Organic evolution is defined as the sum total of adaptive changes from pre-existing or old form that have taken place over a long time, resulting in diversity of forms, structures and functions among organisms.

The concept behind organic evolution is that all living things came into existence from pre-existing living things and that the various species have arisen by a gradual process of change over a very long period of time. The present day species have not always existed in the present form, but have arisen from preexisting ancestors.



There are many evidences to show that evolution has taken place. These are:

  1. Fossil records: A fossil is a part or an impression of a plant or animal that lived a very long time (up to millions of years) ago. Scientists through the use of special techniques called radio-isotope dating are able to determine the age of fossils. By arranging the fossils according to their age, from the oldest to the youngest (the most recent) they have been able to show an unmistakable evolutionary trend from very simple life forms to complex ones. Fossils are normally preserved in sedimentary rock. Fossil records is also called geological, paleontological, archeological or historical record.
  2. Evidence from geographical distribution: It has been proved that living things (plants and animals) tend to show some variations in form, structures and functions in relationship to differences in climate. In other words, no two types of the same climate produce the same type of plants and animals that have some resemblance. Organisms of one climate tend to differ slightly from organisms of the same climate after several years of isolation.
  3. Evidence from comparative anatomy: There are evidences of evolution in certain anatomical features of vertebrates. There is a progressive evolutionary change in the anatomy of the heart in all categories of vertebrates. The pisces (fishes) have a simple heart with one auricle and one ventricle, amphibians, two auricles and one ventricle, reptiles, two auricles and a partially divided ventricles, and birds and mammals have two auricles and two ventricles. In the process of these changes, the circulation of blood also changed from a single to a double circulation.
  4. Evidence from embryology: Embryo is the organism that develops after fertilisation of the egg before birth or hatching. It is noticed that in the embryology of vertebrates, for instance, there are stages which resemble the embryo of other vertebrates from which they are believed to have evolved. For example, the embryo of man in the womb, at different stages of development resembles embryos of fish, amphibians and reptiles. This is as a result of the fact that man evolved from ancestors whose embryos where like those seen in human embryology.
  5. Evidence from vestigial organs: The presence of vestigial organs also help to explain evolution. Vestigial organs are minute or incomplete organs that have no special function. In line with evolutionary theory, vestigial organs are the remaining parts of once-functioning organs, e.g. the appendix in humans which functions as caecum in herbivore. Again, the rudimentary tail in human is well developed in many other mammals.
  6. Evidence from domesticated organisms: Domesticated organisms have been used as an evidence of evolution. It is known that specific domesticated animals like cats, dogs, hens have lived with human for many millions of years. They have gone through several stages to become what they are today.



The theories of evolution are attempts to explain how evolution have taken place.

Two of such theories are explained below.

1) Lamarck’s Theory of Evolution: Jean Lamarck, a French zoologist was the first to suggest that organisms undergo evolution. His theory which was propounded in 1801 is also called the theory of use and disuse.

Lamarck’s contributions or postulates are as follows:

  • that great changes in environment result in corresponding changes in the species;
  • that these changes cause the organism to form new structures or habits to adjust to the new prevailing environment;
  • that organisms then develop specialized characters by use and disuse of organs;
  • that frequently used organs become well developed while the ones not used degenerate and become atrophied or useless;
  • the well developed or dominant acquired characters are inheritable.


Lamarck used the long neck of giraffe to give example of his theory. The long neck of giraffe arose form the need to browse on tree tops. Since this was a useful adaptation, subsequent generations of giraffe have been inheriting this acquired character.

Lamarck’s theory is not accepted by modern scientists because their findings show that physical features of a parent acquired through use and disuse are not inheritable by offspring. Only characteristics represented in the genes are inheritable.


2) Darwin’s Theory of Evolution: Charles Darwin, a British naturalist in 1859 also propounded another theory of evolution. His theory is known as Darwin’s theory of natural selection.

Darwin’s contributions or postulates are as follows:

  • that species have the ability to produce large number offspring into environment with limited resources;
  • that this process then lead to competition by these offspring;
  • that the survivors must have inherited the useful traits.
  • they then reproduce and pass on these good traits to their offspring;
  • those that could not survive the struggle die off or are eliminated;
  • that there is great variability with population as this process continues through many generations;
  • that the population gradually became better adapted to the environment leading to the origin of new species.


It should be noted that Darwin’s theory is popularly upheld by many scientists even up till today.

Both Lamarck and Darwin recognised the importance of environment n evolution.

It is generally accepted by many scientists that it is by natural selection of the better adapted organisms by chance and the elimination of the much less adapted ones that evolution or changes take place.



Darwin’s theory of natural selection

  • Variations appear by themselves in a population.
  • The environment selects from members of a population only those with favourable adaptations or variations.
  • Heredity variations are transmitted to offspring.
  • Species that are fittest for the environment contribute more offspring to the next needs of the generation.


  • Lamarck’s theory of use and disuse Variations appear because organisms are trying to satisfy needs which exist in the environment.
  • The environment causes variations to be maintained, variation that actually arose from members of the population trying to satisfy the needs of the environment.
  • Characteristics acquired through use or disuse are transmitted to offspring.
  • Offspring inherit characteristics acquired by their parents through striving to satisfy the environment.




Modern evolutionary theory is based on the combination of natural selection and genetic origins of variation.

  • The modern evolutionary theory can be summarised in the following statements: There exist variations in a species’ population.
  • Some of the variations have special survival advantage.
  • Individuals with favourable variations are more adapted to the environment than others.
  • The individuals have to struggle for existence in the environment.
  • The fittest contribute more offspring to the next generation than the unfit ones. This brings about a gradual shift in the features of the population.
  • The main cause of variation are mutation and recombination of genes.


Formation of New Species

The process by which new species are formed is called speciation. Speciation is a continuous process because new species always evolve from old ones. Speciation occurs through various ways which are mutation and isolation.


(a) Formation of a New Species by Mutation

Mutation is a sudden change in the structure of DNA. This causes a change in the protein formed and hence a change in the phenotype of the species concerned.

The sudden appearance of distinct trait or character in a large population of organisms can cause variation in that population. Mutation can only be inherited when it occurs in the gene or gametes leading to production and perpetuation of new species. Mutation elsewhere in the body cannot be passed on or inherited.


(b) Formation of a New Species by Isolation

If members of a species are separated into two or more population by natural barriers like desert, mountains, oceans, rivers, continental drift etc for a long period, If their genes may change to the extent that they cannot interbreed again to produce viable offspring and the population thereby may have produced new species.



Different evolutionary trends have been identified in plants and animals. These are:

  1. Simple to complex organisms: In both plants and animals, simple or unicellular organisms appear before the multicellular ones. In plants, the evolutionary trends from the simple to the complex organisms are in this order. Algae, liverworts, mosses, ferns, gymnosperms and angiosperms.In animals, the evolutionary trends from the simple to the complex organism are in this order: potozoa, coelenterate, flatworms, earthworms, snails, anthropoids and vertebrates.
  2. Aquatic to terrestrial life: In both plants and animals, life started in water and then moved onto the land, e.g. organisms which appeared first were mainly aquatic (mainly Algae, Protozoa) or inhabitants of moist places such as liverworts, mosses and ferns.
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