Chemical evolution resulted in the formation of life on earth about 3.8 billion years ago. The primitive cells that resulted from this process gradually gave rise to primitive forms of organisms such as bacteria, algae, fungi and protozoan. These organisms in turn, gave rise to a variety of forms of life that exist today on Earth.
The occurrence of evolution is undeniably established by the various types of evidences that are available today. However, how it has come about is explained by the several ideas that have been put forth from time to time. These ideas are known as 'theories on organic evolution'. Following are the main theories on the mode of evolution.
The fact that evolution has taken place can be established by taking several kinds of evidences that are available. These evidences can be either 'direct' or 'indirect'. Direct evidences interestingly are provided by organisms, which have now become extinct, while indirect evidences are available from the study of organisms that are existing today - extinct organisms.
Variations can be defined as the differences that occur in the characteristics between members of the same species. Variations occur with reference to every character. In the absence of variations, every species would have continued to exist in the same form and no new species would have arisen from the existing one. Thus, variations are the raw materials for organic evolution.
A species is a group of similar organisms, which can breed among themselves producing fertile offsprings. The members of a species not only share a common gene pool, but also share a common ancestry.
solation is the segregation or separation of populations by certain barriers, which prevent interbreeding. As a result gene flow between populations is prevented. Each population on isolation, develops genetic divergence independently leading to the formation of new species.
Organic evolution refers to the slow and gradual process by which living organisms have changed from the simplest unicellular form to the most complex multi-cellular forms that are existing today.