Organic evolution is the development of newer types of organisms from the pre-existing ones through gradual or abrupt changes in their components. When not specified, evolution means organic evolution. The term was coined by Spencer.
Darwin has termed evolution to be “descent with modifications”.
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 with major lineages”.
In other words, evolution may be viewed as the change in descent, as a change in the genetic composition of populations as morphological differentiation exhibited a set of animals, or as the progressive diversification of taxa in a larger taxonomic 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 an 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 for Human 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-
Evidences from Morphological 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 are called morphology. By comparing organs, musculature, and tissues, it can be said that man and some other vertebrates have developed from the same stock following evolution.
The 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 a common origin and are built on the same fundamental pattern but perform varied functions and have a different appearance. The homology is seen in limb structure, brain structure, the structure of the heart, which is, or way evident that there is a common descent with modification. The analogous organs have an almost similar appearance and perform the same function but these develop in totally different groups on totally different patterns. For example- the wing of a butterfly, bird, and bat serve the same purpose of uplifting the body in the air 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 the same in man and apes.
Evidences from Embryology:
Embryology is another source of evidence to support evolution. Embryos of different animals very closely resemble each other at their early stage. One is not easily distinguishable from another at that stage. Differentiation starts only when they start to mature. This phenomenon is known as Baer’s Law. The embryos of different animals start developing to take different forms as per the animals. Following Ernst von Haeckel this may be described as the ontogeny (individual development of a given embryo) is governed by its phylogeny (that is, the evolutionary development of the species to which the embryo belongs). For example– the embryo of fish and also that of a man at their early stages have similar gill arches. In the case of fish, it develops to gills through which fishes breathe, while in man that arch eventually changes and develops to certain features useful to man. This gives evidence that a very distant past ancestor of man had gills. The presence of gill arches in the human embryo simply repeats an ancestral form of life.
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. Wiedersheim has listed nearly 100 vestigial characters in man. Vermiform appendix, nictitating membranes, wisdom teeth, ear muscles, etc. are some of them in man.
Evidences from Paleontology:
Paleontology is the study of fossil remains, which provides 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 the past and the course of the evolution of plants and animals.
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.
Evidences from Physiology and Biochemistry:
In recent years several works have been done on physiology and biochemistry and the findings reinforced the relationship of man with other animals, which provide evidence for evolutionary development. Man and several other animals have similar body functions, which are performed in similar manners. Some of these are reproduction, digestion, respiration, etc. The studies on blood chemistry reveal that the reaction between antiserum and blood serum of man and that with the blood of an ape is similar. But when the same test is done, taking the blood of a monkey instead of an ape, the results are less alike, indicating that man is more closely related to apes than to monkeys. In this way, several experiments have been done taking into consideration some other biochemical materials to ascertain the position of man in relation to other higher primates, or in other words to trace the evolutionary line.
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.