The formation of complex organisms through gradual change from simple ancestral types over the course of geological time is termed Evolution or Organic Evolution.
Theory of Organic Evolution
- The various present day organisms were not created in the same form in which they exist today, but have gradually evolved from much simple ancestral forms from a common ancestor.
- The characteristics of organisms had been changing in the past. They are changing even today, and will continue to do so in the future as well. This is due to the fact that the environment in which organisms live also changes and organisms need to adapt to the changed environment in order to survive.
- Several living organisms of the past have become extinct.
- The origin of the various forms (species) found on earth has been a gradual and extremely slow process, requiring hundreds or even thousands of years.
The process of slow and gradual change is called Organic Evolution.
Evidences of Organic Evolution
The evidences supporting organic evolution are derived from a number of fields of Biology such as:
- Morphological evidences
- Embryological evidences
- Palaeontological evidences
- Molecular evidences
Evidences from Morphology
Though organisms of different species and groups are quite different from each other, they still retain certain common features. The comparative study of various organs in different groups of vertebrates exhibit common features which show that they evolved from a common ancestor.
Evidences from Embryology
Embryology is the study of development of an organism. All the different aspects of embryology strongly support the fact that the different classes of vertebrates had common ancestors.
Evidences from Paleontology
Paleontology is the study of fossils. Fossils are the remains or traces of animal and plant life of the past, found embedded in rock either as petrified hard parts or as moulds, casts or tracks.
Molecular Evidence of Evolution
All organisms have cell as the basic unit of life. The cell is made of biomolecules common to all organisms. Ribosomes, the cellular organelles are of universal occurrence in organisms. DNA is the hereditary material of all organisms, except for some viruses.