Organic evolution is the process by which the lower forms of life by gradual modifications accumulated through successive generations over million of years give rise to higher forms of life thereby generating organic diversity and organic progress.
Darwin described organic evolution as “descent with modification”. Weinberg depicts it as “Changes in the gene pools of populations”. Buettner-Janusch defines it as “Changes in gene frequencies between ancestral and descendant populations” or “Changes in the morphology of organisms through time” or “Changes in numbers and kinds of animals within major lineages”. In other words, evolution may be viewed as the change in descent, as change in genetic composition of populations as morphological differentiation exhibited a set of animals or as the diversification of taxa in a larger axonomic set.
All these definitions reveal the basic elements of evolution. First organic evolution involves “a fairly continuous, long-term multiplication of species and therefore a corresponding increase in organic diversity”. Secondly, organic evolution involves organic process or “a raising of the upper level of biological efficiency”. Evolutionists recognize that both the genetic information and the information stored in memory are “basic internal adaptive mechanisms” regulating the growth patterns and “physical development of organisms”. Thus, organic evolution reflects the emergence, divergence and progress of life across millions and millions of years.
Evidences of Evolution:
The theory of organic evolution appears to be the most accepted explanation for the occurrence of varied forms of plants and animals. But an absolute proof in this connection is lacking. The circumstantial proof is the only type of evidence available. The convincing evidences are as follows:
(1) Evidences from Morphology and Comparative Anatomy: The structure of animals, both external and internal, provides a source of evidence for evolution, the branch of biology that deals with the form and external structure of plants and animals is called morphology. By comparing organs, musculature and tissues, it can be said that man and some other vertebrates have developed from a same stock following evolution.
Evolution of the organisms can be better demonstrated with the help of ‘homology’ and ‘analogy’. Homology is similarity due to inheritance from an ancestor with the trait. The homologous organs have common origin and are built on the same fundamental pattern but perform varied functions and have different appearance. The homology is seen in limb structure, brain structure, structure of heart, which is, or way evident that there is a common descent with modification. The analogous organs have almost similar appearance and perform the same function but these develop in totally different groups on totally different pattern, the wing of a butterfly, bird and bat serve the same purpose of uplifting the body in aim, but their basic structure is totally different. Anatomy deals solely with the internal structure of organisms. A comparative study of anatomy reveals the similarities and dissimilarities between man and the other higher primates. The overall skeletal structure and bones are more or less same in man and apes.
(2) Vestigial Evidences – The vestigial or rudimentary organs are the useless residue of structures or organs, which might have been large and functional in the ancestors. These are under size, degenerated and non-functional. Weidersleim has listed nearly 100 vestigial characters in man. Vermiform appendix, nictitating membranes, wisdom teeth, ear muscles etc. are some of them in man.
(3) Evidences from Embryology– Embryology is a specialized branch of biology, which deals with the formation and development of the embryo. Comparative embryology furnishes several outstanding evidences for evolution. earnest Haeckel formulated the ‘Biogenetic Law’ more after known as ‘The theory of Recapitulation”. The theory states that ‘Ontogeny recapitulates phylogeny’, which means the embryos in their development repeat the adult stages of their ancestral forms. Von Baer’s principles of embryonic differentiation constitute a better guide to embryological evidence for evolution.
(4) Evidences from Paleontology – Palaeontology is the study of fossil remains, which provides a very reliable information regarding the fauna of particular periods. Any type of remain of living forms from the remote past as obtained from the earth may be considered as a fossil. The study of fossils reveals the existence of life in past and the course of evolution of plants and animals.
(5) Evidence from biochemistry and physiology – The most convincing evidence of descent from a common ancestor comes from the similarities in the biochemical composition and physiological activities of organisms. All the animals including primates are made up of similar elements and bio-chemical substances such as carbohydrates, protein and fats etc.
fundamental similarity among different groups of animals so far digestive, respiratory and reproductive physiology is concerned.
(6) Evidences from taxonomy – Taxonomy is the science of classification. The natural system of classification is based upon similarity and such similarities of structure could be only due to an origin from common ancestors. More facts that animals could be graded in order of increasing complexity is an evidence of evolution.
(7) Evidences from genetics – The final line of evidence for evolution is drawn from genetics, ‘the science of heredity’. It has been established now that genes are quite constant and are inherited unchanged generation after generation. But genes undergo changes producing mutations and variations. These changed genes determine the character in a different direction than the original. The natural forces of isolation and natural selection operate on these mutations. The same selection and interbreeding might have occurred and is occurring on large scale in nature and, thus would have established new species.