Organic evolution is defined as “a change in population of organisms over time”. The history Earth is a billion of years old. The earth was inhabited by different population during different times of its history. Life evolves just as an individual has a family history. Each species has one tip on a branching tree of life. These branches extend back to some ancestral species. Species those are very similar, have a common ancestor, for example, horse and zebra. Such similar Species forms a relatively recent branch point on the tree of life. But there is an ancestor that lived much farther back in time, Therefore, horses and zebras are also related to rabbits, humans, and all other mammals by these ancestors. Thus mammals, reptiles, birds, and all other vertebrates share a common ancestor. This ancestor may be more ancient.
Only primitive prokaryotes inhabited the earth more than three billion years ago It means they ‘are ancestors • of other species. All life is connected to these ancestral prokaryotes through evolutionary tree. Evolution transformed life from simple to complex one. Evolution has produced unending diversity today. This diversity is the one biological theme. Evolution ties together all organisms.
Concept of Darwin about oneness and diversity of life
Charles Darwin published his book On the Origin of Species in 1859. Darwin’s book had two objectives:
- First objective: Descent with modification
Darwin described that the species are arised through evolution in their ancestors He called this process as “descent with modification”. He used these words for evolution. Many additional evidence has come since Darwin’s time. Thus nearly all biologists now see evolution as a mechanism of origin of life.
- Second objective: Natural selection
Darwin also evolved a mechanism of evolution. It is called natural selection. Darwin based natural selection on following facts and conclusions:
(i) Individuals in a population vary in many heritable traits.
(ii) These populations has the potential to produce far more offspring than the environment can support it with food, space, and other resources.
(iii) The struggle.for survival starts among the various members of a population due to over production.
(iv) Those individuals which suited to the local environment are selected. They leave a large number of surviving offspring. This selective reproduction increases the number of certain heritable variations in the next generation.
(v) Darwin called this as differential reproductive success.. It is a cause of avolution.
Darwin proposed that natural selection is a mechanism of evolution. It can )roduce new species from ancestral species. This can occur when a population
is divided into several populations. These populations are isolated in different environments. In these various forms of natural selection, one organism begins as one species. It may gradually form many geographically isolated populations. These populations adapt over many generations to different sets of environmental problems.
Thug Descent with modifidation explains both the unity and diversity in life. zeatures shared by two species are due to their descent from common ancestors. The differences between the species are due to natural selection. It modifies the ancestral characters in different environments.