Organic Evolution: Meaning and biological evidences
Do you know that the diversities of plants and animals found today are evolved from a common ancestor? This article highlights the meaning of evolution and also provides the pieces of evidence of organic evolution. The article will also explain the connecting link, homologous organs, analogous organs, atavism, and vestigial organs.
Meaning of Evolution
Evolution is a naturally occurring process. It is believed that all the organism are evolved from a common ancestor through the process of evolution. Thus every kind of living organism has the potential to evolve. Complex forms are evolved from the simplest forms over millions of years.The term evolution is derived from two Latin words evolvere ( e=form, volvere= to roll) and means that act of unrolling or unfolding. The term evolution was first used by Charles Bonnet. According to Savage (1969), the term evolution was first used by English philosopher Herbert Spencer. Evolution has been defined as a slow, gradual, orderly and irreversible process of change in the organism from one form of life to another over a long period of time.
Biological Evidences of Evolution
Evolution is a complex process accounting for the present-day diversity among organisms. But it has clearly maintained the basic unity among them since it occurred over a period of millions of years, no one would have seen or recorded evolution and hence various pieces of evidence have been provided by the scientists to prove evolution.Paleontological Evidences
Paleontology (Gk. Paliaos=ancient + onta=existing things +logos=discourse) is the study of ancient life or fossils which are the relics of the extinct organism. The science of palaeontology links biology with geology and is concerned with the finding, cataloguing and interpreting of fossils. Modern palaeontology was established by George Cuvier therefore he is regarded as the founder of modern palaeontology.Fossil records show that evolution has taken place from simple to complex forms in a gradual manner. The palaeontological aspects which support evolutions are:
Number and nature of fossils in early rocks
The rocks of the early Proterozoic era contain a lesser number of fossils than the rocks of later era and only simple marine invertebrates are represented in these fossils. It is because of the fact that the life originated first in the sea as simple forms. Therefore, fossils were not as plentiful in the beginning as it becomes in the later stage.Distribution of fossils in the successive strata
Moving up through the various strata, from older to recent formations, there is a succession of the higher and more complex form of life. The rocks of the archaeozoic era do not show fossils. The fossils are very few in the rocks of the palaeozoic era; it contains the fossils of invertebrates, fishes, and amphibians. The rocks of the Mesozoic era contain the fossils of great reptiles, primitive birds and mammals. Finally the rocks of the recent era, the Cenozoic show the presence of the fossils of various mammals.Disparity between the past and present form of life
Fossil study shows that early or extinct organisms were very different from their modern forms. E.g., the early man lived in the caves like animals without any social life and obtained food by hunting wild animals and by gathering fruits and seed of wild plants but man progressed and the modern man has civilized and having a rich social life, cultivating crops for food and domesticating animals for food and fulfil other needs. Thus, life has been changing since its appearance. This supports that evolution has been taking place.Transitional forms (Missing links)
The fossils contain characters of two different groups of organisms are called a missing link or a transitional form. It is also known as a connecting link. E.g., Archaeopteryx is an excellent example of a missing link between birds and reptiles. The fossil of Archaeopteryx was found in the rocks of the Jurassic period and it was about the size of a crow and in certain respect was quite like a reptile. It was discovered by Andreas Wagner in 1861 from Solnhofen, Bavaria, in Germany.Avian (bird-like) characters of Archaeopteryx are:
- Presence of wings and feathers in the body.
- Its jaw were equipped with conical teeth.
- Its tail quite unlike that of modern birds, was long with a row of feathers on each side.
- Its forelimbs are modified into wings, three fingers, four toes, first directed backwards, 2-4 forward and "U" shaped furcula and were also used for climbing.
- The hind limbs are built on the typical avian plan.
Evidences from comparative anatomy
A comparative anatomical study of any organ system in the diverse members of a given phylum reveals a basic similarity of form which is varied to some extent from one class to another. Such studies provide many pieces of evidence of organic evolution. We can explain comparative anatomy under the following heads:- Connecting link
- Homologous organs
- analogous organs
- Vestigial organs
- Atavism or Reversion
Connecting link
The animals standing between two groups or resembles with two groups of animals are called connecting links. The connecting link between plants and animals is Euglena. Similarly, the connecting link between reptiles and birds is Archaeopteryx.Homologous organs
The organs which are similar in origin and development but perform different functions are called homologous organs. For example, the flipper of a seal, wings of a bat, forelimb of horse and arms of human are the homologous organs as they are morphologically similar but different in function. These organs also suggest that evolution has taken place.Analogous organs
The organs which are similar in function but different in origin and development are known as analogous organs. For example, wings of insects, wings of birds and bat perform the same function but these organs are different in origin and development. The presence of analogous organ is also a proof of organic evolution.Vestigial organs
There are many organs that have no apparent function and are useless in one animal but are well developed and functional in others. These organs are called vestigial organs. More than 100 vestigial organs are found in the human body. For example, vermiform appendix, muscles of the external ear, nictitating membrane, semilunaris in the human eye, wisdom teeth in man, wings of Ostrich, Hind limb of snakes.Atavism or Reversion
Atavism or Reversion is the reappearance of an ancestral trait which is presently absent or vestigial, e.g., small tail in some babies, addition mammae below the normal ones, dense body hair in males, ability to move pinnae somewhat irregularly and enlarged canines in humans. Atavism supports organic evolution that living organisms have the ability to develop even lost or non-functional structure.Evidences from Embryology
It became evident from the comparative embryological studies that there was one developmental pattern. In all organisms, life begins with a unicellular structure. The embryos of a fish, frog, turtle, bird and man resemble one another so closely that it becomes difficult to distinguish them. Mammalian embryo possesses through a fish-like amphibian-like, reptiles like and birds like stages during the development. It means that during the process of development of an organism same of the evolutionary steps are repeated in different groups of organisms. The sequence of embryonic development I vertebrates show striking similarities:- Gill clefts and notochords appear in the embryonic development of all vertebrates from fish to man.
- Vertebral column is formed by the replacement of notochord in adults.
- Gills are replaced by lungs in adults.
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