Individuals of a population or of different populations of a species have different genetic constitution. These also exhibit unequal reproductive capacity. Any given generation of a population is descended from only a small fraction of previous generation. Also in a given generation, the genes transmitted by those individuals who reproduce successfully reproduce, will predominate. It means generic composition of a population becomes altered each successive generation. It has changed in past, is changing today and will continue to change in future. This ever changing phenomenon is evolution. It can be defined as �the changes in the genetic composition of a population with the passage of each generation.�
Charles Darwin�s name is closely associated with the concept of evolution, but the concept for the first time appeared in the writings of ancient Greek philosophers.
Greek Theories:
1. Thales (624-548 B.C.) propounded the theory of aquatic or marine origin of life.
2. Anaximers (588- 524 B.C.) described air to be the source of life. He proposed the theory of abiogenesis.
3. Aristotle (384-322 B.C.) was vitalist and his ideas dominated the biological thoughts well over a thousand years. He suggested that various organisms constitute a series the so called ladder of life in which organisms can be arranged in a sequence of increasing complexity, from non-living matter through plants to lower and then to higher animals. He placed man on the top of this ladder.
Pre-Modern Theories:
1. Francis Bacon (1561-1626) who reviewed Aristotalian idea and presumed that new species could arise from the old species by degenerative process caused due to mutability in the species.
2. Maupertius (1698-1759) was the first to propose a general theory of evolution. He proposed that hereditary material was particulate and transmitted through both maternal and paternal sides of the family.
3. Linnaeus (1707- 1778) is known as father of taxonomy. He believed in special creation. He presumed that species are created by God and are immutable and fixed entities.
Modern Theories:
1. Lamarck�s Theory of Inheritance of Acquired Characters (1744-1829) Lamarck�s theory emphasizes the influence of environment on living beings. The changes introduced by the environment are acquired by the living beings and are inherited to the next generation.
2. Darwin�s Theory of Natural Selection (1809-1882) Darwin formulated the �Origin of Species by Natural Selection.� To explain some phenomenons which were not suitably explained by natural selection Darwin proposed some more theories- Theory of Pangenesis, Theory of Sexual selection, Artificial Selection.
3. Weisman�s Theory of Continuity of Germplasm- Considering the futility of Darwin�s Theory of Pangenesis, August Weisman (1892) proposed that the cytoplasm of the animal body is differentiated into somatoplasm and germplasm. The germplasm produces gametes which transmits the characteristics of parents into the offspring�s.
4. De Vries Theory of Mutation- Darwin in his Theory of Natural Selection described occurrence of variations but did not explain the method of their origin. De Vries (1848- 1935) suggested that variations which are important for evolution, are sudden and large, which he called mutations or saltations.
5. Recapitulation theory of Haeckel Ernst Haeckel (1811) proposed that �Ontogeny recapitulates phylogeny� i.e. the development of the individual repeats the evolutionary history of race, condensing some stages and eliminating the others.
Thus, it could be concluded that the process of evolution is an ever continuous process; it has not stopped but is occurring more rapidly today than in many of the past ages. Although, the process is too slow to be observed, there are notable examples of evolutionary changes which have occurred in past, and recorded in the form of fossils inside the rocks.
