The Process of Evolution
The process of evolution was detected in ancient times. Both in India and in Greece, there were important ideas of evolution. In modern times, the theory of evolution is mainly the work of Linnaeus (1707-78), Buffon (1707-88), Erasmus Darwin (1731-1802), Lamarck (1744-1829), Charles Darwin (1809-82) and his followers.
On the Origin of Species written by Charles Darwin (1859) gave details and demonstrations of his scientific theory of evolution, according to which, life on the earth evolved by a gradual and yet continuous process from the earliest forms of living organs to the latest product, man.
Natural selection, variation and heredity are said to be the factors through the operation of which new species arise out of existing ones. When new characters are produced by the variability of organisms, natural selection decides their survival or death. If the characters do not adapt to their environment, they are eliminated in the competition. If, on the other hand, they equip themselves better for the struggle, they tend to survive. The offsprings of the successful tend to resemble the parents in exhibiting the favoured variation to a greater degree than the parents, and a new type becomes established by a continuous piling up of small useful accretions through many generations.
The two original components of Darwin's theory were (I) that evolution is gradual, and (ii)that the nature of the change is dictated by natural, not divine, selection. Both of these are closely interlinked, and both are at the heart of controversy today, as they were in Darwin's time.
Many naturalists accepted Darwin's gradualism because it accorded well with what they saw in living species. But critics could not accept that all the world's marvellous species and their extraordinary structures, such as those of the eye, could have arisen only by chance. Some biologists accepted that minor changes might be the result of natural selection, but held that beyond extremes within a range of variation, a new species could not arise by natural selection alone. The only way in which the boundaries of species might be breached, they contended, would be through a sudden jump.
Paleontologists who dug up and classified the remains of extinct species raised another major objection to gradualism. They argued that if Darwin were right, they should be able to find a series of specimens that could be laid out in a gradual continuum from one major type of animal to another. If, for example, reptiles evolved into mammals, there should be fossils representing every gradation between these two groups. Instead, the paleontologists found more gaps than continua. Darwin conceded this, but he thought that further researches would reveal the intermediate links. As it turned out, only a few links have been found, and this issue is a part of today's controversy.
There are biologists today who maintain that the evolutionary process jumps from one species to another. Their theory is called 'saltationism' (from Latin saltare to leap.)
The early geneticists maintained that plants and animals sometimes produce offsprings with unusual abnormalities or variations that could be considered well outside the normal range of variation. These odd offsprings were called sports. Hugo de Vries, an early Dutch geneticist, also observed that the sports undergo some kind of parmanent, largescale alteration of the hereditary units. He called the change a mutation. On the other hand, graudal changes or variations were called by him fluctuations.
In the early twentieth century, evolutionists were divided into two camps. There were geneticists, who saw only evidence for sudden discontinuous change or mutation.. They supported the saltationist view. On the other hand, there were naturalists who supported Darwinian gradualism. By the 1930s, however, the rift between these two camps came to be healed by a new evolutionary theory that Julian Huxley named the 'modern synthesis'. As part of the new theory, Dobzhansky emphasized the need for what he called isolating mechanisms. He recognized that a new species could not emerge from an old one in the wild, if its early members continue to breed with the parent stock. The novel features would either be swamped by the parent stock, or they would be spread throughout the existing species, causing the entire species to evolve slightly. If part of the species population is to split from the parent stock, it must be isolated from the larger population of stock. A river, mountain range, or some other geographic feature must prevent the small variant group from breeding with its original stock. Eventually, the isolated population would become so different that biological differences would prevent inbreeding.
In 1972, Stephen Jay Gould and Niles Eldredge asserted that evolutionists had become too rigid in insisting on gradualism. They put forth a new theory that reduced gradualism to a rare event and named the dominant phenomenon 'punctuated equilibria'. According to them, species are, for most of their existence, in evolutionary equilibrium or stasis. They change very little, if at all. But once in a while the stasis is punctuated by a sudden 'speciation event', somehow, a small population of the parent species begins evolving rapidly and, within a relatively few generations, becomes a distinct species.
However, there are evolutionists who continue to stick to the gradualist view, and at the moment, there is no clear resolution in sight. The present debates point to the possibility of the emergence of a new scientific theory which might give a better understanding of the intrinsic 'how' of the evolutionary process.