Evolution, also known as organic evolution, can be defined as the total adaptive changes in the inheritable genetic make-up of organisms, which results in the diversity of functions, forms and structures among the organisms. It usually takes a long period of time.
[Photo credit: Волков Владислав Петрович.]
All living things started existing from pre-existing ones. This has led to the the existence of various species by a gradual process of change, over a long period of time.
Evidence of Evolution
The evidences to prove the occurrence of evolution include:
1)Embryology: In the embryology of vertebrates, it is noticed that some stages resemble the embryo of some other vertebrates, which we believe they have evolved from. For instance, some stages of development of embryo of man resembles that of fish, amphibians and reptiles. This is because, man is believed to have evolved from ancestors, whose embryos were like those seen in human embryology.
2) Geographical distribution: Climatic differences is believed to lead to some changes in the function, structure and form of plants and animals. Different climates produce different types of plants. If one of two organisms from the same climate is isolated, it tends to differ from the other organism after several years.
3) Fossil records: A fossil is a remain of plants or animal, that existed in the past. An instrument, called radio-isotope dating, is used to determine the age of fossils. Evolutionary trend has been shown from simple life forms to complex life form, by arranging the fossils according to their age. Fossil records are also known as historical record,
4) Vestigial organs: These are small organs with no special function. They are usually incomplete organs. Their presence helps in the explanation of evolution. In accordance with evolution, they are the remaining parts of once-functioning organs. For instance, the appendix in man functions as cecum in herbivores, the rudimentary tail in man is well developed in many other animals.
5) Comparative anatomy: In the anatomy of the heart in all categories of vertebrates, progressive evolutionary change is said to occur. The Pisces possess one simple heart with one ventricle and auricle; amphibians, one ventricle and two auricles; reptiles, two auricles and a partially divided ventricle. In the case of birds and case, they have two ventricles and two auricles. Blood circulation also changed from single to double circulation in the process of the changes.
6) Domesticated organisms: Some domesticated organisms like dogs, hens, cats, have lived with man for millions of years. They are said to have gone through series of stages to become what they are today.
EVOLUTION THEORIES
There have been some theories that have tried to explain how evolution has taken place. The two common evolution theories are:
1) Lamarck's Theory of Evolution: Jean-Baptiste Lamarck (1744-1829) was a French naturalist. He was the first scientist to suggest that organisms undergo evolution. In 1801, he propounded a theory, known as theory of use and disuse. His postulates are as follows:
Lamarck used the neck of giraffe as an example. He said the neck of giraffe became long as a result of the need to browse on tree tops. Because the adaptation was useful, it started becoming inherited by subsequent generations.
From new research by modern scientists, the physical features acquired by use and disuse are not inheritable. It is only the characters that are represented in genes that are inheritable. As a result of this, Lamarck's theory is not accepted by modern scientists.
2) Darwin's Theory of Evolution: Charles Darwin (1809-1882) was a British naturalist. In 1859, he propounded a theory, known as Darwin's theory of natural selection. His postulates are as follows:
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All living things started existing from pre-existing ones. This has led to the the existence of various species by a gradual process of change, over a long period of time.
Evidence of Evolution
The evidences to prove the occurrence of evolution include:
1)Embryology: In the embryology of vertebrates, it is noticed that some stages resemble the embryo of some other vertebrates, which we believe they have evolved from. For instance, some stages of development of embryo of man resembles that of fish, amphibians and reptiles. This is because, man is believed to have evolved from ancestors, whose embryos were like those seen in human embryology.
2) Geographical distribution: Climatic differences is believed to lead to some changes in the function, structure and form of plants and animals. Different climates produce different types of plants. If one of two organisms from the same climate is isolated, it tends to differ from the other organism after several years.
3) Fossil records: A fossil is a remain of plants or animal, that existed in the past. An instrument, called radio-isotope dating, is used to determine the age of fossils. Evolutionary trend has been shown from simple life forms to complex life form, by arranging the fossils according to their age. Fossil records are also known as historical record,
4) Vestigial organs: These are small organs with no special function. They are usually incomplete organs. Their presence helps in the explanation of evolution. In accordance with evolution, they are the remaining parts of once-functioning organs. For instance, the appendix in man functions as cecum in herbivores, the rudimentary tail in man is well developed in many other animals.
5) Comparative anatomy: In the anatomy of the heart in all categories of vertebrates, progressive evolutionary change is said to occur. The Pisces possess one simple heart with one ventricle and auricle; amphibians, one ventricle and two auricles; reptiles, two auricles and a partially divided ventricle. In the case of birds and case, they have two ventricles and two auricles. Blood circulation also changed from single to double circulation in the process of the changes.
6) Domesticated organisms: Some domesticated organisms like dogs, hens, cats, have lived with man for millions of years. They are said to have gone through series of stages to become what they are today.
EVOLUTION THEORIES
There have been some theories that have tried to explain how evolution has taken place. The two common evolution theories are:
1) Lamarck's Theory of Evolution: Jean-Baptiste Lamarck (1744-1829) was a French naturalist. He was the first scientist to suggest that organisms undergo evolution. In 1801, he propounded a theory, known as theory of use and disuse. His postulates are as follows:
- changes in environment leads to corresponding changes in species;
- as a result of this change, organisms form new habits to adjust to the new environment;
- specialized characters are then developed by the organisms by use and disuse of organs;
- the organs that are regularly used become well developed, while the unused ones degenerate and become useless;
- the well developed acquired characters are inheritable.
Lamarck used the neck of giraffe as an example. He said the neck of giraffe became long as a result of the need to browse on tree tops. Because the adaptation was useful, it started becoming inherited by subsequent generations.
From new research by modern scientists, the physical features acquired by use and disuse are not inheritable. It is only the characters that are represented in genes that are inheritable. As a result of this, Lamarck's theory is not accepted by modern scientists.
2) Darwin's Theory of Evolution: Charles Darwin (1809-1882) was a British naturalist. In 1859, he propounded a theory, known as Darwin's theory of natural selection. His postulates are as follows:
- species are capable of producing large number of offspring in the environment but with limited resources;
- this gives rise to competition by the offspring;
- the survivors must have inherited the useful character(traits);
- the useful traits are then passed to their offspring;
- those with bad traits can not survive and they die;
- this results in the survival of the fittest (natural selection);
- as this process continues, there is variation within the population;
- the population gradually become adapted to the environment, which leads to the origin of new species.
This theory is popularly used by many scientists, even up till today.
The importance of environment were recognized by both Lamarck and Darwin in evolution.
Many scientists accept that evolution takes place through natural selection of the better adapted organisms by chance and the elimination of the much less adapted ones.
MODERN EVOLUTION THEORY
Modern Evolution Theory is based on the composite of natural selection and genetic origins of variation. It can be summarized as follows:
- variations exist in populations.
- some of the variations have special survival advantage.
- organisms with the favorable variations are more adapted to the environment than others
- the organisms in the environment have to compete for survival
- the fit organisms contribute more offspring to the next generation more than the unfit ones. This results in a gradual shift in the features of the population
- the main causes of variation are mutation and recombination of genes.
How New Organisms Are Formed
New species always form from the existing ones. The process is called speciation. It is a continuous evolutionary process. Speciation is the process by which new species are formed from existing ones. It can occur through two ways:
- mutation, which is a sudden change in the structure of DNA;
- isolation
Evolutionary Trends in Organisms
In plants and animals, it is believed that there are evolutionary trends. The trends are:
1)simple to complex organisms: In plants, the evolutionary trends are in the following order: Algae, liverworts, mosses, ferns, gymnosperms and angiosperms
And in animals: protozoa, coelenterata, flatworms, earthworms, snails, anthropoids and vertebrates
2) aquatic to terrestrial life: The evolutionary trend in this aspect is the same in plants and animals. The trend is that: life started in water and then moved to land. For instance, the organisms to have exited first were mainly aquatic.
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