What are 5 Scientific Theories?


What are 5 Scientific Theories?

The scientific method — a process of acquiring knowledge about nature — was discussed. If you will recall, the process involves a sequence of steps, which are: observation, problem definition, hypothesis formulation, experimentation, conclusions and theory formulation. 

The material in this article will explain to you in detail what a scientific theory means and also list some of the basic scientific theories.

Emphasis in this post will be on the theories of evolution.

This topic is important because it will expose you to the different views people, right from the beginning of recorded history, and have held about how life began and how the great variety of earth’s organisms came to be.

It is expected of you that at the end of the article, you will have achieved the objectives listed below.

At the end of this article, you should be able to define clearly a scientific theory, list clearly three characteristics of a scientific theory, identify three events that led to the revival of discussion on evolution, describe briefly evolution by natural selection and explain concisely how other fields of inquiry have led to a better understanding of the mechanism of evolution.


What is a Scientific Theory?

A scientific theory is an explanation about the cause or causes of a broad range of related phenomena (Starr and Taggart: 1992). 

A theory explains how things are related or their common properties. Scientific research (using the scientific method) leads to the accumulation of facts about nature. As the facts accumulate, organizing them into higher knowledge becomes imperative. This is because our minds require a rationalized, logically consistent body of knowledge to help retain and use information (Beveridge: 1970).

To help in the organization, a working hypothesis is usually required.

Thus, theories start as hypotheses or tentative formulations meant to explain the phenomenon under investigation. When a hypothesis is confirmed through experimentation, it becomes a theory.

Theories take various forms, which may be as diagrams, equations, statistical and propositional formulations. A theory is formulated in such a way that its range of application is indicated (Nwala: 1997). It says, ‘given such and such conditions’, any phenomena, which satisfy those conditions, are subject to the theory under reference. Let’s take for instance that a new drug called ‘Relief tablets’ against bacterial disease ‘X’ is manufactured in Nigeria [‘The nature of science’ (see p. 29)]. To test the effectiveness of that drug, Nigerian patients of different ages, sex, eating habits, hereditary background, etc, are used.

Additionally, the experiment is carried out under specified hospital conditions with proper allowance for unspotted errors and with different doses of the drug. At the end of the experiment the drug might be found to have an effectiveness of 60% against the bacteria. If this result is confirmed by many independent researchers in different localities, a theory may be proposed.

The theory might state that ‘in bacterial disease X, “Relief tablets” are effective in 60 per cent of the cases’. This statement is very much broader than the experiment on which it is based, and theories are always like that.

The statement has not given any limitations in terms of who manufactures the drug, where it is manufactured, where it can be used and whom it can be used on.

Therefore that statement implies, for example, that ‘Relief tablets’ no matter who manufactures it will be 60% effective anywhere in the world, under any conditions and can be used for both man and other animals. You can understand that direct evidence for the added implications does not exist.

But so far as ‘Relief tablets’ is already known to work within certain limits, the theory expresses the belief, the probability that it may also work within certain wider limits. Every good theory therefore has a predictive value. It prophesies certain results.

Scientific prophecies always have a considerate amount of evidence to back them up.

Additionally, a scientific prophecy does not say that something will certainly happen, it only says that something is likely to happen and with a stated degree of probability. Theories that have proved to be universally valid and have a high degree of probability are called natural laws. Laws also do not pronounce certainties.

In conclusion, theories enable us to explain, predict and control phenomena. They also provide us with a new way of looking at a familiar object or phenomenon. The theory of optics, for instance, provides us with a new way of looking at the phenomenon of light (Nwala: 1997).

Before this theory was discovered, people thought of light in the form of patches, shades of colour, etc. With the theory of optics, people’s way of looking at light changed. Light is now viewed as travelling (propagation) and this gives room for explaining a wider range of phenomena than the old way of looking at light.

Since the advent of modern science in the sixteenth and seventeenth centuries AD, many scientific theories have been proposed.


5 Basic Scientific Theories

These theories include:  

1.     The theory of universal gravitation

2.     The theory of evolution by natural selection

3.     Atomic theory, genetic theory

4.     Relativity theory

5.     Quantum theory,

The theories of evolution will be discussed in the next section.


Evolution before Darwin

Ever since the beginning of recorded history, man has speculated on the origin of life. In ancient times, it was generally believed that organisms originated spontaneously from non-living matter.

Thus, the ancient Egyptians believed that rats came from garbage. Renaissance scientists knew better than this, but in the seventeenth and eighteenth centuries AD, it was widely believed that bacteria (microorganisms discovered in 1676) originated spontaneously. It was only in 1862 that Louis Pasteur disproved, once and for all, the theory of spontaneous generation (Roberts: 1971).

Have you ever wondered how the earth came to be populated by a great variety of organisms? The search for explanations has proved to be challenging. The answers have been found to point to one direction — to evolution, which means the progressive change of living things through time. The idea of evolution was not originated by Darwin in the nineteenth century as some people think (Raven and Johnson: 1986).

Some ancient philosophers observed that organisms ranged from very simple to relatively complex ones. Each group of organisms, according to them, was created by God. Modern individuals of each group trace their ancestry to the individual created by God.

The Judeo-Christian culture also promoted such thinking. Most early scientists believed fully that each kind of organism with all of its individual adaptations was created by God. They studied these organisms, their structures, and the relationships between them as a way of learning more about the creator.

In the seventeenth century, for example, the English scientist-clergyman, John Ray (1627- 1705) clearly declared his belief that each kind of animal and plant had remained unchanged from the day it was created (Raven and Johnson: 1986).

All these views are collectively known as the doctrine of fixed species or creationism. It was never convincingly challenged before Darwin (Moore et al: 1995). 

During Darwin’s time, biology was dominated by natural theology. The natural theologians believed that the variations and adaptations of organisms proved that each species was fashioned by God for a particular purpose. 

Besides they believed that the earth was only a few thousand years old. For them, this was not long enough for significant evolutionary change. What is your own view? Do you believe in the theory of spontaneous generation or in the doctrine of fixed species?

What are your reasons? Gradually, some discoveries led to the revival of discussions on evolution.

These include:

1. The discovery of many more kinds of organisms by the first part of the eighteenth century (Raven and Johnson: 1986).

2. The study of fossils which was begun in the eighteenth century by Georges Cuvier (Moore et al: 1995).

Fossils as you might be aware are remains or body impressions of dead organisms that lived in the past. They are usually found within sedimentary rocks, which occur in layers. Cuvier, from his studies, found that different layers of the rocks held different kinds of fossils.

Also the fossils appeared in chronological order that is the deeper the layer, the older the fossil it contains.

To him, these observations seemed to be boundaries for dramatic change in ancient environments. His attempt to explain these changes came to be known as catastrophism (Starr and Taggart: 1992).

1. From 1707-1788, a French biologist named Georges-Louis Comte de Buffon studied many mammals and observed that all of them common features. He then suggested that these could be explained in terms of their evolution from a common ancestor.

2. In 1795, a geologist, James Hutton argued that the earth was older than a few thousand years (Moore et al: 1995). His hypothesis was based on the fact that he believed that sedimentary rocks that encased fossils were formed by the gradual accumulation of sediments in lakes, rivers and oceans.

This idea is known as gradualism. His explanation indicated that the earth was millions rather than thousands of years old. Catastrophists such as Cuvier and gradualists like Hutton and Lyell (1797-1895) were good geologists (Moore et al: 1995).

They were convinced of an ancient earth but they found it difficult to explain the appearance and disappearance of species in the fossil record. They were all creationists. Lyell, in particular attributed the gradual addition of new species to the earth’s flora and fauna to an unspecified creator.

3. In 1809, the year Charles Darwin was born, Jean Baptiste de Lamarck (1744-1829) proposed that all species, including human beings were descended from other species (Moore et al: 1995; Raven and Johnson: 1986).

He believed that life changed progressively from the simple to the more complex.

He was the first naturalist to present a unified theory that attempted to explain the changes in organisms from one generation to the next.

Lamarck’s explanations were based on the theory of inheritance of acquired characteristics, which he propounded.

The theory states that ‘changes acquired during an individual’s life are brought about by environmental pressure and internal “desires” and that offspring inherit the desired changes’ (Starr and Taggart: 1992).

Have you heard of an animal called giraffe? If your answer is yes, do you know why it has a long neck? Lamarck’s theory explained it as suppose the ancestor of the modern giraffe was a short-necked animal. Pressed by the need to find food, this animal constantly stretched its neck to feed on the leaves on high tree branches.

According to Lamarck, stretching increased the length of the neck and this acquired characteristic was passed to its offspring. These offspring in turn stretched their necks to reach higher leaves. Thus generations of animals desiring to reach higher leaves led to the modern giraffe.

Using similar reasoning, he also proposed that the use and disuse of a feature governed the fate of that feature in successive generations (Moore et al: 1995).

According to him organs of the body that were used extensively to cope with the environment became larger and stronger, while organs that were not used deteriorated.

Presently, scientists know that the mechanism he proposed for changes was wrong. Acquired characteristics cannot be inherited.

His theories, however, stimulated people’s interest in evolution. As a result of this, the stage was then set for the acceptance of the much simpler explanation that was developed by Darwin half a century later (Raven and Johnson: 1989). Darwin’s greatest contribution was the laying of the foundation for an evolutionary theory by noting that species change over time and the environment is a factor in that change (Starr and Taggart: 1992).

Darwin and modern evolutionary thought

In this section, you will learn about Darwin’s ideas on evolution. His ideas have had a great impact on a wide array of human endeavours than any scientific advancement of the past 150 years (Moore et al: 1995). His research work on evolution provided the first scientific explanation for the diversity of life.

His explanation of how evolution works has attracted more controversy than most scientific ideas. This is because it has affected not only science but also philosophy, religion and human attitudes.

Charles Robert Darwin (1809-18882) was an English naturalist. At the age of eight, his interest in observing the natural world was manifested. At that age, he was an enthusiastic but haphazard collector of shells. At ten, his interest shifted to the habits of insects and birds (Starr and Taggart: 1992).

At fifteen, he preferred hunting, fishing and observing the natural world to doing his schoolwork. At the university, he could not complete his study of medicine, because that was not his area of interest. He later graduated from Cambridge University as a clergyman but was still interested in natural history.

At the age of 22, one of his professors at Cambridge University, John Henslow arranged for him to take part in a training expedition led by an eminent geologist.

Henslow had earlier noticed and respected Darwin’s real interest in natural history. He therefore arranged that he be offered the position of naturalist in the ship they were about to sail in. Do you know that right from the 1770s naturalists had always been posted on all British voyages to distant lands? 

The aim was to gather more knowledge of nature to add to the store of human knowledge (Raven and Johnson: 1989; Starr and Taggart: 1992).

The ship they boarded was called H.M.S. Beagle. The main aim of the voyage was to complete an earlier work of mapping the coastline of South America. While on the expedition, Darwin had chances to study many diverse forms of life on the islands where they stopped, near mountain ranges and along rivers. He returned to England in 1836, after nearly five years at sea and he began a long life of study and contemplation.

When Darwin came back to England, the question that disturbed him was, ‘What could explain the remarkable diversity among organisms?’ Luckily, field observations he had made during his voyage enabled him later to recognize two clues that pointed to the answer (Starr and Taggart: 1992).

First, while the coast of Argentina was being mapped, he repeatedly got off the ship for exploratory trips inland.

On these trips, he made detailed field observations and collected fossils. He saw for the first time many unusual species, including an armadillo.

Among the fossils were remains of the now extinct glyptodonts. Glyptodonts were very large animals that closely resembled the living armadillos.

Darwin became puzzled and started wondering: If both kinds of animals had been created at the same time, if they lived in the same part of the world, and if they were so much alike, then why were armadillos still moving about while the glyptodonts were long gone and buried (Starr and Taggart: 1992)? Nothing else in the world resembled either animal.

Although nobody including Darwin had ever seen one species evolve into another, he later wondered whether armadillos were descendants of glyptodonts.

Second, he had observed that populations of similar kinds of organisms that lived in different geographic regions often showed remarkable differences in some of their traits (that is, characteristics).

For instance, Darwin saw giant land tortoises on the Galapagos Islands which were off the coast of Ecuador.

To his surprise, all the tortoises were not identical. It was recorded that even natives of those islands and the sailors could tell which island a particular animal had come from just by looking at it (Raven and Johnson: 1989).

Darwin reasoned that perhaps all those species descended from the same ancestral form and had become slightly modified after they became isolated on different islands. Darwin again wondered how such modification could occur. He got a clue from a book published by Thomas Malthus in 1798.

The title of the book was ‘Essay on the Armadillo Glyptodont principles of population’. 

According to Malthus, ‘any population tends to outgrow its resources and its members must compete for what is available’. This statement struck Darwin and he thought about all the populations he had observed during his voyage. 

He thought about how the individual members of those populations had differences in body size, form, colouring and other traits. It then dawned on him that some traits could lead to differences in the ability to secure scarce resources (Starr and Taggart: 1992).

Darwin got a clue of how modifications could occur from Thomas Malthus’s ‘Essay on the principles of population’. With that clue, Darwin declared that it was natural selection that is nature selecting the ‘fit’ and rejecting the ‘unfit’ that led to modifications in members of a species. He described the process of natural selection as follows (Starr and Taggart: 1992):

If there were struggles for existence (competition) within a population, then individuals that possess superior physical, behavioural or other attributes might have an edge in surviving and reproducing.

In other words, Nature would select individuals with advantageous traits and eliminate others — and so a population could change. Favoured individuals would pass on the useful traits to offspring; their offspring would do the same and so on.

Gradually, descendants of the favoured individuals would make up most of the population, and less favoured individuals might have no descendants at all.

Darwin published his work in 1859 and it caused an immediate sensation. Many people were very disturbed because the theory implied that humans probably evolved from apes, monkeys, etc, since all share similar characteristics. Surprisingly, in 1858, Darwin received an essay from a young English naturalist named Alfred Russel Wallace (1823-1913).

The essay was on ‘Theory of evolution by means of natural selection’! What a coincidence! Both of them had independently reached the same conclusion about evolution.

However, Darwin was given the credit of propounding the theory because of the amount of evidence he marshaled out.

Thus, Darwin’s theory of evolution is one of the main unifying themes of the biological sciences. It provides an explanation for three main sets of facts about life on earth, which we observe (Rutherford and Ahlgren: 1988). These are:

1. The incredible display of different types of living things we see about us.

2. The different degrees of likeness among the living things. The likeness could be anatomical or molecular.

3. The fossil record that shows a sequence in the kinds of organisms that have lived on earth over billions of years.

At the heart of the theory of evolution is the concept of natural selection. That means, natural selection is the mechanism that results in evolution.

However, Darwin’s theory still faced a crucial test which worried him. It did not explain the mechanism of heredity - that is the way desirable traits or characteristics was transmitted to offspring. You will read about the solution to this problem in the next section.


Evolution after Darwin

Before Darwin’s death in 1882 and even after his death, many important discoveries have been made and these have led to a better understanding of the mechanism of evolution.

These discoveries were made in other fields of inquiry such as paleontology, genetics, biochemistry, embryology, geology and ecology. You will learn about the contributions of the first three fields in this section.

Darwin before his death predicted that the fossil record should yield intermediate links between the great groups of organisms. I believe you know the great groups of organisms? They are fishes, amphibians, reptiles, birds and mammals. Paleontologists (those who study fossils) confirmed this prediction two years after Darwin’s book was published. In 1861, an early bird-reptile called archaeopteryx was discovered. It resembled reptiles and birds.

Like fossils of small two legged reptiles, it had teeth and a long bony tail. Like modern birds, its body was covered with feathers (Starr and Taggart: 1992).

Discoveries of microscopic fossils by paleontologists have extended back the known history of life on earth to more than 3.5 billion years. The discovery of other fossils has shed light on the ways in which organisms have evolved from the simple to the complex over the course of this enormous time span.

The evolution of the major groups of vertebrates and their relationships to another have become reasonably well understood since their bones and teeth are often seen in the fossil record (Starr and Taggart: 1992).

Among the most important developments in evolutionary biology since Darwin is the application of genetics to the theory of evolution by natural selection. Genetics is the science of inheritance. If you would recall from section 3.2.2, Darwin was worried because his theory could not explain how parents pass on their characteristics to offspring.

In the early part of the century, Gregor Mendel’s laws of inheritance were rediscovered. Scientists found that those laws accounted for the origin of new variations in organisms and how these variations were passed on from parents to their offspring.

Biochemistry is the science that examines the chemical processes and substances that occur in living things. Its techniques are being used understand evolution better (Raven and Johnson: 1989). Would you know how this is done? Samples of the same kind of protein are obtained from different organisms.

Then biochemical tools are used to determine the sequences is into acids in those protein samples. Individuals, populations, species and even larger groups have sequences that are characteristic of them. When sequences are compared for, let’s say, 10 different species, their degree of relationship can be specified precisely.

The following information might interest you very much (The Guardian: 2001): An international team of scientists from Australia, Great Britain and the United States found that human evolution is still operating according to the forces of natural selection which was first identified by Charles Darwin.

The main findings of the study are that:

1. Natural selection is leading women to have the first child earlier in life.

2. This tendency is partly inherited.

3. As a result, evolution is leading to an increasing biological pressure on women to start families earlier than later.

In the study, Roman Catholic women are found to have a 20% higher ‘reproductive fitness’ than women of other religions. University educated women, on the other hand, had 35% lower fitness than those who left school early.

The study, however, found that such cultural influences could not explain all reproductive differences between women. Dr Owens, a member of the research team from Imperial College, London said that it was surprising to find that genes played almost as much of a role in deciding these issues as religion or social class. Genes were found to explain about 43% of the differences in age of first child within the female population. Their next step according to Dr Owens is to try to discover what these genes are.


Read on: Functions and Types of Stem

Conclusion on what are 5 Scientific Theories?

In this article you have learnt what a scientific theory really means and its various characteristics. You have also been introduced to the various ideas on evolution before Darwin proposed his own theory. 

It is good to be aware that Darwin’s theory of evolution by natural selection is one of the most comprehensive themes in biology. This is because it explains how life began and how diversified into the organisms of today. If you had thought that human evolution had stopped as a result of improvements in diets, housing and medicine, you would have been pleasantly surprised with the information in section 3.2.3. This shows that science is never finished. It is a dynamic activity.

The main points in this unit include the following:

• A scientific theory is an explanation about the cause or causes of a broad range of related phenomena. A theory explains how things are related or their common properties.

• Theories take various forms, which may be as diagrams equations, statistical and prepositional formulations. 

• A theory is formulated in such a way that its range of application is indicated.

• Every good theory has a predictive value.

• Theories enable us to explain, predict and control phenomenon.

• They also provide us with a new way of looking at a familiar object or phenomena

• Scientific theories include the theory of universal gravitation, the theory of evolution by natural selection, atomic theory, relativity theory, quantum theory, etc.

• Theories of evolution propounded before Darwin include the theory of spontaneous generation and the doctrine of fixed species or creationism.

• Events that led to the revival of discussions on evolution include:

(a) The discovery of many more kinds of organisms by the first part of the eighteenth century.

(b) The study of fossils which showed that layers of sedimentary rocks held different kinds of fossils. The fossils also appear in chronological order - the deeper the layer, the older the fossil it contains.

(c) Comte de Buffon’s observation that all mammals he studied had common features.

(d) James Hutton’s hypothesis that sedimentary rocks that encased fossils were formed by the gradual accumulation of sediments in lakes, rivers and oceans — thus indicating that the earth was millions rather than thousands of years old.

(e) The proposition of de Lamarck’s theory of inheritance of acquired characteristics.

10. Some of Darwin’s pieces of evidence that evolution occurs include the following:

(a) Extinct species, such as the glyptodonts shown in figure 1.3 most closely resemble the living armadillos in the same area, suggesting that one had given rise to the other.

(b) Layers of sedimentary rock held different kinds of fossils and they appeared in chronological order.

(c) Populations of similar kinds of organisms that lived in different geographic regions often showed noticeable differences in some of their characteristics.

For instance, the giant land tortoises on the Galapagos Islands were not identical; thus indicating to Darwin that all those species might have descended from the same ancestral form but had become slightly modified after they became isolated on different islands.

• Darwin got a clue as to how those modifications could occur from Thomas Malthus’s ‘Essay on the principles of population’.

• With the aid of those pieces of evidence and the clue from Malthus, Darwin declared that it was natural selection, that is, nature selecting the ‘fit’ and rejecting the ‘unfit’ that led to modifications in members of a species. That means that natural selection is the mechanism that leads to evolution.

Thus Darwin’s theory consists of two major parts

(a) Concept of evolutionary change

(b) The concept of natural selection.

• Darwin’s theory, however could not explain how desirable characteristics or modifications were transmitted from parents to offspring.

• After Darwin’s death, fields of inquiry such as paleontology, genetics, biochemistry, embryology, geology, ecology, etc, have produced results that have led to a better understanding of the mechanism of evolution.

• The field of genetics, particularly, through the laws of inheritance proposed by Gregor Mendel, has given evidence of how desirable characteristics are transmitted from parents to offspring.


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