and reproduce and so it cannot be claim as origin from the state of earlier life form.

However virus and other organisms that are dependent on other cell can be regarded as formed after the start of the evolution of living things.

We have to consider something physical as living thing on this earth when it takes nutrients from others and starts breathing and that is to consider from the formation of amino acid through chemical reaction in the oceans and lakes on the planet dating back to 3 billion years. But some scholars say that life originated from other exo-planet and not from this earth.

Some researchers have found the evidence of the formation of RNA molecule through chemical reactions in the early stage of the world. And some researchers estimate that life has to be considered from the point of conception of reproduction ability to a combination of chemical substances. At present, so many scientists conduct different kinds of experiment in laboratory to obtain new understanding on the formation of life from the beginning and strive to make the type of chemical substances or things to generate similar substances or things.

Before proceeding to the next lesson, answer these questions by doing reasearch:

1. What is Evolution?

2. What relation is there between living things according to Evolution?

3. How does gene influence evolution?

4. What are the three conditions of natural selection?

5. Does evolution prove the existence of creator or negates it?

6. What evidence can you bring forth in claiming the theory of evolution?

7. How does natural selection happens?

8. Why is it that while one species of organism gets extinct, the other survives?

9. How would fish species remain when organisms living in water evolve into both land and water animals in the process of evolution?

10. Did humans evolve from monkeys?

11. How do you differentiate if its organism or not?

12. How did the origin of life start on the planet?