There are different forms of animals and plants in the nature. The development of new complex living organism both plants and animals on the surface of earth as a result of slow but continuous changes. This mechanism is called the organic evolution. The term evolution mean to hardly change from one form to another (Latin e = out + volvere = to roll).
(a) Evidences from Comparative Anatomy and Morphology: Comparative anatomy of animals provides the most extensive evidences of organic evolution. Though the structure and functions of different organs of animals are varied, the basic pattern of organisation are aim the same in a group or a class of animals or plants.
A few examples anatomical similarities are as follows:
(i) Homology: Homologous organs are structurally and ontogenetically similar due to common ancestry but may have varied functions. For example, there appears little similarity between flippers of whale, wings of bats, forelimbs of horse and the hands of human being. All the structures are dissimilar in look and the functions are also quite different. But all these structure possess almost same number of bones, muscles, nerves and blood vessels and their mode of development are also almost similar. All these structures have common origin and are built on the same basic pattern, but due to their adaptation for different functions they are superficially dissimilar. The existence of homologous organs suggests that all these vertebrate animals have inherited the basic structure from 4 common ancestral stock.
Forelegs of Amphibia
Flippers of Sea
Wings of Birds
Wings of Bat
Forelegs of Horse
Hands of Human
(ii) Analogy: They have structural differences but have functional (1) similarities means they are coming from the same environment.
Wings of Bat and wings of Butterfly are the common example of analogous organs.
(iii) Vestigial Organs: The most convincing morphological evidence of evolution is provide by the vestigial organs. Sometimes an organ which is well developed in one group may be present in rudimentary and functionless condition in another group. Such reduced and functionally useless organs are called vestigial organs. Vestigial organs were once functional and fully developed in the ancestral form but have lost their significance and are reduced structurally in the present form.
The presence of all these vestigial organs in different animals can only be explained by the fact that the ancestors of these animals had used these organs. Because of the change of environment or mode of life these organs became nonfunctional and so became reduced in size.
The common vestigial organs of Man are:
(a) Vermiform appendix.
(b) Coccyx.
(c) Nictitating membrane.
(d) Wisdom teeth.
(e) Body hair.
(f) Ear muscle.
(g) Abdominal muscle.
(h) Nipples of Male (Man)
(i) Clitoris of female.
(b) Evidences from Comparative Embryology: The study of the development of an organism from egg to the adult is known as embryology. Amongst the evidences of organic evolution, the embryological evidences are found to be the most conclusive evidences of evolution. It has been observed that the embryos of different groups of animals have many similarities in their development than the adults. For example, the embryonic development of all groups of vertebrates have striking resemblance during their early stage of development. All multicellular animals start their development is a single zygote. The zygote divides mitotically to form the blastula. The cleavage, blastula and gastrula are almost similar in all the groups of vertebrates.
Embryos of all groups of vertebrates possess gill pouches in their early development but; functional gills are found only in fishes and in tadpoles of amphibians. The avian and mammalian embryos possess simple tubular heart and same pattern of aortic arches that are found in fishes. The vertebrate embryos possess notochord which is present only in protochordates.
Why all these structures found only in lower chordates, develop in the embryos of reptiles, birds and mammals? It is because these animals have evolved from a common ancestor.
(c) Evidences from Palaeontological Records or Geological Evidences: Palaeontology means the study of fossils of ancient plants and animals. The direct evidence of organic evolution is provided by fossils. Fossils provide the missing links in the evolution of the present day flora and fauna.
Fossil remains are found deep under the soil. They are therefore have to be excavated carefully. Different types of plant and animal fossils have been discovered in different geological strata. Whenever any kind of fossil is discovered in any strata it becomes necessary to ascertain the geological time at which the organism was in existence. It is also necessary to ascertain its evolutionary position by linking it up wich fossils of related forms found in its lower and upper strata in the same place or in other places. It is also necessary to know whether such fossils have been found in other places and in other geological strata. The period in which the animals or plants were existed can be determined by study of fossil records.
