According to theory of evolution organisms living today have arisen from earlier types of organisms by a process of genetic change that has occurred over a period of several billion years. The fact that all organism have arisen from a common ancestor explains why they have the same mechanisms for the storage and utilization of genetic information, many of the same types of cellular organelles and similar types of enzymes and metabolic pathways. Evolution explains how a single species can give rise to numerous other species. The biologists may argue over the mechanisms of evolution; they agree that all life descended with modification from single common ancestor.
(1) EVIDENCES FROM PALEONTOLOGY
It is study of fossils. Fossils are remains of life from the past. Fossils are formed when organisms become buried in sediment, the calcium in bone and other hard tissue is mineralized and the sediment is converted into rock. They may include entire organism, hard skeletal structures and casts petrifications, impressions, imprints and fossilized faecal pellets. Earliest known fossil plants are about 410 million years old. Seed bearing plants with feather like leaves similar to living cycads were abundant in Mesogoic era. Oldest fossils of flowering plants or angiosperms are from early cretaceous period.
(2) EVIDENCES FROM COMPARATIVE ANATOMY
Comparing the structures of the parts of the bodies of different organisms is probably the most commonly used evidence of evolution. In order to gather comparative evidence for evolution, biologists study external characteristics, examine bones and teeth dissect organs systems, study sections of tissue under the light microscope and examine finer details of cells and tissues under the election microscope.
(i) Homologous Structures:
Comparative anatomy reveals that certain structure features are basically similar. For example the basic structure of all flowers consist of sepals, petals, stamens, stigma, style and ovary yet the size colour, number of parts and specific structures and different in different species. Organ having similar basic pattern but modified into different forms to perform characteristics functions are called homologous structures.
(ii) Adaptive radiation:
When a group of organisms share a homologous structure which is differentiated to perform a variety of different functions, it illustrates a principle known as adaptive radiation variations between different species within the class enable them to adapt to different habitat.
Analogous Structure: Similar structures, physiological process or modes of life in organisms that differ in their structural but show adaptations to perform the same functions are called analogous structures. E.g. Presena of thorns o plant stens and spines on animals. The existence of analogous structures suggest convergent evolution. In this type of evolution. The environments act through the agency of natural selection favouring advantageous variations.
(3) EVIDENCE FROM COMPARATIVE EMBRYOLOGY
The evidence of progressive development on the basis of embryology can be seen in plants. the gametophytes of primitive mosses and forms are represented by Protonema produced by germination of the spores has similar structure, physiology and pattern of growth to filamentous green algae from which they thought to be evolved. Alternation of generation in plant life cycles and homologous variation in it are considered to be due to adaptations to various environmental conditions e.g. Gymnosperms are regarded as a group in-between land plants and plants which still requires water for fertilization. In cycas male gametophyte resemble pollen grains of angiosperms in that it is distributed by wind. As it develops pollen tube is formed but instead of carrying gametes it act as hanstorium for absorption of food. The gametes are flagellated and swim to ovule to fertilize the egg present there. Cycas therefore represents intermediate group of plants. it suggests that gymnosperms are adapted to different environment and are phylogenic intermediate forms between non vascular plants and angiosperms. Many of these intermediate forms are extinct and are represented by fossils.
(4) EVIDENCE FROM COMPARATIVE BIOCHEMISTRY
As techniques of biochemical analysis have become more precise, this field of research provided evidences for evolution. The occurence of similar molecules in complete range of organisms suggest the existence of biochemical homology in similar way to the anatomical homology shown by organs and tissues. Most of the research which has been carried out on comparative biochemistry has involved analysis of primary structure of protein molecules such as cytochrome, haemoglobin and most recently nucleic acid molecules particularly r RNA.
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